Interpreting Spatial Structure, Visual Axes and Borrowed Scenery of Sui–Tang Luoyang Within the Historic Urban Landscape Framework

Abstract

Sui–Tang Luoyang represents a classic achievement in Chinese capital planning, yet research remains dominated by archaeological and historical–geographical approaches, lacking a unifying theoretical framework. This research addresses this gap by applying the Historic Urban Landscape (HUL) approach to systematically interpret the city’s integration of built form and natural landscape. This research developed a three-dimensional analytical scheme comprising spatial structure, visual axis, and borrowed scenery and implemented it using historical documents, archaeological data, GIS, and cross-validation methods. The results reconstruct the city’s triple spatial structure (Palace City, Imperial City, Outer City) and identify a near-north–south central axis that connects the palace with the Longmen Yique and Mount Mang, forming a dominant view corridor and ritual sequence. Further analysis examines how multi-layered borrowed scenery embodies and articulates the traditional “Harmony of Nature and Humanity” philosophy. This research supports the plausibility of the applicability of HUL to sites with scarce surface remains and provides a transferable framework for the holistic conservation, view-corridor management, and digital reconstruction of historic cities.

1. Introduction

Historically, Sui–Tang Luoyang represents a quintessential achievement in the evolution of ancient Chinese capital planning and remains a classic model for East Asian urbanism. The city is renowned for its rigorous axial symmetry and its sophisticated integration of the built environment with the surrounding natural topography. Since the Western Zhou dynasty, six major capitals have been established in this region, creating a profound spatial layering of history, as vividly illustrated in Figure 1. In 605 CE (the first year of the Daye era), Emperor Yang of the Sui dynasty proclaimed Luoyang the Eastern Capital, a strategic status maintained into the Tang dynasty. The city attained its urban zenith during the reign of Empress Wu Zetian, emerging as one of the most sophisticated and representative secondary capitals of the flourishing Tang period [1]. However, despite its historical grandeur, Sui–Tang Luoyang now presents a significant challenge for heritage conservation: it is a “surface-remains-scarce” site. Although archaeological excavations have revealed a rigorous, nearly square urban plan organized into a characteristic grid of li-fang (wards) and a three-tier spatial hierarchy comprising the Palace City, the Imperial City, and the Outer City [2], subsequent geopolitical turmoil has left no above-ground structures intact.

The Historic Urban Landscape (HUL) framework, ratified by UNESCO in 2011, represents a significant paradigm shift to address such challenges: moving from the isolated preservation of artifacts to treating the city as a dynamic, layered system resulting from the long-term interaction between human agency and the natural environment [3,4]. By transcending the conventional focus on “historic centers,” the HUL approach advocates for a holistic perspective that integrates broader urban–rural geographical contexts into the conservation discourse [5]. This conceptual evolution is particularly salient for heritage sites where surface-level remains are scarce, as it shifts the analytical focus from tangible entities to intangible dimensions, including spatial relationships and historical stratification [6,7]. In the specific context of Luoyang, conventional heritage preservation tends to prioritize individual monuments or artifacts while overlooking the continuity of the overall spatial structure, which leads to a gradual separation between the physical remains and their humanistic spirit [8].

The HUL approach conceptualizes historic cities as palimpsests in which tangible fabric and intangible practices are interwoven across time. For surface-scarce capitals, HUL’s emphasis on layering and context is particularly relevant because it allows researchers to infer continuity through relationships rather than through surviving objects alone. Furthermore, the HUL framework advocates for balancing the relationship between conservation and sustainable development [9,10]. Within this perspective, the cultural landscape tradition underscores the inseparability of nature and culture, situating urban form within the regional matrices that conditioned Luoyang’s siting [11].

To operationalize these theories, this research delineates three core elements that define the spatial logic of Luoyang: spatial structure, visual axes, and borrowed scenery. In this context, a “Visual Axis” is conceptualized as a regional visual alignment rather than a continuous physical street. Functionally, the tripartite spatial configuration of the Palace, Imperial, and Outer Cities delineated urban zones through a rigorous hierarchical sequence to accommodate governance and residence [12]. Culturally, the north–south central axis established a ritual sequence that symbolized unified imperial authority and the ideological fusion of governance and education [13]. The urban ensemble further reflected a Daoist cosmological paradigm the harmony between nature and humanity conceptualizing the capital as a microcosm of the universe [14]. Planning thus emphasized strategic alignment with natural topography, where distant mountain peaks were integrated into visual corridors. Visual analysis provides a tractable bridge from these intangible meanings to measurable spatial signals, enabling the transition from analyzing physical loss to documenting spatial continuity through the identification of the “urban spirit” (genius loci).

Extant scholarship on Sui–Tang Luoyang has traditionally been anchored in archaeological and historical–geographical methodologies, which have been instrumental in reconstructing the physical framework and the distribution of key relics [15]. However, a significant theoretical and methodological lacuna persists. On the one hand, while traditional archaeology has yielded a wealth of empirical data, it often struggles to interpret the underlying planning logic and cultural intentionality [12]. On the other hand, research predominantly focuses on the identification of discrete sites, often overlooking the city’s spatial integrity and its organic connectivity to the surrounding natural environment. A robust interdisciplinary framework remains elusive, and theoretical support for a unified “space–time–landscape” approach is notably inadequate [16].

The operationalization of the HUL framework for surface-scarce Chinese capitals remains nascent. Previous reconstructions have often neglected the role of topography, thereby obscuring critical planning relationships between the urban form and its environmental setting. Although the reconstruction of historical axes has been proposed to integrate fragmented urban forms [7,17], a systematic methodology that synthesizes spatial, visual, and natural dimensions is currently absent. Accordingly, this research contributes an operational framework that integrates archaeological data with DEM-based visibility analysis to formalize “spatial structure, visual axes, and borrowed scenery” as a measurable triad. This approach moves beyond purely qualitative descriptions to provide reproducible metrics that can be tested against terrain constraints and compared across cases.

Addressing the aforementioned gaps, this research seeks to systematically interpret the spatial logic of Sui–Tang Luoyang by applying an integrated HUL-informed analytical scheme. By moving beyond purely qualitative descriptions, this research aims to formalize how key nodes acting as landscape catalysts activate the connectivity between the urban fabric and the surrounding natural environment [11]. The first objective is to reconstruct the spatial hierarchy of the city tiers to understand how they expressed ritualistic and political order through the deliberate siting of catalytic nodes [13]. The second objective is to operationalize visibility in reproducible terms by translating intangible heritage concepts into testable spatial parameters through analysis based on digital elevation models. The third objective is to bridge the gap between historical interpretation and contemporary management by mapping these spatial–visual indicators to practical conservation tools such as view corridor protection and height control buffers.

Furthermore, grounded in memory theory [7], this research explores heritage reinterpretation for remains-less capitals by introducing a sociocultural memory perspective to activate public imagination and enhance emotional resonance [15]. Theoretically, this research extends the HUL approach to sites with scant physical remains through a novel interpretative framework [18]. Practically, the findings provide applicable insights for the sustainable management and presentation of Luoyang and other globally significant historic cities [19].

2. Materials and Methods

2.1. Theoretical Framework: Operationalizing the HUL Approach

The Historic Urban Landscape (HUL) approach provides a comprehensive framework for integrating the layered natural and cultural values of urban heritage into sustainable development strategies, thereby aligning heritage conservation with broader socio-economic and environmental objectives [3,20]. However, for a “surface-remains-scarce” capital like Sui–Tang Luoyang, the HUL principles must be translated from broad policy guidelines into an analytically tractable research design. Rather than strictly adhering to procedural checklists, this research distills the HUL perspective into three core analytical dimensions spatial structure, visual axes, and borrowed scenery that capture the city–landscape coupling most relevant to sites with minimal physical remains [21]. These domains do not replace the official elements of the HUL recommendation but represent a systematic synthesis of international scholarship on the city as a dynamic cultural landscape [22,23,24,25,26].

To bridge the gap between theoretical values and empirical evidence, this research groups the core attributes of Luoyang’s historical landscape into domains that structure both the analysis and the resulting management strategies [27,28,29,30]. The logical mapping from theoretical rationale to measurable proxies is summarized in

Table 1. Operationalization matrix: From theoretical dimensions to measurable proxies.

Analytical Dimensions	Theoretical Rationale	Operational Proxy
Spatial Structure	Urban form as historically layered organization (HUL: layering and institutional order); city tiers encode ritual and functional hierarchy [22].	Digitized palace, imperial, outer-city polygons, and ward grid; morphometrics (area, standard compactness index) and adjacency-based connectivity indices used as the spatial skeleton for analysis.
Visual Axes	Axes and framed termini translate ritual and cosmological logics into perceivable alignments; visual relations act as legible carriers of intangible value [23].	True-north azimuths (°) between key nodes; corridor polygons generated by intersecting viewsheds along the axis bearing with an ±θ angular tolerance (specified in Section 2.4); terminal-landmark intervisibility (binary) and skyline continuity ratio (%) computed along equal-angle bins as a legibility indicator.
Borrowed Scenery	Cultural–landscape view: mountains and rivers are co-constitutive heritage elements shaping orientation and meaning rather than mere backdrops [24].	Georeferenced peaks and rivers; counts of intervisibility between urban nodes and natural features; subtended angular extent (°) to approximate perceived salience along corridors; corridor–feature overlap statistics.

. This operationalization ensures that the intangible dimensions of heritage are consistently measured and cross-validated through the historical, cartographic, and GIS-based workflows detailed in Section 2.2, Section 2.3 and Section 2.4.

Our framework builds upon recent HUL-based reconstructions of Luoyang’s layered evolution [20,31] by transforming qualitative arguments into testable, reproducible indicators, including axis azimuths, intervisibility, and skyline continuity. Critically, the HUL paradigm exhibits a profound ontological resonance with the traditional Chinese philosophical ethos of “Harmony between Nature and Humanity” (Tianren Heyi). The morphology of Sui–Tang Luoyang characterized by its rigorous axial order and strategic landscape integration provides a historically grounded context for the indicators used in this research [30], allowing the research to reconcile ancient planning cosmology with contemporary conservation instruments.

To ensure methodological clarity and replicability, the subsequent section (Section 2.2) provides precise scholarly definitions for the key terms used throughout this research including visibility, landscape catalysis, and borrowed scenery followed by their specific measurement rules and parameters. This is followed by a description of the data acquisition process (Section 2.3) and the analytical workflows (Section 2.4) that implement the proxies established in this framework.

2.2. Operational Definitions and Parameters

To ensure analytical consistency and methodological replicability, this section provides scholarly and operational definitions for the key concepts applied within the HUL framework. Each concept is defined in both theoretical and operational terms to establish measurable constructs for spatial–visual analysis.

Visibility and Perceptual Threshold: Visibility refers to the geometric intervisibility between an observer and target within a three-dimensional terrain, capturing the portion of space unobstructed by intervening topography or built elements [32]. In landscape and urban visibility analysis, viewshed methods are commonly used to compute the spatial extent of visible areas, reflecting how landscapes are perceived from specific vantage points [33]. Research on landscape visibility further indicates that perceivable targets in outdoor environments must achieve a sufficient angular presence within the observer’s field of view to be recognized as meaningful features, and quantifiable visibility metrics form the basis for evaluating visual prominence and scenic impact in planning and design contexts. Accordingly, this research establishes an operational threshold for visibility based on documented analytical practices in visibility modeling and perceptual studies. Visibility is modeled using digital elevation data within a defined analysis radius to ensure computational consistency, and features that satisfy both geometric visibility and perceptual prominence criteria are considered meaningfully visible for subsequent analysis.

Visual Axis and Alignment: A visual axis is defined as a symbolic and structural alignment connecting key urban nodes with natural landmarks, functioning as an experiential and ceremonial vector. In this research, the term “visual axis” is used to denote a regional visual alignment an analytically testable landscape-management frame within the HUL approach rather than implying the existence of a continuous extramural paved avenue. This concept builds upon HUL and cultural–landscape theories that interpret axiality as a medium of spatial meaning and political power articulation [22]. Operationally, visual axes are represented by azimuthal bearings derived from the directional relationship between reconstructed archaeological nodes and corresponding topographic termini. Axial stability is determined by the minimal angular deviation between the historical orientation (as recorded in texts) and GIS-reconstructed geometry.

Landscape Catalysts: Landscape catalysts are specific urban architectural or infrastructural nodes that “activate” the experiential linkage between the city and its natural surroundings. This aligns with the HUL emphasis on identifying “key heritage nodes” as focal points for urban management [29]. In this research, major archaeological sites specifically Dingding Gate, Mingtang, and the Tiantang Palace are designated as Primary Observation Points (POPs) from which all visibility and skyline simulations are performed.

Borrowed Scenery (Jiejing): Borrowed scenery denotes the intentional incorporation of external natural features into a designed or urban visual composition, such that distant landscape elements become integral to the perception and experience of a place. Originally articulated in the seventeenth-century Chinese garden treatise The Craft of Gardens (Yuanye) as jiejing [34], the concept is found across East Asian landscape traditions, often translated in Japanese as shakkei and widely used in garden and landscape design to frame distant mountains, water bodies, or other natural features as compositional components of a foreground space. Importantly, borrowed scenery goes beyond a simple scenic background to reflect a philosophical and perceptual strategy that contextualizes human-made environments within a larger topography and cultural narrative.

Skyline Continuity Ratio (SCR): The SCR measures the degree of visual coherence at the mountain–city interface, serving as a proxy for the integrity of the urban–landscape boundary [23]. It is calculated as the proportion of the horizontal field of view occupied by discernible natural terrain when observed from a POP. A higher SCR indicates a more intact perceptual connection between the historical urban core and its natural matrix.

2.3. Data Sources and Processing

To ensure the reliability and reproducibility of the results, this research utilizes a multi-tiered data structure integrated within a standardized geospatial environment. All spatial analyses were conducted using ArcGIS Pro 3.3.3 (Esri, Redlands, CA, USA), ensuring full documentation of analytical parameters and compatibility with current geospatial standards.

2.3.1. Textual and Cartographic Sources

Primary sources for this research fall into two main categories. The first encompasses a range of historical texts, including administrative codes, geographical records, and literary works, which provide foundational information on the city’s systems, layout, and cultural context. The second category comprises ancient maps and modern archaeological reconstruction diagrams. Archaeological base plans were adapted from the Archaeological Atlas of Luoyang (Institute of Archaeology, CASS, 2015) and verified against the Atlas of Chinese Cultural Relics: Henan Volume (Cultural Relics Press, 2019). Literature analysis was strictly employed as a data acquisition procedure: published papers and monographs were systematically screened to extract toponyms, feature attributions, and plan annotations that could be cross-checked against archaeological mapping [12]. Where available, official sheets issued by Luoyang cultural heritage authorities were consulted for feature positions and naming conventions.

2.3.2. Digital Topographic and GIS Data

This research incorporates GIS spatial data to analyze the topographical conditions of the Luoyang Basin and their relationship with site selection [35]. The topographic foundation was derived from the SRTM 1 Arc-Second Global DEM (Version 3, 2023 update), accessed via the USGS EarthExplorer platform. This dataset provides a horizontal resolution of approximately 30 m, with a vertical RMSE of less than 10 m and horizontal accuracy within 15 m. For precise metric calculations, the DEM was reprojected to the WGS 84/UTM Zone 49 N (EPSG 32649) coordinate system. To verify visibility stability, the dataset was cross-validated against the 2015 ALOS AW3D30 (v3.2) model, showing a mean elevation deviation of less than ±8 m across the basin. GIS technology is now widely applied in Chinese archaeology, demonstrating significant potential in settlement analysis and environmental reconstruction [36]. To maintain consistency with Section 2.2, the DEM mosaic was clipped to a 30 km radius to provide a buffer that prevented edge effects during convolution- and viewshed-based operations, while all visibility queries and distance screens were strictly evaluated within the 25 km effective radius.

2.3.3. Data Preparation and Georeferencing

A multi-tiered verification strategy secured a trustworthy basis for analysis. Textual evidence was cross-checked across historical sources, with conflicts resolved via archaeological data. For spatial alignment, historical and reconstructed maps were georeferenced using 10 authenticated control points, confining the mean registration error (RMSE) to approximately 38 m at a 1:10,000 scale, in accordance with archaeological standards. These 10 control points were evenly distributed across the Sui–Tang urban extent (covering the north–south axial termini and the eastern–western ward limits) and prioritized excavated gate foundations, palaces, imperial corners, bridge abutments, and river confluences. First-order polynomial transforms were preferred to preserve global bearings relevant to axial analysis; localized spline warping was avoided.

2.3.4. Integration for Analysis Inputs

The harmonized datasets yielded several standardized outputs for subsequent modeling. These included a vector spatial skeleton encompassing the palace, imperial, and outer city tiers, along with the li-fang grid for form and layout measures. Additionally, urban reference nodes, or Primary Observation Points (POPs), such as Dingding Gate and Mingtang, were established for azimuth extraction and the seeding of viewsheds. A natural features layer containing major peaks and ridgelines, including Mount Mang and the Longmen Yique, was also generated for intervisibility and skyline computations. These processed inputs directly implemented the operational proxies defined in Section 2.2 and supported the analytical procedures reported in Section 2.4.

2.4. Research Methods

The analytical methods detailed below translate the theoretical constructs and operational definitions established in Section 2.2 into a reproducible, GIS-based workflow. This framework was specifically designed to empirically test hypotheses regarding spatial order, long-distance visibility, and the perceptual linkages between the urban core and its natural matrix. Section 2.4 articulates the end-to-end pipeline: from the geometric reconstruction and azimuthal validation of putative axes (Section 2.4.2) to systematic viewshed modeling utilizing an explicit parametric design (Section 2.4.3). This was followed by the computation of perceptual metrics formalizing “borrowed scenery” and skyline continuity (Section 2.4.4) and a multi-criteria evaluation procedure for identifying landscape catalysts (Section 2.4.5). Each subsection delineates the underlying theoretical rationale, precise computational execution, and primary analytical outputs, including cartographic layers, tabular manifests, and raster products. To ensure the robustness of the findings, all critical parameters including observer heights, analysis radii, atmospheric refraction, and Just Noticeable Difference (JND) thresholds were subjected to a full-factorial sensitivity sweep and documented to ensure computational reproducibility (Section 2.4.6).

2.4.1. Workflow Overview

The analytical research was structured into four sequential stages to transform raw geospatial data into interpretable landscape metrics. First, the process began with the spatial reconstruction of the urban skeleton, where georeferenced archaeological datasets were utilized to delineate city tiers and the ward grid. This geometric foundation supported the second stage of alignment extraction, which tested the azimuthal stability between urban nodes and landscape termini. Subsequently, the workflow proceeded to systematic visibility modeling by generating cumulative viewsheds from defined Primary Observation Points across the digital terrain. These simulations served as inputs for the fourth stage: the calculation of perceptual metrics, including the skyline continuity ratio (SCR) and the subtended angular extent of borrowed scenery. Finally, to ensure methodological rigor, the procedure concluded with a sensitivity analysis to validate result stability under varying observer parameters.

2.4.2. Reconstruction of Geometric Order (Axes)

The primary visual axes of Sui–Tang Luoyang were digitally reconstructed to translate the historical concept of “celestial alignment” into measurable geometric vectors. The central organizing line was delineated from archaeologically attested nodes that anchor ceremonial movement: extending southward from Cuiyun Peak on Mount Mang, passing through the Mingtang and Yingtian Gate, and terminating at the natural gateway of Longmen Yique. Historical descriptions implying precise cosmological orientation are hereby treated as testable hypotheses regarding geometric intentionality. To mitigate projection distortions inherent in planar coordinate systems over long-distance vistas, this research computed geodesic azimuths (referencing the ellipsoid) for all POP terminal pairs rather than relying solely on planar grid bearings. These geodesic metrics provided the “true” directional values reported in

Table 2. Operational parameters for visibility and viewshed analysis.

Parameter	Value	Description
Software Version	ArcGIS Pro 3.3.3	Core computational platform for geospatial analysis
Observer Height	15 m	Based on archaeological heights of Tang gate platforms [37]
Target Height	0 m	Bare-earth baseline to evaluate terrain-level visibility
Analysis Radius	25 km	Defined extent covering the Palace City to terminal landmarks
Refraction Coeff.	0.13	Standard atmospheric correction for curvature-adjusted LOS
Curvature Correction	Enabled	Compensation for Earth’s curvature in long-distance views
DEM Resolution	30 m	Derived from SRTM 1 Arc-Second (Version 3) input
Azimuth (Measured)	173.82°	Quantified alignment from Yingtian Gate to Longmen Yique

, while the node network was simultaneously maintained in the projected WGS 84/UTM Zone 49 N system to ensure that subsequent distance measurements and buffering operations remained consistent with the uniform metric space used for visibility modeling.

Axis stability was adjudicated using a dual-tolerance classification system reflecting differing functional roles. For the ceremonial core, such as the north–south axis traversing the Palace City, a strict tolerance of ±1° was applied to isolate high-precision ritual alignments. In contrast, a lenient tolerance of ±3° was utilized for subsidiary structures and landscape penetration corridors, acknowledging that historical siting in these zones likely admitted minor deviations. Each POP terminal pair was assigned a stability flag based on these thresholds to map the spatial distribution of robust geometric order. To validate the vertical integrity of the central axis, line-of-sight (LOS) profiles were generated from Yingtian Gate to Longmen Yique using the 30 m DEM. This profiling confirmed a clear geometric corridor free of topographic obstruction. As detailed in Table 2, the measured azimuth of the central alignment was 173.82°, quantifying the intentional orientation as modestly west of the true meridian. This procedure converted textual claims about axial cosmology into replicable bearings, establishing the geometric skeleton for the comprehensive viewshed modeling in Section 2.4.3.

2.4.3. Viewshed Modeling and Parametric Setup

Viewshed modeling was employed to translate hypothesized associations between axial geometry, urban nodes, and distant landforms into quantifiable evidence of visual connectivity. This analysis rigorously tested whether the putative termini of ceremonial lines remained discernible from Primary Observation Points (POPs) under terrain-constrained line-of-sight conditions, thereby grounding historical interpretations in spatially verifiable outcomes.

To ensure the rigor of the spatial simulation, the geodesic distances of the primary visual terminals were strictly measured: 14.5 km from the urban core (Yingtian Gate) to Longmen Yique in the south and 4.0 km to Cuiyun Peak in the north. Utilizing the pre-processed 30 m DEM described in Section 2.3, the analysis domain was defined by a 30 km radius centered on the urban core. This topographic buffer ensured redundancy around the analysis window, effectively mitigating edge artifacts during horizon scanning and raster convolution operations. Within this domain, visibility queries were strictly evaluated within a 25 km effective analysis radius, which was purposefully calibrated to encompass the aforementioned terminal landmarks (14.5 km and 4.0 km) while providing a sufficient background buffer. The 25 km limit provided a necessary background buffer to capture the continuous ridgelines of the Mount Wan’an and Mount Mang ranges, ensuring that the mountain silhouettes behind the landmarks were correctly rendered without “floating island” artifacts, a structural stability further confirmed by the sensitivity analysis in Section 2.4.6.

To address potential uncertainties in historical reconstruction and variable atmospheric conditions, a systematic simulation matrix was implemented. The primary model adopted a standard atmospheric refraction coefficient (k = 0.13), applied as a modeling convention consistent with the default curvature–refraction correction settings in GIS-based visibility analysis. This coefficient did not represent a fixed atmospheric condition, as refraction varied with temperature, pressure, and humidity. Observer heights were varied from 1.7 m (pedestrian level) to 20 m (representing monumental towers such as the Mingtang). Computations were executed using the ArcGIS Pro Spatial Analyst extension, with earth curvature and atmospheric refraction corrections strictly enabled. For every POP, a binary visibility raster was generated across the designated parameter combinations. These outputs were subsequently intersected with narrow buffers along the geodesic axis bearings to evaluate visual continuity within the specific ceremonial corridors. All analytical settings were standardized across runs to ensure the internal consistency and comparability of the visibility results.

2.4.4. Quantitative Metrics for Landscape Perception

The binary visibility results derived from the viewshed analysis were further processed to calculate quantitative metrics that represented human perceptual experience rather than simple geometric existence. To operationalize the concept of borrowed scenery, this research computed the angular extent subtended by the silhouette of each key landscape feature as viewed from the Primary Observation Points. Calculations were derived from the Digital Elevation Model by projecting rays to the extrema of the target landform to determine its apparent size.

A conservative operational threshold of 0.5 degrees was employed as the lower perceptual bound in order to establish a minimal condition of distant landmark legibility. While psychophysical research places the limit of human visual resolution (minimum angle of resolution) at approximately 1 arcminute (0.017°) for high-contrast detail [38], mere detectability does not equate to the structural dominance required for “borrowed scenery.” Therefore, the 0.5° threshold was applied here as a heuristic filter to distinguish dominant landscape catalysts from background visual noise, ensuring that the identified links represented significant morphological entities rather than faint pixels on the horizon.

Complementing the specific feature analysis, the skyline continuity ratio measured the overall visual coherence between the urban core and its surrounding mountain matrix. The index was computed by rotationally sampling the horizon at a fixed azimuthal interval of one degree ($∆\theta =1^{°}$) to determine the proportion of the field of view occupied by discernible natural terrain. The calculation was formalized as a summation function where the horizon was scanned stepwise. If N represented the total number of sampling intervals within the designated field of view and $v_i$ was a binary variable that equaled one only when natural terrain was visible within the effective 25 km radius and exceeded the minimum discernible threshold, the ratio was calculated as

$$\mathrm{S}\mathrm{C}\mathrm{R}={\displaystyle \frac{1}{N}}\sum _{i=1}^N{v}_i\times 100\%$$

To ensure metric stability, the underlying horizon analysis employed a 30 km terrain buffer to eliminate edge artifacts, alongside morphological filtering to suppress isolated pixel noise below the perceptual threshold. This formula was applied to both the full panoramic horizon and the restricted southern corridor facing the Longmen mountains, serving as a standardized proxy for the urban–landscape interface. For the assessment of this interface, a provisional heuristic threshold of SCR ≥ 70% specific to the Luoyang Basin was applied. Similar to the 0.5° visual filter, this benchmark was not a universal constant but a site-specific heuristic derived from the high topographic enclosure that historically defined the capital’s fengshui structure. While subject to future cross-site calibration, this threshold served in this research as a conservative baseline to quantify the intended “continuous mountain screen” and identify segments where modern encroachment has disrupted the historical horizon.

2.4.5. Multi-Criteria Identification of Landscape Catalysts

This analytical step translated the qualitative concept of catalytic nodes into a transparent, reproducible scoring system. To ensure a balanced assessment, each candidate node was evaluated across four measurable dimensions that synthesized visual, spatial, historical, and practical factors. Visibility Centrality quantified the cumulative visual frequency of a node across the ensemble of Primary Observation Points and simulation parameters. This metric was computed as the proportion of successful intervisibility instances relative to the total model iterations and was normalized to a scale from zero to one. Spatial Connectivity assessed the degree of structural integration between the urban skeleton and natural corridors. This score was derived from the geometric overlap between axial view sheds and node buffers, as well as the Euclidean proximity of the node to historical ward boundaries and street centerlines. Archaeological Significance functioned as an evidentiary weighting factor based on data reliability. Scores were assigned according to documented excavation status, stratigraphic certainty, and recurrence in authoritative atlases, ensuring that confirmed heritage sites received higher priority than conjectural locations. Management Feasibility evaluated the implementation viability of protection and interpretation strategies. This metric favored nodes that achieved high visual impact within compact, manageable footprints by penalizing extensive buffer requirements or complex land ownership patterns that would hinder conservation efforts.

To construct the composite index, all four component metrics were standardized onto a common zero-to-one scale using min–max normalization. The baseline weighting vector assigned 0.35 to Visibility Centrality, 0.30 to Spatial Connectivity, 0.25 to Archaeological Significance, and 0.10 to Management Feasibility. This specific distribution emphasized the legibility of the landscape and the functional role of nodes within the reconstructed axial system while incorporating necessary checks for evidentiary reliability and implementation prospects. Recognizing that weighting choices can be subjective, the selected values were not treated as fixed constants. A systematic sensitivity sweep was performed by varying each weight by plus or minus ten percentage points under the constraint that the sum remained unity. The resulting rank order of top-tier candidates remained stable across the tested ranges, indicating that the identification of primary catalysts was robust and not an artefact of a single weighting scheme. Variations in rank were strictly limited to lower-tier nodes with marginal scores that fell within the inherent uncertainty band of the component metrics, as detailed in the full sensitivity table in Section 2.4.6.

The procedure output a ranked catalogue of candidate catalysts accompanied by their individual component scores, composite indices, and final rankings. These results were visualized through mapping outputs that overlayed high-scoring nodes against the reconstructed axis corridors and intervisibility fields. A tabular manifest recorded all input values and normalization constants to ensure that the scoring process remained fully auditable and reproducible.

2.4.6. Sensitivity Analysis and Model Validation

To ensure that the identified spatial patterns were robust structural features rather than artifacts of specific parameter choices, a comprehensive sensitivity analysis was performed alongside a triangulated cross-validation with historical data. A full-factorial parameter sweep was conducted across observer heights (10, 15, 20 m), analysis radii (20, 25, 30 km), and atmospheric refraction coefficients (k = 0.00, 0.13, 0.20).

The stability of visual corridors was tested by comparing the binary visibility maps generated under these conservative and liberal scenarios. As summarized in

Table 3. Summary of sensitivity analysis results illustrating the robustness of visual metrics under parameter variations.

Parameter Scenario	Visible Terminals	Visible Area	Skyline Continuity (SCR)	Axis Azimuth	Stability Assessment
Baseline (H = 15 m; R = 25 km; k = 0.13)	Reference Set	Reference Baseline	Reference Baseline	Reference Baseline	Standard parameters for clear-day analysis.
Obs. Height: 10 m	Stable (Count unchanged)	Marginal Decrease (<3%)	Stable (Deviation < 1%)	Invariant	Lower height affects foreground occlusion but preserves major landmarks.
Obs. Height: 20 m	Stable (Count unchanged)	Marginal Increase (<3%)	Stable (Deviation < 1%)	Invariant	Higher vantage expands ground visibility; axial geometry remains fixed.
Radius: 20 km	Reduced (Terminals lost)	Reduced (Area truncated)	Reduced (Distal segments cut)	Invariant	Truncation excludes key terminals, validating the need for ≥25 km.
Radius: 30 km	Stable (No new nodes)	Increased (Noise added)	Stable (Saturation reached)	Invariant	Extending beyond 25 km adds terrain noise but no new structural nodes.
Refraction: k = 0.00	Stable	Negligible Shift (<1%)	Stable	Invariant	Atmospheric variation has minimal impact on macro-scale visibility.
Refraction: k = 0.20	Stable	Negligible Shift (<1%)	Stable	Invariant	Curvature effects are negligible for established linear alignments.

, the results indicate that the primary visibility connections between the urban core and the landscape termini persisted across the majority of tested parameter combinations. Specifically, variations in observer height resulted in marginal changes to the total visible area (<3%), while the expansion of the analysis radius beyond 25 km introduced extraneous terrain noise without altering the fundamental axial azimuth. This confirms that the “city–mountain” alignment is a stable geomorphological reality.

Following the quantitative validation, the reliability of the reconstructed landscape depended upon a structured cross-verification process that integrated textual evidence, archaeological mapping, and digital topography, as synthesized in Figure 2. A primary “dual-verification” rule was applied to the identification of all spatial nodes, requiring that any historical landmark integrated into the model be corroborated by both documented administrative records and verified archaeological foundations.

Furthermore, the visibility model was framed as an ideal-type bare-earth simulation. By establishing a target height of 0 m and intentionally excluding transient variables such as historical vegetation or non-extant building massing, the analysis isolated the fundamental geomorphological logic of the urban plan. This approach provided a replicable baseline that evaluated the permanent structural potential of the landscape while allowing for objective measurement of long-term spatial continuity.

Table 4. Sample subset of toponym–feature correspondence and spatial validation.

Toponym	Textual Source	Archaeological/Geo Evidence	Validation Criterion	Spatial Function
Yingtian Gate	Book of Sui (Suishu): South gate of the Palace City, with paired towers.	Excavated remains (MT-1); 1:2000 survey maps.	Site-specific coordinates.	Point-of-Origin
Cuiyun Peak	Record of Buddhist Monasteries (Luoyang Qielan Ji): Northern vista toward Mangshan.	Mangshan peak elevation data; historical records.	Subtended visual angle ≈ 0.56° (>0.5°).	North Terminal Peak
Longmen Yique	Yuanhe Maps and Records (Yuanhe Junxian Tuzhi): Two facing peaks with the Luo River flowing between.	DEM-extracted gap; historical field photography.	Subtended visual angle ≈ 0.85° (>0.5°).	South Terminal Peak

Italicized text indicates historical site names and terms derived from ancient literature.

provides a representative sample of this cross-validation protocol, illustrating the specific correspondence between historical toponyms, archaeological evidence, and the spatial criteria used to anchor the model.

3. Results

3.1. Urban Structure and Symbolic System

Through a synthesis of spatial stratification, historical GIS geoparsing, and axial intervisibility analysis, this research reconstructed the distinctive spatial organization of Sui–Tang Luoyang and deciphered its underlying symbolic apparatus. Historical corpora consistently delineate the city as being “fronted by the Longmen Yique and backed by Mount Mang, with the Chan and Jian rivers flanking its peripheries and the Luo River traversing its core like the celestial Milky Way.” As visualized in Figure 3, the three-dimensional topographic model located the capital in the western Luoyang Basin, clarifying a basin-scale arrangement structured by a northern mountain screen and a southern water gateway. Rather than treating these landforms as a mere scenic backdrop, the reconstruction supported the plausibility of how mountains and rivers operated as formative layers that oriented and constrained the urban footprint. This layout profoundly operationalizes the classical Chinese philosophy of “Harmony between Nature and Humanity.” In the vocabulary of this research’s HUL-informed framework (Table 1), this configuration substantiates the Form and Layout dimension underpinned by the natural context, operationalized here through a spatial skeleton derived from digitized palace imperial outer city polygons, morphometric indices, and adjacency-based connectivity metrics.

The urban form was not merely superimposed upon the terrain; rather, it transformed natural topographies such as the Luo River and the flanking mountain ranges—into structural constituents of the city, thereby emulating the cosmic order. By employing “borrowed scenery” techniques, the planners fused regional landscape attributes with the urban spatial fabric, effectively materializing a monumental realm where anthropogenic and natural systems achieved a holistic symbiosis [39]. In analytic terms, these relations were evidenced by intervisibility counts between urban nodes and terminal landmarks, subtended-angle measures that established perceptual prominence, and the skyline continuity ratio (SCR), which quantified the degree of enclosure provided by the northern ridgeline within the observer’s horizon window.

The spatial distribution of the Palace City and the Imperial City further exemplified the deliberate integration of Confucian ritual order with Daoist natural symbolism. Axial analysis revealed that the central meridian of Sui–Tang Luoyang was slightly offset to the west, structured by a sequence of core nodes explicitly named after “Heaven” (Tian). The Palace City was strategically anchored on a plateau at the southern foot of Mount Mang, occupying an elevated vantage point that commanded the entire urban vista and governed the hierarchical progression of the central axis [40]. From south to north, the axis successively traversed Longmen Yique (Heavenly Gate), Tianjie (Imperial Avenue), Dingding Gate (known as Jian Guo Gate during the Sui Dynasty, symbolizing the “Gate of Heaven”), Tianjin Bridge, Duan Gate, and Yingtian Gate, extending into the ritual complex within the Palace City that includes the Mingtang Palace and Tiantang Palace [41]. Collectively, these nodes constructed a ceremonial corridor rich in cosmological imagery, recognized as a quintessential manifestation of the Chinese capital axis [42]. As demonstrated in Figure 4, the axis extended beyond the city walls to anchor the twin peaks of Longmen Yique to the south and Cuiyun Peak of Mount Mang to the north. Read through the HUL lens, this sequence translates ritual norms into perceivable alignments, thereby addressing the visual axes dimension with operational proxies that include geodesic azimuths between node terminus pairs, intersected viewsheds along the corridor bearing, and subtended angle thresholds that filter perceptually meaningful connections.

Crucially, the calibrated deviation of the axis is not interpreted as a cartographic error but as an adaptive symmetry that preserves visual integrity under geomorphological constraints. The basin’s relief and river incision required a slight angular adjustment to maintain clear, long-distance sightlines between the palace precinct and terminal landforms. Sensitivity tests reported in the methods confirm that the corridor’s intervisibility remained stable across plausible observer heights, analysis radii, and atmospheric refraction coefficients. This strengthens the claim that axial ordering in Luoyang privileged perceptual continuity over rigid orthogonality, aligning the city’s ceremonial narrative with legible landscape anchors consistent with HUL’s emphasis on visual relationships within broader environmental settings.

The organization of residential quarters within the Outer City reflected a sophisticated mechanism of social management and spatial discipline. Outside the administrative core, the city implemented a rigorous “Ward System” (Li-fang), which effectively segregated political functions from civilian life [43]. Archaeological plans and legal codices together delineate an orthogonal grid of walled residential units served by a hierarchical street network and controlled gateways. Each fang was enclosed by fortifications subject to strict curfew protocols, a system characterized as “walls around the wards and gates with restrictions” [44,45]. Tang Code with Commentary (Tang Lv Shuyi ) explicitly stipulated that “anyone violating the nighttime curfew shall receive twenty strokes of the cane,” legally ensuring the enforcement of this system [46]. In functional terms, the grid concentrated exchange in designated markets, regulated nocturnal mobility, and embedded surveillance into the everyday circulatory fabric. Within the HUL scheme, this governance choreography materializes Socio-Cultural Context through Form and Layout, while also speaking to Governance and Participation. This is supported by the reconstructed ward polygons, market centrality, gate distribution, and connectivity measures that formed the spatial skeleton used for subsequent analyses (Table 1). The comprehensive configuration of this managed urban apparatus is illustrated in Figure 5, which synthesizes the ward grid and market system on the reconstructed outer-city plan.

Linking these strands, the Palace City and Imperial City are read not only as loci of authority but as catalytic nodes that orchestrate perception and movement across the grid. The axial sequence focused attention on distant landforms, while the ward lattice modulated access, timing, and crowd composition within the daily urban dramaturgy. Together, they constitute a layered apparatus that converts cosmological and legal ideas into durable spatial routines. The mountains, rivers, gates, bridges, and halls do not merely symbolize power; they co-produce a perceivable structure in which authority is rehearsed and remembered through repeated alignments, controlled passages, and framed vistas.

In summary, Sui–Tang Luoyang’s urbanism emerges as a multi-layered system in which environmental structure, ceremonial vision, and everyday governance are mutually conditioning. Its axial framework linked the physical expression of power with celestial symbolism, while the rigorous ward system facilitated efficient social governance. What distinguishes this reconstruction is that each component is tied to HUL analytical dimensions and measurable proxies. The macro configuration evidenced in Figure 3 supports natural context, the calibrated procession mapped in Figure 4 substantiates visual axes, and the regimented outer-city grid shown in Figure 5 confirms Form and Layout intertwined with Governance. By articulating these correspondences explicitly, the results reposition Luoyang’s plan as socio-cultural layering within a historic urban landscape rather than as a collection of isolated relics. This provides a reproducible basis for contemporary conservation, where protecting view corridors, maintaining skyline integrity, and managing historic grid connectivity can be aligned with the layered socio-cultural logics that originally shaped the capital.

3.2. Visual Axis Analysis

GIS-based spatial analysis confirms that the urban plan of Sui–Tang Luoyang was structured around a clearly defined central axis. This near-strict north–south alignment traversed the entire city, connecting a sequence of significant natural and cultural nodes. Building on the operational chain established in Table 1, the analysis tested whether this axis functioned as a perceivable sequence by combining geodesic azimuths between node–terminus pairs, binary intervisibility derived from DEM viewsheds, a conservative subtended-angle threshold of 0.5 degrees, and the skyline continuity ratio (SCR) as an indicator of horizon integrity along the ceremonial bearing. From the palace precinct, vistas were evaluated toward the southern mountain gateway of Longmen Yique and the northern ridge of Mount Mang in order to determine whether the axial narrative was sustained by the terrain and legible from principal observation points within the imperial core [13,16].

The geometric order of the procession was first established using geodesic azimuths to mitigate projection distortion over long distances. The measured bearing of the reconstructed central corridor was 173.82 degrees, which produced a westward deviation of approximately 6.18 degrees from the true meridian. Crucially, this deviation was interpreted not as a cartographic error but as a deliberate act of adaptive symmetry that preserves visual integrity under geomorphological constraints. The basin’s relief and river incision required this calculated angular adjustment to maintain clear, long-distance sightlines between Yingtian Gate and the terminal landmarks of Longmen Yique and Cuiyun Peak. By tying the procession to the actual relief, the plan ensured that the long view remained open and that the termini retained angular salience. The sensitivity analysis reported in the methodology confirms that these core visual connections persisted across varying observer heights (10–20 m), analysis radii (20–30 km), and refraction coefficients (0.00–0.20), proving that visibility is a structural property of the basin. This interpretation aligns with the HUL dimension of visual axes, demonstrating that Tang planners privileged perceptual continuity along the corridor over rigid orthogonality, actively reconciling cosmological alignment with the pragmatic realities of the basin.

The viewshed analysis further validates the deliberate visual effect of this adaptive arrangement [47]. As shown in Figure 6a, from the vantage point of the principal southern gate of the palace precinct, the simulated visible field extended coherently southward toward the Longmen Yique without interruption. Within the 25 km analysis window, a distinct far-view focal point emerged on the southern horizon, while to the north, the ridgeline of Mount Mang remained a stable backdrop. Complementing this raster-based test, the line-of-sight (LOS) analysis in Figure 6b precisely delineated the direct visual corridor between Yingtian Gate and the two terminal landmarks. The LOS profiles confirm that there was no intervening terrain to interrupt the clear view between the palace gate and the axial ends. This places the palace precinct at the functional center of the visual apparatus, reinforcing the ceremonial logic where the gaze is guided across built thresholds toward a natural gate and retained by a mountain screen.

To translate geometric existence into perceptual significance, the analysis applied a conservative subtended-angle threshold of 0.5 degrees at the principal observation point. Both the southern twin-peaked landform and the northern peak attained angular extents above this just-noticeable bound, indicating that the termini would be legible under typical clear-sky conditions. This subtended-angle measurement was complemented by the skyline continuity ratio along the axial bearing, which quantified the proportion of the horizon occupied by natural terrain within the corridor window. The results show a high continuity ratio to the south, where the landform narrows to the Longmen pass, and a stable continuity level to the north, where Mount Mang provides a continuous ridgeline. Together, these metrics indicate that the axis is not merely a line on a map but a perceivable corridor with dense and coherent horizon features. This reliance on angular prominence and horizon integrity ensured that Visual Integrity was assessed as an experiential quality, consistent with the visual relationships dimension in Table 1.

Beyond spatial evidence, Tang Dynasty poetry and prose provide cultural annotations that triangulate these quantitative findings. When depicting Luoyang, poets often emphasized the resonant relationship between the capital’s central axis and the Longmen Yique or Mount Mang. In “Viewing the Ancient Sites of Luoyang,” Liu Yuxi extols the city’s grandeur through the “Celestial Gate” motif, with the majestic palace gates resembling heavenly portals, while Longmen at the southern end stands as a natural gateway facing them, creating the magnificent scene of “Celestial Gate facing Longmen” [48]. These literary testimonies confirm that the visual corridor verified by GIS was historically experienced and remembered. They frame the termini not just as geographic endpoints but as meaningful anchors that sustained the ceremonial narrative of the city, corroborating the measured alignment with contemporaneous symbolic accounts.

In summary, the central axis of Sui–Tang Luoyang transcends mere geometric alignment; it represents an integrated spatial practice that combines geomatic precision, visual construction, and symbolic representation. The emphasis on natural landmarks as axial termini is rooted in cosmological and political rationales: aligning the urban structure with sacred geography expresses the doctrine of “Harmony between Nature and Humanity” and situates imperial power at the center of a legible order. From a planning perspective, the 6.18° adjustment exemplifies the translation of cosmological doctrine into spatial practice through measured tolerance rather than rigid orthodoxy. In HUL terms, the evidence supports the plausibility of the idea that visual axes understood as perceptual relationships between urban nodes and natural anchors served as the operative mechanism by which the capital’s natural context was woven into the everyday experience of the city.

3.3. Borrowed Scenery Analysis

Based on historical textual analysis and topographical examination, this research classified the natural borrowed scenery techniques employed in Sui–Tang Luoyang into three principal types. In this research, borrowed scenery was treated not as an ornamental device but as a mechanism for integrating external landforms into the visual and ceremonial system of the city. The analysis therefore evaluated whether distant and peripheral terrain was intentionally enrolled to sustain axial legibility and stabilize the experiential frame of the capital. To make this determination, the results are interpreted through the operational chain established in Table 1. Intervisibility mosaics reveal where natural anchors recurred across multiple observation points within the imperial and ward precincts. Subtended angle measurements tested whether those anchors achieved perceptual salience above a conservative 0.5-degree bound. Skyline continuity ratios along near, mid, and far windows indicate whether horizon features cohered into a readable background rather than appearing as incidental silhouettes. All computations were undertaken within the 25 km analysis window supported by a 30 km DEM clip to provide terrain redundancy at the computational margins, ensuring that edge effects did not compromise the diagnostics.

The first type, frontal borrowed scenery, involved incorporating distant vistas into the primary north-to-south central axis of the city. The southern terminus of the Palace City axis aligned precisely with the Longmen Yique, where opposing peaks formed a natural gateway to the capital. In the frontal window, the twin peaks at the southern pass recurred with high frequency across palace-facing observation points and retained visibility from representative ward edge positions oriented toward the ceremonial bearing. Subtended angle measurements at the principal palace gate exceeded the perceptual threshold, indicating that the paired landforms read as an unmistakable focal device rather than as a faint backdrop. The skyline continuity ratio in the same corridor rose markedly as the horizon narrowed toward the pass, which concentrated attention at the ritual culmination of the axis. These convergent signals confirmed that the urban sequence was calibrated to a distinct natural aperture and that the terminal scene at Longmen was engineered to deliver a strong visual climax aligned with the south-facing orientation of major palace structures. This geometric and visual relationship is clarified in the axial profile of Figure 7, which traces the descent from the palace precinct toward the pass and shows the unbroken horizon line that supports the intended procession.

The second mode was lateral borrowed scenery, which used medium to near-scale terrain on the flanks of the city to build depth around the everyday field of view. Luoyang is backed by the Han’gu Pass range to the west and opens onto the Luoyang Plain to the east. The Chan River and the Jian River meander along the eastern and western perimeters, respectively, while the low hills flanking the Luo River further articulate the horizontal interface of the city. Viewsheds generated from representative observation points at ward edges and market fronts show recurrent exposure to these lateral landforms under the main parameter set. Subtended angles for the nearer hills exceeded the perceptual bound at short to intermediate distances, and skyline continuity values registered a moderate band that remained stable across changes in observer height and atmospheric refraction. This indicates that lateral elements did not merely delimit the spatial extent of the city but thickened the urban scene and supplied a layered frame for routine movement along the street and market lattice. Read through the HUL lens, this lateral integration translates socio-spatial routines into a persistent and legible horizon that residents and visitors would have recognized as part of the cultural landscape rather than as a neutral environmental backdrop.

The third technique was distant borrowed scenery, which installed a persistent topographic backcloth that stabilized the composition at the metropolitan scale. The Mount Mang range extended across the northern margin of the city and its continuous ridgelines established a natural skyline for the Palace City and the Imperial City. High skyline continuity ratios along the northern window confirm that the Mount Mang ridgeline operated as a durable horizon structure across both imperial and outer city observation points. Angular measurements show that the principal peak remained perceptible above the conservative bound, which supports textual descriptions that framed the northern skyline as solemn and protective. This persistent background stabilized the axis by supplying a fixed reference on which the sequential spaces of the capital unfolded from the palace to the ward grid. In effect, Mount Mang functioned as the environmental screen of the city, ensuring that the narrative of advance and arrival retained a coherent visual grammar from everyday life to ceremonial events.

These three modes operated together as a near-to-mid-to-far gradient that converted borrowed scenery from a stylistic citation into structural integration. Frontal alignment at the southern pass provided focalization at the ritual terminus. Lateral frames on the east and west thickened the daily field of view along the orthogonal street network. The far northern skyline supplied a constant backcloth that anchored the city in its basin setting. The quantitative results show that this gradient was not a product of a single observation point or a single parameter setting. Intervisibility frequency remained high at the southern corridor under the sensitivity sweep of observer heights between 10 and 20 m, analysis radii between 20 and 30 km, and refraction coefficients between 0.00 and 0.20. Subtended angles at the principal southern and northern anchors remained above the perceptual bound across these scenarios, and skyline continuity ratios preserved their relative ordering from frontal to lateral to distant windows. This robustness indicated that the borrowed scenery system derived from structural features of the basin and deliberate siting choices rather than fragile parameterization, which reinforces the reading of the city as a carefully composed cultural landscape.

Borrowed scenery in the construction of Sui–Tang Luoyang fulfilled both aesthetic and political functions. On the one hand, it incorporated landscape imagery to establish a balanced and resonant spatial order, reflecting the traditional ideal of harmony between nature and humanity. On the other hand, rulers transformed natural topography to support narratives of power. The twin peaks of Longmen were rendered as a symbolic gateway, while the profile of Mount Mang was likened in literary sources to cosmic figures, reframing the skyline as an emblem of mandate and stability. These cultural annotations visible in Tang prose and poetry corroborate the quantitative evidence by documenting how the termini and the backcloth were experienced as meaningful anchors rather than as incidental scenery. They also show that the visual system extended beyond the ceremonial axis to the rhythms of everyday life, where lateral frames and river terraces structured the perceptual texture of the ward and market milieu. In this way, borrowed scenery unified symbolic expression and daily experience into a single environmental order.

From a conservation perspective, the indicators employed here created a direct bridge to management actions that are consistent with the HUL framework. The intervisibility corridor toward Longmen defined the logical extent of a view protection zone at the southern terminus. Elevated skyline continuity along ward edges supported lateral open space safeguarding and the calibration of street wall heights so that the mid-range frame remained legible. Persistent continuity on the northern horizon motivated height control bands on the lower hillslopes to preserve the ridgeline silhouette that stabilized the metropolitan composition. These applications do not impose a fixed historical image but encode the structural roles through which the environment and the city co-produced a legible landscape. In this sense, the borrowed scenery system exemplified the integration of natural context into cultural narrative that the HUL approach advocates, where conservation measures are guided by measurable relationships rather than generic aesthetic preference. The synthetic configuration of this system is mapped in Figure 8, which aggregates intervisibility mosaics from representative observation points, overlays the axial buffer where frontal alignment concentrated angular prominence, and visualizes skyline continuity bands for the lateral and distant windows.

4. Discussion

This research reconstructed the spatial logic of Sui–Tang Luoyang through a multi-dimensional lens, integrating archaeological data, historical semantics, and geospatial analysis. By operationalizing the Historic Urban Landscape (HUL) framework, this research reveals that the city’s morphology was not merely a functional response to hydrology or defense but a sophisticated “semiotic landscape” where cosmological order and geographical reality were reconciled. The findings are discussed below in terms of their comparative positioning within historic urban research, their methodological contributions to analyzing non-extant heritage, their technical applications for contemporary urban governance, and the limitations inherent in the reconstruction process.

4.1. Comparative Positioning

From a theoretical perspective, the planning of Sui–Tang Luoyang exemplifies the holistic vision advocated by the HUL framework, integrating the analytical dimensions of form, natural environment, and socio-cultural context [27,41,49]. This research reveals that the adaptation of the city to the terrain was not merely a functional response to hydrology or defense but a sophisticated process of cultural semiosis where topographic choices were rationalized through a cosmological meaning-making system. In comparative terms, Luoyang should not be read as a derivative variant of a canonical capital plan but as an adaptive project calibrated to basin geomorphology. The measured westward rotation of the central axis by approximately 6.18 degrees signals a design strategy that privileges perceptual continuity to terminal landmarks over rigid orthogonality. This reframes long-standing assumptions about imperial capitals by showing that visual legibility governed axial decisions when strictly geometric orthodoxy was in tension with the landscape. In this sense, Luoyang performs as a perceptual priority case that maintains the recognizability of mountain termini and skyline enclosure while preserving ritual coherence.

The core precincts were deliberately located on the northwest highlands, creating a topography-responsive layout that differed significantly from the rigid symmetry of Tang Chang’an [50]. Chang’an was situated on an open plain and could sustain an idealized north-to-south orthogonality with minimal compromise to distant alignment. By contrast, Luoyang was embedded within a basin and converted topographic constraint into visual advantage by anchoring the axis in a sequence of natural and architectural nodes that stabilized long-distance vistas. The anchoring is interpreted as a terrain-defined regional visual alignment, rather than as proof of a continuous extramural physical avenue. The contrast clarifies that the operative variable is not a civilizational preference for symmetry but the mediation between ideal models and real geography. The Luoyang evidence therefore advances the HUL conversation by demonstrating how the visual axes and natural environment dimensions co-determine spatial structure under complex terrain rather than allowing any single dimension to dominate interpretation.

This research does not seek to establish a deterministic causal model wherein geomancy physically drives urban construction but rather provides an interpretive framework. Geomancy functioned as a semiotic language and a cultural logic that aligned the physical hierarchy of the Palace City, Imperial City, and Outer City with celestial patterns. The clustering of structures bearing celestial titles and the rigorous grid of the Ward System or Li-fang articulated Confucian ethics and imperial authority as a single spatial ideology [45]. Read through a comparative lens, this semiotic calibration resembles other traditions in which urban order was framed to secure perceptual legibility at key vistas. This integration of built form and natural scenery resonates with the broader East Asian tradition of borrowed scenery or shakkei, reflecting a shared regional logic of embedding aesthetic judgment within spatial governance [51]. By foregrounding skyline continuity and terminal recognizability, Luoyang converts cosmological discourse into an empirically detectable set of visual relationships.

The intercultural parallels further situate Luoyang within a wider repertoire of historic urbanism that negotiated between invariant ideas and variable sites. Comparative analysis validates the universality of this HUL dimension. Just as Rome evolved from hilltop settlements to formalized Baroque axes that translated dispersed topography into convergent sightlines [52], and Kyoto or Heian-kyō developed axis-based vista framing to stabilize mountain silhouettes as ritual markers [53], Luoyang supports the plausibility of how historical urbanism mediates between ideal models and real geography. Luoyang extends this pattern in a remains-scarce context by showing that axial intent can be verified through consistent terminal intervisibility even when surface fabric has been erased. This alignment across cases supports a cross-cultural proposition that the operative equilibrium among form, nature, and visual order is best understood as a structural and perceptual relationship rather than as a purely material taxonomy. This confirms that the equilibrium between form, nature, and visual order extends the HUL framework from a focus on material stratification to a deeper understanding of the structural–perceptual relationship [54,55].

Positioned this way, the contribution of Luoyang to comparative urbanism is twofold. First, it clarifies that adaptive symmetry is not a deviation from principle but a technique for preserving perceptual integrity under terrain constraints. Second, it supports the plausibility of how socio-cultural layering is spatially carried by the ward grid and ceremonial sequence while remaining visually closed by mountain horizons. By tying axial rotation, skyline continuity, and node sequencing to the four analytical dimensions summarized in Table 1, the case provides a concrete articulation of how HUL reasoning can be operationalized in archaeological landscapes. The result is a comparative positioning in which Luoyang stands with the exemplary capitals of world urban history precisely because it makes perceptual priorities explicit and measurable within a theoretically coherent framework.

4.2. Methodological Contributions

Methodologically, this research addresses the specific challenge of applying the HUL approach to archaeological landscapes where physical remains are scarce and spatial logic survives primarily in collective memory [56]. Building on this premise, the research develops an integrated workflow that translates cultural concepts into replicable geospatial procedures. The contribution lies not only in reconstructing a plausible plan for Luoyang but in specifying a chain of operations that allow perceptual claims to be tested against terrain-constrained evidence. By coding these lenses as procedures rather than narratives, this research moves HUL reasoning from descriptive synthesis to testable models that can be reproduced, inspected, and adapted in comparable remains-scarce contexts.

This research establishes a systematized triadic analytical model that transforms abstract cultural concepts into actionable and quantifiable protocols. This triad couples spatial structure, visual axes, and borrowed scenery as mutually constraining lenses. Spatial structure is fixed through georeferenced archaeological baselines and a consistent registration error budget. Visual axes are reconstructed with geodesic azimuths to avoid projection distortion over long distances. Borrowed scenery is operationalized as intervisibility to named landforms with measurable prominence. Central to this approach is the operationalization of visibility through a two-tier model that integrates geometric calculation with perceptual thresholds. By distinguishing between binary line-of-sight and perceptual legibility, which requires a minimum subtended visual angle of approximately 0.5 degrees, the model avoids overestimating visual connectivity and accurately identifies landscape catalysts that functionally link micro-scale architecture with macro-scale geomorphology [57]. In practice, the binary viewshed establishes the maximum theoretical field of sight on a bare-earth DEM while the legibility filter removes tenuous connections that would be imperceptible to a human observer. This separation of geometry and perception clarifies why certain corridors matter historically even when multiple lines are mathematically unobstructed, supporting the interpretive claim that the axis behaves as a perceptual system tuned to terminal landmarks rather than as a purely abstract geometry.

To ensure rigor, this research implemented a triangulated cross-validation protocol involving a strict acceptance rule where spatial nodes are only confirmed when textual toponyms, archaeological foundations, and topographic logic converge. The protocol assigns each candidate node a documentary anchor, a mapped foundation or excavation footprint, and a visibility rationale derived from geodesic azimuths and legibility thresholds. By explicitly defining the error budget, including the Root Mean Square Error or RMSE of approximately 38 m in map registration, the methodology transforms vague historical descriptions into a verifiable spatial dataset. This framed uncertainty keeps the results falsifiable. If future excavations move a gate by a known offset, the model can be rerun and its axial metrics re-evaluated without redesigning the workflow.

A further contribution concerns parameter transparency and robustness. This research reports observer heights, analysis radii, and atmospheric refraction coefficients as an explicit parameter set and evaluated alternatives through a sensitivity sweep. The principal radius of 25 km was selected to contain the furthest identified terminals while minimizing extraneous terrain. Auxiliary runs at 20 km and 30 km demonstrated that the core intervisibility pattern was stable under reasonable expansions or contractions of the research window. Similarly, the refraction coefficients bracket common horizon conditions, and the invariance of terminal connectivity across this bracket strengthens the claim that the axial system was a structural property of the basin rather than an artifact of a single parameter setting. Because all steps were specified within a mainstream GIS environment, the workflow is portable and auditable across software versions and datasets.

The framework also contributes a set of quantitative indicators that link perception to management, such as the skyline continuity ratio for mountain-city enclosure and the angular tolerance for axial alignment. By embedding these indicators in the analytical sequence, the approach keeps methodological choices coupled to policy-relevant outputs, which is consistent with the emphasis of the HUL framework on managing change rather than freezing form. This modular configuration offers high transferability and can be adapted for comparative studies of other ancient capitals where the meaningful forms of the urban plan persist despite the loss of surface architecture, with potential applications ranging from Delhi to Athens. The transfer provides a scaffold that other teams can recalibrate to local climates, landforms, and documentary corpora while preserving the same audit trail. By coupling macro-scale spatial reconstruction with micro-scale interpretive analysis, this research provides a robust methodological pathway for assessing the visual and structural integrity of complex historical landscapes. In sum, the contribution of the method is to render claims about cultural meaning empirically tractable by setting out a reproducible sequence from data provenance and registration through geodesic alignment and perceptual filtering to indicators that can be compared across cases and carried forward into heritage-sensitive planning.

4.3. A Technical Toolkit for Heritage-Sensitive Planning and Governance

From the HUL perspective, the essential value of this research lies in moving beyond historical reconstruction to guide contemporary conservation under real development pressures. The disruption of the central axis or key sightlines irreversibly weakens the semantic dimensions of the landscape. Therefore, this research proposes a comprehensive technical toolkit for heritage-sensitive urban design that emphasizes precise quantification and regulatory embedding. Building on the results, the first application is the translation of axial geometry and perceptual thresholds into an enforceable visual integrity zone. The verified azimuth of 173.82 degrees defines the corridor spine, while the legibility filter anchored at a minimum subtended angle provides the lower bound for meaningful detection of terminal landmarks. Together they specify where view protection is necessary and why it is justified. The corridor is not conceived of as a uniform band but as an alignment condition that can be tested at designated observation points and along continuous street segments using geodesic bearings. This allows planners to adjudicate proposals with a clear compliance test that is reproducible across software platforms and survey campaigns.

To preserve skyline coherence, height management is calibrated to the skyline continuity ratio. A continuity threshold acts as a trigger for control intensity so that areas with higher enclosure, such as Mount Mang and the southern gateways, maintain stricter height caps while transitional districts accept moderated envelopes that still respect the silhouette of distant ridgelines. The intent is to protect the perceptual frame that anchors the ceremonial sequence rather than to freeze every parcel at a single value. In practice, this means mapping continuity surfaces, identifying breakpoints where the ratio falls below acceptable levels, and adjusting overlay districts so that cumulative change does not erode the dialogue between the mountain and the city. International experiences, including the landscape ordinances of Kyoto and the visual integrity plans of Chiang Mai, demonstrate that such parametric governance is technically feasible and socially legible when supported by clear maps and predictable review procedures [58,59].

Where the historic fabric is already compromised, the emphasis shifts from strict preservation to visual rehabilitation and narrative compensation. Rather than pursuing unrealistic physical restoration, soft conservation tactics can be deployed. Alternative vantage points can be formalized in the public realm to reconstitute lost alignments, with paving bands and ground-level markers inscribing the invisible axis through contemporary spaces. Small geometric adjustments in street furniture, tree placement, and lighting can re-open narrow cones of vision that restore long-distance recognition of terminal landmarks even when full corridors cannot be recovered. Immersive media provide an additional layer where augmented-reality overlays at Yingtian Gate and Tianjin Bridge can reconstruct the ceremonial vista toward Longmen Yique at human eye height, while fixed digital viewers at elevated terraces can simulate historical skyline profiles under clear-air conditions. In development negotiations, a visual balance-sheet approach can require projects that interrupt priority sightlines to fund view platforms, rooftop access, or curated public lookouts that maintain the public ability to perceive the axis and its borrowed scenery.

Implementation depends on integrating these indicators into statutory processes so that cultural values become operational constraints rather than aspirational statements. The HUL approach advocates for managing change rather than freezing the city, which implies that metrics such as line-of-sight availability and skyline continuity should be integrated into Regulatory Plans and Environmental Impact Assessments or EIAs for new projects. In practical terms, the azimuth tolerance and continuity thresholds become review criteria in design approvals where project applicants submit geodesic viewshed tests from specified observation points at the observer heights defined in the methods. Third-party audits can then verify compliance post-construction using the same parameter set. A monitoring regime can be instituted by publishing annual or biennial visual integrity maps that track incremental change in corridor openness and skyline continuity, supported by transparent data repositories so that civil society and professional bodies can replicate the calculations.

These applications align with the HUL emphasis on process and participation. The indicators are intelligible to non-specialists because they tie abstract heritage values to observable vistas and measurable alignments. They also enable adaptive management, where parameters can be recalibrated as local atmospheric clarity or where urban form evolves without abandoning the underlying logic. By coupling axial azimuths, perceptual thresholds, and skyline continuity to clear governance instruments, the approach connects historical interpretation to contemporary decision-making. It reconciles the protection of the calibrated westward orientation of Luoyang with legitimate demands for growth by prioritizing perceptual continuity over rigid form, thereby sustaining the structural–perceptual equilibrium that defines the heritage significance of the city while providing a practicable pathway for heritage-sensitive urban development.

4.4. Limitations and Future Work

This research advances a replicable pathway for interpreting a remains-scarce capital through a structural–perceptual lens, yet several constraints qualify the scope of its claims and identify directions for further inquiry. The visibility analyses are based on a bare-earth digital elevation model and intentionally exclude vegetation canopies, historical building massing, and transient atmospheric conditions. This modeling choice isolates the enduring geomorphological signal and establishes a maximum theoretical potential for intervisibility rather than the precise views available to historical observers. As a result, the mapped corridors should be understood as baseline capacities embedded in the terrain onto which period-specific layers of planting, construction, and air clarity variably amplified or attenuated the experience of long-distance vistas.

Parameterization likewise reflects context-specific judgments that require careful transfer when applied elsewhere. The perceptual legibility threshold and the 25 km analysis window were selected as conservative heuristics for the Luoyang Basin to filter spurious line-of-sight links and encompass the farthest terminal anchors while limiting extraneous terrain. Although sensitivity tests indicate that the principal connections remain stable under plausible variation, these parameters are not universal constants. Coastal cities with high aerosol loads, plateau cities with greater horizon distances, or monsoon regions with pronounced seasonal visibility would warrant local calibration of both subtended-angle thresholds and radius limits, which should be coupled ideally to empirically observed atmospheric ranges and historically attested viewing practices.

Interpretive uncertainty also persists at the level of cultural explanation. This study reads cosmological nomenclature, axial naming conventions, and textual descriptions alongside spatial evidence to argue for a coherent meaning-making system that aligned the urban plan with sacred geography. This is an interpretive synthesis rather than a causal demonstration. The analyses substantiate correlation and structural consistency between terrain, axial ordering, and literary tropes, but they do not prove that geomancy deterministically drove every siting decision. Alternative or coequal factors, including administrative logistics, hydrological management, defense, or construction expediency, may have contributed to specific choices even when the final composition was rationalized through cosmological language.

Crucially, because the analysis relies on a bare-earth DEM, the modeled intervisibility strictly reflects macro-scale topographic potential rather than a definitive reconstruction of micro-level historical visual experience. Inherent uncertainties regarding untraceable 7th-century canopy structures, alongside strict ancient architectural sumptuary laws (which ensured that paramount imperial structures like Yingtian Gate remained visually unobstructed), preclude the precise simulation of line-of-sight occlusion by period features. Thus, the results must be interpreted as identifying persistent, terrain-defined visibility corridors that provide a structural spatial substrate for cultural interpretation within the HUL framework, rather than as empirical proof of micro-level perceptual experience or psychological intent.

These limitations point to several concrete priorities for future work. First, while the macro-scale structural alignment is established herein, investigating the micro-level lived experience would require layering hypothetical obstacle scenarios (such as localized vegetation and detailed building massing). This transition from topographic potential to localized perceptual simulation could be approached through the fusion of historical ecology, palynological proxies, and typological massing derived from excavated footprints and tested with ray-tracing on higher-resolution terrain data where available. Second, empirical calibration of perceptual parameters is needed. Systematic field campaigns using repeat photography, photogrammetric sky-horizon extraction, and atmospheric measurements can tie modeled thresholds to observed legibility under a range of conditions, while experimental studies can test recognition rates for distal landmarks at controlled subtended angles. Third, documentary triangulation can be strengthened by expanding the corpus of dated texts, steles, paintings, and route narratives to evaluate whether reported views align with modeled corridors across different reigns and seasons. Finally, comparative transfer studies should apply the triadic protocol to other archaeological capitals and extant historic cities by explicitly reporting how parameter recalibration affects corridor detection, catalyst ranking, and skyline continuity metrics. By iterating between model refinement, empirical calibration, and cross-site comparison, future research can convert the present framework from a robust baseline for Luoyang into a generally applicable instrument for managing structural–perceptual heritage across diverse urban landscapes.

5. Conclusions

This research systematically reconstructed the spatial logic of Sui–Tang Luoyang, fulfilling the objective of interpreting how ritual hierarchy was reconciled with a complex basin geography. By decoding the intricate relationships between the urban grid and the surrounding terrain, this research reveals that Luoyang’s morphology was defined by a strategy of “adaptive symmetry.” The empirical identification of the 6.18 degree westward rotation confirms that the central axis was not a rigid imposition of abstract orthodoxy but a calibrated alignment designed to capture the “borrowed scenery” of the Longmen Yique. This finding supports the plausibility of the siting of catalytic nodes such as the Palace City on the northwestern highlands serving to anchor political legitimacy within the regional topography, thereby activating a “semiotic landscape” where cosmological order and environmental constraints were dynamically balanced.

Theoretically, this research successfully operationalizes the Historic Urban Landscape (HUL) framework for archaeological sites where surface remains are scarce. Addressing the objective of translating intangible heritage concepts into testable parameters, this research establishes a “structural–perceptual” analytical model. By distinguishing between theoretical line-of-sight and legally meaningful legibility thresholds, the methodology moves beyond qualitative description to provide a reproducible protocol for assessing visual connectivity. This validates the HUL premise that the significance of a historic city resides not only in its extant material fabric but also in the invisible yet quantifiable visual relationships that bind the urban core to its hinterland.

Practically, this research bridges the gap between historical interpretation and contemporary management by mapping these spatial–visual indicators to concrete conservation tools. The verified axial azimuth and the skyline continuity ratio (SCR) are translated here into operational mechanisms for visual integrity zones and height control buffers. These tools provide urban planners with a scientific basis for regulating development density and protecting critical view corridors, ensuring that the “mountain–city” dialogue is preserved against the pressures of modern urbanization. This transition from descriptive history to prescriptive governance fulfills the critical need for heritage-sensitive planning instruments that are both theoretically grounded and technically enforceable.

Looking forward, the integrated framework developed here opens broader trajectories for the digital humanities and comparative urbanism. Beyond the specific technical refinements noted in the discussion, the future application of this model lies in leveraging immersive technologies such as Virtual Reality (VR) and Augmented Reality (AR) to transform static spatial data into public experiences that reactivate the collective memory of the lost capital. Furthermore, extending this triadic protocol to other imperial capitals, such as Tang Chang’an or Heian-kyō, would allow for a systematic cross-cultural comparison of how different civilizations mediated the tension between ideal urban models and real-world geography. Ultimately, this research offers a replicable pathway for recognizing and sustaining the deep structural resilience of historic urban landscapes in the twenty-first century.