RTK-GNSS Characterization of Raised Coastal Terrace-like Morphology Along Southern Java, Indonesia

Abstract

The southern coast of Java, Indonesia, is situated along the active Sunda subduction margin where raised coastal landforms may record the combined influence of relative sea-level change, wave processes, sedimentation, and vertical land motion. This study presents field-based RTK-GNSS topographic profiles from four coastal sites: Pantai Ajah, Kalijali, Kulon Progo, and Wingko. Profiles were used to locate terrace treads, risers, slope breaks, residual topographic highs, and possible raised coastal surfaces. The results show spatially varying coastal morphology. Pantai Ajah has a marked riser and probable terrace tread at about 7–8.5 m elevation. Kalijali shows a lower terrace-like surface at about 4–5 m, an upper surface at about 7–9 m, and a higher local topographic high at about 12–13 m. Kulon Progo is characterized by a low-elevation coastal surface that is only weakly expressed in the topography, whereas Wingko contains a distinct slope break and a broad landward surface at approximately 5–6.5 m elevation. Across the four profiles, broad low-gradient surfaces recur within two elevation ranges, approximately 4–6.5 m and 7–9 m. These ranges are treated as provisional morphometric groupings rather than correlated or coeval terrace levels. Higher isolated elevations are described as ridge-like or residual topographic highs whose origin and age remain unresolved. No direct chronological or sedimentological constraints are currently available, so correlations with Holocene or older sea-level highstands are only tentative. The results show the usefulness of RTK-GNSS profiling for the documentation of local coastal terrace morphology and for the identification of priority sites for future dating, sedimentological analysis, and coastal-hazard assessment.

1. Introduction

The southern coast of Java is located along the active Sunda subduction margin, where the Indo-Australian Plate converges beneath the Eurasian or Sundaland plate (Figure 1). This tectonic setting produces earthquakes, tsunamis, vertical crustal deformation, and long-term coastal uplift. The coastal zone of southern Java is therefore an important area for studying the relationship between Quaternary sea-level change, active tectonics, coastal geomorphology, and natural-hazard mitigation [1,2]. This tectonic activity produces both persistent forearc uplift and transient coseismic deformation, which sculpt geologic features throughout the Late Quaternary [3,4]. As a result, the southern coastal region of Java has turned into a natural laboratory for the investigation of Quaternary tectonic events, paleoseismic activity, and coastal landform evolution.

Marine terraces are among the most useful landforms for reconstructing relative sea-level history and vertical land movement [5,6]. They commonly form as wave-cut platforms, abrasion surfaces, beach deposits, or coastal benches during periods of relative sea-level stability. If these surfaces are later uplifted, they may be preserved above present sea level as terrace treads separated by risers or paleo-sea cliffs. In tectonically active margins, marine terrace morphology may record both long-term uplift and short-term coseismic deformation. In this case, the long-term uplift rate is estimated at approximately 2.4 mm/yr, while short-term coseismic uplift ranges from about 1.4 to 3.1 m per event, with an average of around 2.1 m per event [7,8,9]. However, terrace interpretation can be complicated by erosion, sediment cover, karst processes, fluvial modification, and anthropogenic disturbance.

One of the key contributions of this study is to improve understanding of the tectonic and geomorphic context of earthquake and tsunami hazards along the densely populated southern coast of Java, where more than 600,000 people live in tsunami-prone zones, according to Yunus et al. [10]. Geological records and historical accounts document immense destructive waves generated by megathrust events along the plate boundary [11,12]. Along the Sunda subduction zone, questions regarding fault segmentation, the development of uplift patterns, and recurrence timescales have attracted considerable scientific attention because strong interplate coupling and recurrent megathrust earthquakes produce significant forearc uplift and coastal deformation. These tectonic processes are recorded by marine terraces and other uplifted coastal landforms preserved along the southern Java margin [3,4,5,6,7,11,12]. Significant uncertainties remain regarding recurrence intervals, rupture segmentation, and the temporal evolution of uplift patterns along the Sunda subduction zone [13,14]. Coral microatolls have previously been used to show that megathrust earthquakes along the Sunda subduction zone, especially in western Sumatra, cause variable coseismic uplift and longer-term deformation of forearc islands during interseismic periods. For example, Natawidjaja et al. [13] reconstructed substantial uplift and tilting associated with the 1797 and 1833 Sumatran megathrust earthquakes; coral records from the Mentawai Islands show repeated cycles of strain accumulation and release, including clustered earthquake sequences or “supercycles” [11]. Some rupture boundaries such as the transition between the 2004 and 2005 rupture areas near Simeulue may persist through multiple earthquake cycles [15]. Collectively, these studies suggest that fault segmentation, the development of uplift patterns, and recurrence timescales are important controls on seismic cycle deformation along the Sunda margin. Because contemporary instrumental seismic records along the southern coast of Java are insufficient to fully characterize the long-term behavior of the seismic cycle, geologists rely on proxy records to reconstruct its temporal and spatial variability.

Marine terraces are key indicators of past sea-level variations and tectonic movements [3,4]. Their association with Marine Isotope Stages (MIS) helps distinguish interseismic deformation from coseismic megathrust activity [16], while elevation and chronological data allow estimation of Pleistocene-to-Holocene uplift rates [17]. Despite important advances in previous studies of marine terraces and neotectonic deformation along the southern coast of Java [18,19,20,21,22,23], several gaps remain. Most earlier works relied on regional geomorphological mapping, remote sensing, DEM-based interpretation, literature compilation, or broad geophysical synthesis, while direct field-based topographic measurements across individual terrace profiles are still limited. Previous studies [18,19,20,21,22,23] report spatial variability in raised coastal morphology along southern Java, although the age and tectonic significance of many surfaces remain uncertain. The Pangandaran–Pacitan area is marked by stair-stepped marine terraces, suggesting uplift rates of about 0.1–0.4 mm/yr, with locally higher rates in some tectonic blocks. In the Gunung Kidul–Gunung Sewu region, the terraces point to moderate long-term uplift, while the abrasion platforms and marine notches around Pacitan–Klayar may reflect more recent Holocene or coseismic uplift [18,19,20,21,22,23]. Taken together, these studies suggest that the southern Java margin is tectonically segmented and has been shaped by both gradual forearc uplift and episodic earthquake-related deformation. Consequently, because many previous interpretations were based on regional-scale mapping or moderate-resolution DEM and remote-sensing datasets, the local morphology of terrace treads, risers, shoreline-angle zones, and residual coastal highs has not been sufficiently constrained at the meter scale. As a result, terrace correlations between sites often remain tentative. In addition, many uplift interpretations depend on assumed correlations with Marine Isotope Stage or Holocene sea-level highstands, but direct chronological control is still sparse, making uplift-rate estimates provisional. Previous studies have not always been able to clearly distinguish true marine terraces from fluvial, aeolian, karstic, erosional, or anthropogenically modified surfaces, particularly in low-elevation coastal settings. The Real-Time Kinematic Global Navigation Satellite System (RTK-GNSS) survey conducted in this study helps reduce this geomorphic uncertainty by providing direct, high-resolution cross-shore elevation profiles from Pantai Ajah, Kalijali, Kulon Progo, and Wingko. Although these data do not provide Holocene-scale chronological constraints, they allow terrace morphology, slope breaks, and relative elevation patterns to be characterized more consistently across the study sites. These measurements allow terrace treads, slope breaks, possible risers, and higher topographic features to be identified more clearly, improve the basis for tentative terrace correlation, and provide a field-based geomorphic baseline for comparing local cross-shore morphology and identifying priority sites for future geological, chronological, and coastal-hazard investigations.

This study applies RTK-GNSS transect measurements to selected coastal sites along southern Java to characterize raised coastal morphology using high-resolution cross-shore topographic profiles. Specifically, this study (1) characterizes coastal profiles using high-resolution, centimeter-scale RTK-GNSS data; (2) identifies key geomorphic features including terrace treads, slope breaks, steeper slopes separating adjacent terrace-like surfaces, and higher remnant surfaces; (3) compares recurring elevation ranges and profile configurations among the four study sites; and (4) discusses possible implications for sea-level interpretation and coastal hazards as hypotheses to be tested in future studies. By integrating RTK-GNSS measurements with the geomorphological analysis of marine terraces, this study provides a field-based geomorphic baseline for raised coastal landforms along southern Java, complementing previous regional-scale interpretations with direct, high-resolution cross-shore elevation profiles. The results contribute to improving terrace interpretation at the local scale and documenting spatial variability in surface elevation, profile geometry, and landform preservation along an active coastal margin, while also providing a geomorphic context that may be relevant to future assessments of tsunami exposure and coastal vulnerability.

2. Geological, Sea-Level, and Geomorphological Background

2.1. Tectonic Setting of Southern Java

Southern Java forms part of the active Sunda Arc (Figure 1), where subduction-related deformation influences the morphology of the coastal margin. The interaction between plate convergence, forearc deformation, inherited crustal structures, and local geomorphic processes can produce spatially variable uplift and subsidence. This variability is important for interpreting marine terraces because terrace elevation is controlled not only by eustatic sea level, but also by tectonically driven uplift and subsidence within the forearc. This regional tectonic setting provides context for the observed coastal morphology but is not used to assign individual surfaces to specific tectonic blocks. Along active margins, the same shoreline surface may be preserved at different elevations if different coastal blocks experience different uplift rates. Conversely, surfaces at similar elevations may not necessarily be the same age if uplift histories differ.

Figure 1. Regional-to-local tectonic framework of Java. (A) Regional tectonic setting of western–central Indonesia, showing the interaction of major plates, continental shelves, marginal basins, and subduction systems. (B) North–south geological cross-section of Java Island derived from the regional framework in Panel (A), showing the active subduction of the Indo-Australian Plate beneath the Eurasian Plate, including the Java Trench, inclined Benioff zone, and volcanic–plutonic arc above the subduction zone [24].

Figure 1. Regional-to-local tectonic framework of Java. (A) Regional tectonic setting of western–central Indonesia, showing the interaction of major plates, continental shelves, marginal basins, and subduction systems. (B) North–south geological cross-section of Java Island derived from the regional framework in Panel (A), showing the active subduction of the Indo-Australian Plate beneath the Eurasian Plate, including the Java Trench, inclined Benioff zone, and volcanic–plutonic arc above the subduction zone [24].

The southern Java coast forms part of a forearc system above the subducting Indo-Australian Plate. Spatial differences in coastal uplift may therefore reflect variations in interplate coupling, forearc structure, and inherited crustal segmentation along the margin (Figure 2).

Figure 2. Conceptual tectonic framework of the southern Java margin, showing the subduction of the Indo-Australian Plate beneath Sundaland and the main forearc, volcanic arc, and crustal deformation domains relevant to coastal uplift and terrace development. The white arrow indicates the direction of subduction.

2.2. Regional Relative Sea-Level Highstand Context

Mid-Holocene relative sea-level highstands varied spatially across Indonesia because of differences in tectonic setting, sediment dynamics, and glacio-hydro-isostatic adjustment [25,26,27,28,29,30,31,32,33]. Some regions record lower-amplitude highstands or more complex relative sea-level histories owing to the influence of these local and regional factors. Therefore, the mid-Holocene highstand is treated here only as a possible age scenario and a working hypothesis, rather than as a definitive age assignment for the RTK-GNSS-derived coastal surfaces. This hypothesis is based on the observed elevation and morphology of the coastal surfaces, together with published regional sea-level records that document a mid-Holocene highstand. However, because this study provides neither numerical age nor direct sedimentological evidence from the terrace surfaces, the proposed correlation with the mid-Holocene highstand remains tentative.

Elevation alone s insufficient to determine a mid-Holocene surface from an older Late Pleistocene shoreline, including surfaces potentially associated with MIS 5, or from a landform unrelated to a former sea level.

2.3. Terrace Morphology and Tsunami Relevance

Raised coastal terraces, beach ridges, and topographic highs may influence local tsunami-inundation patterns through their elevation, width, continuity, and the distribution of gaps or channels [34,35,36,37]. Natural coastal landforms such as dunes, beach ridges, reefs, mangroves, and coastal ridges can reduce wave energy, flow depth, or inland penetration under some conditions, although their effectiveness depends on the height, width, continuity, vegetation cover, and slope of these landforms, as well as on tsunami magnitude and the presence of gaps or channels. However, topographic profiles alone cannot quantify wave attenuation, overtopping, erosion, or reduction in inundation.

For this reason, the elevated terrace ridges and local topographical highs identified in the RTK profiles are important not only for reconstructing coastal evolution, but also for coastal-hazard assessment. However, they should not be regarded as providing complete protection. A large tsunami can overtop, erode, breach, or flow around natural barriers. The measured elevations may provide useful topographic input for future coastal-hazard assessment. Their hydrodynamic significance, however, requires three-dimensional terrain analysis and numerical tsunami modelling and is not evaluated in this study.

3. Historical Background of Terrace Studies

Studies of marine terraces along southern Java have progressed from regional geomorphological observations to integrated interpretations of sea-level change and tectonic uplift. Early studies identified elevated coastal surfaces as evidence of repeated emergence and vertical land movement [38]. Later studies emphasized spatial variations in terrace elevations [39] and used remote sensing to distinguish marine, fluvial, and denudational landforms [40].

Recent approaches integrate terrace morphology, sea-level changes, chronology, and comparisons across tectonic settings [3]. Digital elevation models, statistical clustering [41], and regional geological and geophysical synthesis [42,43] have further improved terrace identification. Previous studies in Pangandaran, Gunung Kidul, Gunung Sewu, Pacitan, Klayar, Lebak, and the Sunda Arc documented terrace-like surfaces, abrasion platforms, marine notches, paleotsunami deposits, and other coastal features [18,19,20,21,22,23].

However, most interpretations rely on regional mapping, remote sensing, or literature synthesis rather than direct field measurements. Therefore, Table 1 is used only as a regional reference, not as a dated terrace chronology. This study addresses remaining uncertainties through RTK-GNSS cross-shore profiles to evaluate local terrace morphology and possible raised coastal surfaces.

Table 1. Synthesis of previous studies on raised coastal landforms, marine terraces, and related coastal-deformation indicators along southern Java and adjacent parts of the Sunda Arc.

Study	Area/Sector	Main Data or Approach	Main Geomorphic or Geological Contribution
Andreini et al. [18]	Southern Java coastal sectors including Pangandaran, Pacitan, Karanghawu, and Pangamalang	Geomorphological mapping and topographic analysis	Reported stair-stepped coastal terrace morphology and spatial variation in raised coastal surfaces.
Haryono and Day [19]	Gunung Kidul	Karst geomorphology and regional morphometric interpretation	Characterized coastal relief and karst landforms relevant to interpreting long-term landscape development in the southern Java margin.
Tjia [20]	Gunung Sewu, Oyo River, and Pacitan–Klayar area	Morphostructural and geomorphological interpretation	Documented marine and fluvial terraces, abrasion platforms, and coastal notches as indicators of coastal emergence and deformation.
Kázmér et al. [21]	Bali and adjacent Sunda Arc examples	Coastal notch and karst-geomorphic analysis	Provided regional analogues for interpreting uplifted coastal notches and shoreline-related deformation in the Sunda Arc.
Putra et al. [22]	Lebak and Pangandaran	Coastal stratigraphy, trenches, and cores	Identified paleotsunami deposits and provided evidence for tsunami recurrence and coastal hazard context along southern Java.
van Gorsel [23]	Java and surrounding regions	Regional geological and bibliographic compilation	Provided broader geological context for interpreting coastal and tectonic features in Java.

Note: This table summarizes published observations from previous studies. It is used as a qualitative regional reference framework and does not provide direct chronological constraints for the RTK-GNSS profiles analyzed in this study.

4. Materials and Methods

4.1. Field Survey and Data Acquisition

This study is designed as a field-based geomorphological survey supported by regional literature synthesis. The primary dataset consists of RTK-GNSS topographic profiles measured across four coastal sites: Pantai Ajah, Kalijali, Kulon Progo, and Wingko (Figure 3). These profiles provide high-resolution cross-shore elevation data that are used to identify terrace morphology and possible raised coastal surfaces. The study is intentionally limited to the topographic characterization of coastal landforms. It does not attempt to establish numerical ages, confirm marine depositional origin, or calculate uplift rates.

Figure 3. Location of RTK-GNSS transects along the southern coast of Java, Indonesia. The transects were collected at four sites: Pantai Ajah, Kalijali, Kulon Progo, and Wingko, with red boxes indicating the specific survey areas.

Field surveys were conducted March to June 2025 using a RTK-GNSS system operated in network RTK mode. Real-time corrections were received through NTRIP NTRIP-based InaCORS/NRTK service provided by the Indonesian Geospatial Information Agency (BIG), Cibinong, Indonesia. The rover antenna was mounted at a measured height of 1.80 m. The recorded ellipsoidal heights were converted to orthometric elevations using the Web InaGeoid service provided by BIG. The resulting data were organized and processed using Microsoft Excel (Microsoft Corporation, Redmond, WA, USA), whereas spatial processing and map preparation were performed using QGIS 3.28 Firenze (QGIS Development Team).

4.2. RTK-GNSS Topographic Profiles

RTK-GNSS profiles were measured from the beachward side toward the landward side (Figure 3). Cross-shore profiles were collected from the beachward side toward the landward side because this orientation best captures elevation changes from the modern shoreline to inland raised surfaces, including terrace treads, slope breaks, possible risers, and higher topographic remnants. We also acknowledge that cross-shore profiles do not fully resolve alongshore variability or three-dimensional terrace continuity. Therefore, interpretations of terrace correlation and coastal segmentation are treated as tentative. In the profile figures, point A represents the beachward end of each transect and point B represents the landward end. This orientation allows the profiles to capture the transition from modern coastal surfaces to older or higher landward geomorphic features.

The profiles were interpreted by identifying:

• Broad and gently sloping surfaces that may represent terrace treads;
• Steep slope breaks that may represent terrace risers or paleo-sea cliffs;
• Shoreline-angle zones marking transitions between former platforms and scarps;
• Isolated topographic highs that may represent residual ridges, bedrock highs, dunes, erosional remnants, or locally uplifted former shoreline features.

The terrace interpretation emphasizes surface continuity rather than isolated elevation peaks. A broad surface is more reliable as a terrace tread than a narrow peak, because marine terrace treads are usually laterally continuous geomorphic surfaces.

Only fixed RTK solutions/solutions meeting the stated quality criteria were retained. Points affected by loss of correction, unstable antenna position, duplicated coordinates, or isolated elevation spikes were removed. The retained measurements had reported horizontal uncertainties of approximately 0.01–0.02 m and vertical uncertainties of approximately 0.02–0.03 m, based on the receiver-reported positioning quality output. Coordinates and elevations were subsequently processed, and cross-shore distance was calculated cumulatively along each transect. Profiles were generated from raw fixed-solution points without smoothing. The unsmoothed profiles were retained to preserve the original field measurements and support quality-control assessment.

4.3. Criteria for Terrace Interpretation

A landform was interpreted as a probable terrace tread when it showed a broad or semi-continuous expression in the profile, a relatively low gradient, and separation from adjacent surfaces by a clear slope break. Because this study relies mainly on RTK-GNSS topographic profiles, terrace interpretation was based on morphology rather than direct geological confirmation. Broad, gently sloping, semi-continuous surfaces separated by clear slope breaks were classified as probable terrace treads, whereas steep elevation changes were interpreted as possible risers or paleo-sea-cliff zones. Narrow peaks were treated cautiously because they may reflect local relief or erosional remnants rather than true terrace surfaces. These features are therefore regarded as preliminary geomorphic candidates that should be tested in future work using geological mapping, sedimentological evidence, and chronological constraints. In this study, the term ‘terrace-like surface’ is used in a strictly morphological sense and does not imply a confirmed marine origin. Alternative explanations considered include beach-ridge or dune construction, fluvial or alluvial surfaces, erosional or denudational benches, bedrock highs, sediment-covered coastal surfaces, and anthropogenic levelling.

Candidate surfaces were assigned a relative interpretive confidence. Higher-confidence candidates are broad, low-gradient surfaces that remain continuous along a substantial part of the surveyed profile and are separated from adjacent surfaces by a clear slope break. Moderate-confidence candidates show only some of these characteristics. Narrow peaks, short irregular surfaces, and visibly modified ground were classified as ridge-like or residual topographic highs rather than terrace treads.

The RTK profiles provide strong morphological information, but they do not provide ages. Therefore, all terrace-like correlations in this study are morphostratigraphic and preliminary. Confirmation requires geochronological dating, sedimentological logging, and field verification of marine indicators such as beach deposits, abrasion surfaces, marine shells, coral fragments, beachrock, or coastal sedimentary structures.

Throughout the manuscript, directly measured features are described using observational terms such as ‘broad surface’, ‘slope break’, and ‘ridge-like high’. The term ‘terrace-like candidate’ denotes a morphological interpretation, whereas marine, chronological, and tectonic explanations are presented only as hypotheses. Along-profile gradient was calculated from elevation change divided by horizontal distance and was summarized using a 50-m moving window. Candidate tread boundaries were assigned where a persistent low-gradient segment was separated from an adjacent steeper segment by a reproducible change in gradient. Linear fits were applied to the adjacent low-gradient and steep segments, and their intersection was recorded as the slope-break or shoreline-angle zone.

5. Results

The RTK profiles show that coastal morphology varies among the four study sites. The profiles do not indicate a single continuous terrace level along the entire coast because equivalent surfaces cannot be traced consistently among the transects based on elevation, width, slope-break position, and surface continuity. Each profile contains a different combination of broad low-gradient surfaces, slope breaks, short-wavelength undulations, and ridge-like topographic highs.

Pantai Ajah and Kalijali show the clearest stepped morphology. Wingko also contains a probable raised landward surface, whereas Kulon Progo shows a weaker and more subdued terrace signal. Overall, the probable terrace surfaces occur mainly between approximately 4 m and 9 m elevation, while higher local peaks reach approximately 12–14 m at some locations.

5.1. Pantai Ajah

The Pantai Ajah profile shows a marked elevation variation from the seaward to the landward part of the transect. At the seaward end, elevation rises rapidly from near sea level to approximately 5–6 m within the first few hundred metres. A distinct foredune occurs at approximately 6–6.5 m elevation, around 100–200 m from the start of the profile. Landward of the foredune, a broad surface occurs at approximately 5.5–6.2 m elevation and extends from around 200 m to 1300–1400 m along the profile (Figure 4).

Figure 4. RTK transect location and topographic profile of Pantai Ajah, Kebumen, Central Java. A marks the seaward starting point of the transect, whereas B marks the landwardmost measurement point.

The most prominent ridge-like high occurs at approximately 550–600 m and reaches around 11–12 m elevation (Figure 4). A second ridge-like high occurs between approximately 950 m and 1350 m, with local elevations of around 8–11 m. Together, these features form a ridge complex approximately 700–800 m wide, although the individual ridge crests are generally only about 50–150 m wide. Beyond approximately 1400 m, the profile descends toward a lower, more subdued surface at approximately 3–4 m elevation. This lower surface extends from approximately 1500 m to nearly 3000 m and is characterized by gentle undulations rather than sharp ridge forms.

5.2. Kalijali

The Kalijali profile displays the strongest relief contrast among the four investigated transects. The frontal part of the profile, from approximately 0 to 200–250 m, contains a cluster of foredunes at elevations of around 6–7 m. These foredunes appear as several low ridges rather than as a single continuous ridge. Behind the foredune zone, the profile drops markedly to a relatively low and flat surface at approximately 1.5–2.5 m elevation. This surface extends from approximately 250–300 m to 600–650 m along the profile.

The most prominent ridge occurs between approximately 650 m and 900–950 m and reaches a maximum elevation of around 13–14 m (Figure 5). It is steep on its seaward side and descends more gradually landward. The ridge is approximately 200–300 m wide, with its main crest located around 800–850 m. Landward of this ridge, the profile forms a broad, gently undulating surface at approximately 7–8.5 m elevation. This broad surface extends from approximately 900 m to 1800 m.

Figure 5. RTK transect location and topographic profile of Kalijali, Purworejo, Central Java. A marks the seaward starting point of the transect, whereas B marks the landwardmost measurement point.

5.3. Kulon Progo

The Kulon Progo profile is more gradual and less sharply dissected than those of Pantai Ajah and Kalijali. At the seaward end, the profile begins at approximately 5–6 m elevation and contains a subdued foredune zone within the first 200–300 m. The foredune reaches approximately 5.5–6.5 m elevation. From the frontal section toward the middle of the transect, a relatively broad surface remains at approximately 5–5.5 m elevation and extends to around 1200–1300 m. A broad, multi-crested ridge complex occurs between approximately 700 m and 1200 m along the profile (Figure 6). Several ridge crests reach elevations of approximately 7.5–8.5 m, and the overall ridge complex is around 400–500 m wide. Beyond approximately 1300 m, the profile descends toward a lower, gently sloping distal surface at approximately 4–4.5 m elevation. This distal surface continues landward to approximately 3200 m and is characterized by low-amplitude undulations and gradual lowering.

Figure 6. RTK transect location and topographic profile of Kulon Progo, Yogyakarta. A marks the eaward starting point of the transect, whereas B marks the landwardmost measurement point.

5.4. Wingko

The frontal part of the Wingko profile contains a broad and subdued foredune zone between approximately 0 and 300–350 m, with elevations reaching around 5.5–6.5 m. Behind this zone, the profile descends to a lower surface extending from approximately 400 m to 1400–1500 m. This surface lies mostly at around 2–3 m elevation and shows gentle irregularities interrupted by several small ridge-like highs.

A distinct ridge occurs at approximately 1200–1350 m, where elevation rises sharply to around 6.5–7 m (Figure 7). The ridge is approximately 100–200 m wide. Beyond approximately 1500 m, the profile rises into a broader and more laterally continuous surface at approximately 5.5–6.5 m elevation. This surface extends from approximately 1500 m to 2500 m and is smoother than the lower, more irregular sector of the profile.

Figure 7. RTK transect location and topographic profile of Wingko, Purworejo, Central Java. A marks the seaward starting point of the transect, whereas B marks the landwardmost measurement point.

5.5. Cross Site Geomorphic Varialility

Across the four transects, the measured profiles contain different combinations of foredunes, ridge-like topographic highs, broad low-gradient surfaces, and slope breaks (Figure 8; Table 2). Pantai Ajah and Kulon Progo contain relatively elevated seaward-to-intermediate surfaces, that transition landward to lower distal surfaces. In contrast, Kalijali and Wingko contain lower seaward or intermediate surfaces, that transition to broader, more elevated landward surfaces. The morphology and elevation of the principal ridge features also differ among the sites. Kalijali contains the highest and most prominent ridge, reaching approximately 13–14 m, whereas Pantai Ajah contains a wider ridge complex with local elevations of approximately 8–12 m. Kulon Progo is characterized by a broad, subdued, multi-crested ridge complex reaching approximately 7.5–8.5 m, while Wingko contains a narrower and more localized ridge at approximately 6.5–7 m elevation. These comparisons describe present-day profile geometry and do not demonstrate that surfaces at different sites are coeval, genetically equivalent, or affected by the same uplift history.

Figure 8. Topographic profiles of the Ajah, Kalijali, Kulon Progo, and Wingko transects showing the distribution of foredunes, ridge-like topographic highs, broad low-gradient surfaces, and major slope breaks.

Table 2. Orphometric characteristics of the four investigated coastal transects, including the position and elevation of foredunes, ridge-like topographic highs, broad low-gradient surfaces, and distal landward surfaces.

Transect	Foredune Morphology	Beach-Ridge Morphology	Seaward Surface	Landward Surface	Cross-Profile Configuration
Ajah	Frontal foredune at approximately 6–6.5 m elevation, located around 100–200 m from the profile origin.	Prominent beach-ridge complex. Main sharp ridge reaches ~11–12 m at ~550–600 m; additional ridge highs occur around 950–1350 m. Estimated ridge-complex width: ~700–800 m.	Higher terrace-like, approximately 5.8–6.2 m, extending broadly from ~200 to 1300–1400 m.	Lower terrace-like, approximately 3–4 m, extending from ~1500 to 3000 m.	Seaward Surface (Possible Terrace 1) is higher than Landward Surface (Possible Terrace 2).
Kalijali	Clustered foredunes at approximately 6–7 m elevation, within the first ~0–250 m.	Highest and most pronounced beach ridge among the transects. Ridge crest reaches ~13–14 m at ~800–850 m. Estimated ridge width: ~200–300 m.	Lower terrace-like, approximately 1.5–2.5 m, extending from ~250–300 to 600–650 m.	Higher terrace-like, approximately 7–8.5 m, extending from ~900 to 1800 m.	Landward surface (Possible Terrace 2) is higher than Seaward Surface (Possible Terrace 1).
Wingko	Broad, subdued foredune at approximately 5.5–6.5 m elevation, located within ~0–300/350 m.	Localized beach ridge at ~1200–1350 m, reaching ~6.5–7 m. Estimated width: ~100–200 m.	Lower terrace-like, approximately 2–3 m, extending from ~400 to 1400–1500 m.	Higher terrace-like, approximately 5.5–6.5 m, extending from ~1500 to 2500 m.	Landward surface (Possible Terrace 2) is higher than Seaward Surface (Possible Terrace 1).
Kulon Progo	Subdued frontal foredune at approximately 5.5–6.5 m elevation, within the first ~200–300 m.	Broad beach-ridge complex between ~700 and 1200 m, with multiple crests reaching ~7.5–8.5 m. Estimated width: ~400–500 m.	Higher terrace-like, approximately 5–5.5 m, extending from the frontal profile to ~1200–1300 m.	Lower terrace-like, approximately 4–4.5 m, extending from ~1300 to 3200 m.	Seaward Surface (Possible Terrace 1) is higher than Landward Surface (Possible Terrace 2).

6. Discussion

6.1. Correlation of Terrace Surfaces Between Sites

Based on the RTK-GNSS profiles, the measured coastal morphology can be provisionally organized into two recurring elevation-based surface groups, accompanied several higher, discontinuous topographic features. This classification is derived from consistent patterns in elevation, surface continuity, and overall profile morphology observed across the four study sites. These groupings describe recurring morphometric patterns only and do not demonstrate that surfaces at different sites are coeval, genetically equivalent, or affected by the same uplift history. This study provides direct field-based topographic measurements that were previously unavailable or limited for these coastal sites. The RTK-GNSS profiles allow subtle differences in terrace elevation, surface morphology, slope breaks, and preservation state to be documented at the local scale. Therefore, the importance of the proposed subdivision lies in providing a reproducible geomorphic baseline for comparison of coastal surfaces among Pantai Akah, Kalijali, Kulon Progo, and Wingko. This baseline can guide future dating and sedimentological work, and help to evaluate if the observed surface groups are related to different ages of formation, preservation histories or spatially variable uplift processes along the southern Java margin.

Comparable terrace studies have used RTK-GNSS to determine tread elevation and profile geometry, but tectonic and sea-level interpretations are generally strengthened by geomorphic mapping, stratigraphic observations, and independent age control. The present profiles are therefore most appropriately treated as a reconnaissance-scale morphological baseline rather than a complete uplift reconstruction.

6.1.1. Interpretive Confidence and Alternative Landform Origins

The RTK-GNSS profiles identify reproducible geometric features, but geometry alone does not establish landform genesis. Broad surfaces may represent marine or coastal terraces, beach-ridge plains, fluvial or alluvial surfaces, erosional benches, or anthropogenically modified ground. The interpretations presented below therefore compare morphological candidates rather than confirmed marine-terrace levels.

6.1.2. Lower Terrace-like Group (Approximately 4–6.5 m)

The lower terrace-like group includes the lower surface at Kalijali, the tentative low surface at Kulon Progo, and the landward surface at Wingko. Kalijali and Kulon Progo occur mainly at elevations of 4–5 m, whereas the Wingko surface is slightly higher, at approximately 5–6.5 m These surfaces occupy comparable elevation ranges, but their ages and genetic relationships cannot be established from the RTK-GNSS profiles. Subsequent modification by local uplift, erosion, and sedimentary processes may have contributed to the observed variability in elevation. The slightly higher position of the Wingko surface may reflect differences in original morphology, sediment accumulation, preservation, vertical land motion, or surface classification.

6.1.3. Upper Terrace-like Group (Approximately 7–9 m)

The upper terrace-like group is best expressed at Pantai Ajah and in the upper surface at Kalijali, where both sites display comparable elevation ranges and well-defined stepped morphology. This represents the most consistent morphostratigraphic correlation identified in the dataset. These surfaces may correspond to an older or more strongly uplifted coastal level relative to the lower terrace group. The comparable elevation ranges at Pantai Ajah and Kalijali constitute the strongest morphological similarity in the dataset. Nevertheless, similar elevation does not demonstrate a common age or uplift history. The surfaces could represent Holocene or older coastal landforms, depositional surfaces formed at different times, or unrelated landforms that have converged toward similar present elevations.

6.1.4. Higher Local Highs (Approximately 12–14 m)

In addition to the two main terrace groups, several higher and more localized topographic features are observed, particularly at Kalijali (~12–13 m) and Pantai Ajah (~13–14 m). Unlike the lower and upper terrace groups, these features are narrower and lack clear lateral continuity, and are therefore not interpreted as regionally developed terrace treads.

Nevertheless, these elevated features are geomorphologically significant and warrant further consideration. They may represent (i) residual remnants of older terrace surfaces; (ii) uplifted beach-ridge or dune-ridge systems; (iii) bedrock or karstic highs modified by marine processes; (iv) erosional remnants of previously more extensive surfaces; or (v) locally uplifted features associated with Holocene or older sea-level positions.

These interpretations remain working hypotheses because the narrow topographic highs are consistent with several possible origins, including older coastal remnants, constructional beach or dune ridges, bedrock highs, and erosional remnants. Their ages and relationships to former sea-level positions cannot be determined from elevation alone.

6.2. Relative Sea-Level Context and Differential Uplift of Elevated Coastal Surfaces

6.2.1. Possible Age and Sea-Level Scenario

The mid-Holocene highstand provides a useful framework for interpreting low to moderate raised coastal surfaces along southern Java. Regional sea-level studies from Indonesia and Sundaland indicate that relative sea level was higher than present during parts of the mid-Holocene, although the magnitude and timing varied between locations [25,26,27,28,32,33]. Some records suggest highstands of around 2–3 m above present sea level between roughly 7000 and 4000 BP [25,26,27,28]. If coastal platforms, beach ridges, or abrasion surfaces formed during this period, they could now be preserved at higher elevations if the coast has been uplifted since formation [3,17]. This is particularly relevant for the 4–6.5 m lower terrace group and possibly for the 7–9 m upper group. The 12–14 m local highs may also be related to this framework, but their elevation would require either stronger uplift, episodic coseismic uplift, or an older origin [21]. The mid-Holocene highstand is therefore retained as one testable scenario rather than an age assignment. Older Late Pleistocene highstands, including MIS 5, and non-marine origins must also be considered. Distinguishing among these alternatives requires numerical dating and diagnostic geological evidence.

6.2.2. Differential Uplift as an Explanation for Variable Elevations

The variation in terrace elevation between the four sites may reflect spatially variable uplift along the southern Java forearc [18]. If similar coastal surfaces formed at approximately the same relative sea level, then differences in their present elevation may indicate different uplift histories (Figure 9). If some surfaces were originally formed at comparable relative sea levels, their present elevation differences could partly reflect differential vertical land motion. However, the same differences could also result from contrasts in original depositional elevation, sediment accumulation, erosion, landform type, or preservation. The profiles therefore identify spatial variation in present morphology but do not quantify uplift magnitude.

Figure 9. Regional tectonic setting of Central Java showing major structural lineaments, inferred faults, fold axes, principal volcanic centers, and the locations of the coastal transects investigated in this study (Ajah, Kalijali, Kulon Progo, and Wingko). The distribution of the study sites relative to the Pamanukan–Cilacap and Muria–Kebumen fault systems shows the regional structural context surrounding the four RTK-GNSS transects. The figure is included as regional background and is not used to infer direct structural control on individual measured surfaces. Modified from Untung and Hasegawa [38], after Satyana and Purwaningsih [42].

However, this model assumes that the surfaces are the same age and marine in origin, which has not yet been proven. The observed elevation differences may also reflect local erosion, sediment cover, lithology, dune formation, anthropogenic modification, or preservation bias. Therefore, the profiles support the idea of spatial variability in terrace elevation and preservation, but they do not yet allow definitive calculation of uplift rates [3,41]. Table 3 below showing the morphostratigraphic correlation between each location.

Table 3. Provisional morphometric comparison of RTK-identified coastal surfaces and topographic highs between Pantai Ajah, Kalijali, Kulon Progo, and Wingko.

Terrace/Landform Group	Elevation Range	Locations Included	Alternative Geomorphic Interpretation	Notes
Lower terrace group	~4–6.5 m	Kalijali lower surface, Kulon Progo, Wingko	Possible low raised coastal platform, young marine terrace, or Holocene-related coastal surface	Recurring lower-elevation surface range; age and correlation unresolved
Upper terrace group	~7–9 m	Pantai Ajah, Kalijali upper surface	Probable higher marine terrace or older/more uplifted coastal surface	Strongest morphological similarity in the dataset; not a chronological correlation
Higher local highs	~12–14 m	Kalijali high, Pantai Ajah high, possibly other isolated highs	Residual ridge, older terrace remnant, uplifted Holocene/highstand-related feature, dune/beach ridge, or bedrock high	Should be discussed as a working hypothesis, not a confirmed terrace level

Elevations are interpreted from RTK topographic profiles and should be regarded as preliminary geomorphic estimates. Correlation is based on elevation range, profile morphology, surface continuity, and slope-break expression. Because no direct age control is currently available, the proposed correlation is morphostratigraphic rather than chronostratigraphic. Higher local peaks at approximately 12–14 m are narrow ridge-like or residual topographic highs whose origin remains unresolved.

6.2.3. Topographic Relevance for Future Tsunami Assessment

Although raised coastal landforms may be relevant for future assessments of tsunami exposure and coastal vulnerability, this study does not directly evaluate tsunami inundation, run-up, or mitigation performance. Therefore, tsunami-related implications are not treated as a primary result, but only as broader geomorphic context for future work.

The raised terrace surfaces and local topographic highs may have practical importance for tsunami mitigation [11,12]. A continuous ridge, terrace scarp, or elevated coastal platform can act as a partial barrier to incoming tsunami flow by increasing flow resistance, reducing inland penetration, or concentrating flow within lower gaps and channels [11,12]. Studies of tsunami impacts have shown that coastal landforms such as dunes and beach ridges can reduce inland tsunami penetration under some conditions, particularly where tsunami heights are moderate and the barriers are continuous.

In this context, the 12–13 m topographic high at Kalijali and other elevated points may [ptentially provide some degree of natural topographic protection. The 7–9 m upper terrace surfaces at Pantai Ajah and Kalijali may also reduce direct inland flow where they form continuous ridges or scarps. The lower 4–6.5 m surfaces may provide more limited protection, but they can still influence tsunami flow paths by creating roughness, elevation contrasts, and local ponding zones. This interpretation represents our preliminary hypothesis based on the observed topographic configuration. The RTK-GNSS profiles document elevation contrasts and slope breaks that could be incorporated into future tsunami-inundation models. However, the present data do not demonstrate that any individual ridge or surface can reduce flow depth or inland penetration. Such effects depend on three-dimensional landform continuity, the presence of low-elevation gaps, land cover, erosion potential, tsunami magnitude, and wave direction.

However, these natural features should not be interpreted as complete tsunami defenses. Their protective function depends on height relative to tsunami run-up, width and continuity of the ridge or terrace, presence of low gaps, river mouths, roads, or drainage channels, surface roughness and vegetation, tsunami wave height, period, and direction, erosion or breaching during the event [11,12].

A large tsunami may overtop or erode even high coastal ridges [11,12]. Therefore, the raised terraces should be incorporated into tsunami-hazard mapping as natural mitigation features, but not used as the only basis for safety planning.

6.2.4. Implications for Land-Use and Hazard Planning

The RTK profiles can help identify areas where natural elevation may reduce tsunami exposure [11,12]. Higher terrace surfaces and ridges may be suitable as evacuation routes, temporary refuge zones, or priority areas for vertical-evacuation planning, provided that access is rapid and the surfaces are continuous. Low-lying areas seaward of the terrace risers are likely to be more exposed to inundation and should be prioritized for hazard zoning. The profiles therefore have two scientific and practical uses. Beside the scientifically part discussed above, practically ractically, they help define natural topographic barriers and low-lying corridors that may control tsunami inundation pathways [11,12].

6.3. Potential Controls on Coastal SurfaceElevation and Geomorphic Development

The contrasting elevation relationship between Terrace 1 and Terrace 2 along the four transects suggest that terrace development along the southern Central Java coast My reflect the combined effect of sediment supply and structural deformation. Importantly, terrace age should not be inferred from elevation alone. In prograding coastal settings, differences between seaward and landward surfaces may reflect successive phases of deposition or erosion. However, their relative ages cannot be established from cross-shore position and elevation alone. The contrasting elevation and cross-shore arrangement of the seaward and landward surfaces across the four transects are shown in Figure 8 and summarized in Table 2. These differences may reflect variations in sediment supply, depositional history, erosion, preservation, and/or vertical land motion. Importantly, terrace age should not be inferred from elevation alone. In prograding coastal settings, differences between seaward and landward surfaces may reflect successive phases of deposition or erosion. However, their relative ages cannot be established from cross-shore position and elevation alone.

Ajah and Kulon Progo show an unusual configuration in which the seaward-to-intermediate surface occurs at a higher elevation than the more landward surface (Figure 8, Table 2). This relationship may reflect substantial vertical aggradation of the coastal plain, potentially associated with enhanced fluvial–volcaniclastic sediment supply from major river systems. Ajah is located west of the Serayu River system, while Kulon Progo is located west of the Progo River system. These rivers are capable of delivering large volumes of volcanic and mountainous sediment to the coast. Consequently, coastal progradation and sediment accumulation may have contributed to the formation of a thicker and more elevated depositional surface, represented by Terrace 1 and the associated beach-ridge morphology. In contrast, Kalijali and Wingko show a more conventional configuration, in which the broad landward surface occurs at a higher elevation than the lower seaward or intermediate surface (Figure 8, Table 2). These transects are not directly associated with nearby major volcaniclastic river systems, and therefore the younger Terrace 1 may have developed under lower sediment-supply conditions. The higher elevation of Terrace 2 may reflect preservation of an older landward surface, possibly enhanced by local uplift or tilting.

Regionally, the study sites occur within the broader structural setting of the Central Java indentation. However, the present profiles do not establish a causal relationship between individual coastal surfaces and specific mapped or inferred fault. The model of Satyana and Purwaningsih [42] proposes that the indentation of Central Java was produced by the interaction of the Pamanukan–Cilacap dextral wrench fault and the Muria–Kebumen sinistral wrench fault (Figure 9). These regional structures provide a tectonic context for future investigation, but their relationship to the measured elevation differences cannot be resolved from the present dataset. Thus, the Ajah–Kulon Progo pattern reflects strong sediment-driven aggradation superimposed on a deforming coastal margin, whereas the Kalijali–Wingko pattern more clearly preserves the expected older-higher and younger-lower terrace relationship under lower sediment-supply conditions.

6.4. Knowledge Gaps and Future Work

An important methodological limitation of this study is that the interpretation is based on a few two-dimensional RTK-GNSS cross-shore profiles, while marine terraces, coastal ridges and associated fluvial-fan surfaces are inherently three-dimensional landforms. Profiles are useful for identifying local slope breaks, broad surfaces, and potential terrace levels along individual transects, but do not fully resolve lateral continuity, planform geometry, terrace width, or spatial relationships between adjacent geomorphic units. As a result, the terrace treads, risers and residual highs identified in this study should be considered preliminary geomorphic candidates rather than fully mapped terrace surfaces. Future work should combine RTK-GNSS profiles together with three-dimensional topographic datasets (UAV photogrammetry, LiDAR, high resolution DEMs) and geological mapping, sedimentological observations and chronological control.

However, the workflow has broader applicability to other tectonically active coastal margins where direct age control and detailed three-dimensional topographic data are limited. RTK-GNSS profiling can provide a rapid reconnaissance tool for the identification of candidate raised coastal surfaces, the prioritization of sites for sampling and dating, and the guidance of more detailed three-dimensional geomorphic mapping.

The principal unresolved issues concern the age and origin of the measured surfaces, their alongshore continuity, and their potential hydrodynamic significance. Future work should combine three-dimensional topographic mapping, targeted sedimentological observations, numerical dating, and, where relevant, tsunami-inundation modelling. These additional analyses are required to test the geomorphic hypotheses generated from the RTK-GNSS profiles rather than to validate the accuracy of the present topographic measurements.

7. Conclusions

This study presents new RTK-GNSS topographic profiles from Pantai Ajah, Kalijali, Kulon Progo, and Wingko along the southern coast of Java. These measurements directly document cross-shore elevation changes, broad low-gradient surfaces, slope breaks, and ridge-like topographic highs. The RTK profiles identify several probable raised coastal surfaces. Pantai Ajah contains a prominent riser and a broad elevated surface at approximately 7–8.5 m elevation. Kalijali contains a lower surface at approximately 4–5 m, an upper surface at approximately 7–9 m, and a localized topographic high at approximately 12–13 m. Kulon Progo shows a weakly expressed low-gradient surface at approximately 4–5 m. Wingko contains a clear slope break at approximately 1450–1500 m along the profile and a abroad, l low-gradientandward surface at approximately 5–6.5 m elevation. A provisional morphometric comparison identifies recurring surface-elevation ranges at approximately 4–6.5 m and 7–9 m. These ranges do not establish chronological correlation or genetic equivalence among sites. Higher local peaks at approximately 12–14 m may represent residual coastal remnants, older terrace fragments, dune or ridge features, or locally uplifted shoreline-related landforms. A possible hypothesis is that some of these features were originally associated with the mid-Holocene relative sea-level highstand, when sea level in parts of Indonesia and Sundaland was higher than present, and were subsequently uplifted to different elevations. However, this interpretation remains preliminary and requires direct dating and sedimentological confirmation. The raised terrace surfaces and topographic highs may also have practical importance as partial natural barriers to tsunami inundation. Where continuous and sufficiently high, these features may reduce inland tsunami penetration or influence flow pathways. Nevertheless, they should not be treated as complete protection, because large tsunamis may overtop, breach, or flow around natural barriers. Overall, the RTK profiles provide new field evidence for spatially variable terrace morphology along the southern Java coast. They demonstrate marked spatial variation in present coastal morphology, but they do not independently establish landform age, marine origin, uplift rate, or neotectonic segmentation. The age and origin of the identified coastal surfaces are not resolved in this study. The main contribution of this study is therefore the direct, field-based documentation of coastal profile geometry and the identification of sites suitable for targeted geological and chronological investigation. Future sedimentological observations, numerical dating, three-dimensional mapping, and geodetic analysis are required to test the sea-level and tectonic interpretations discussed in this study.