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Achieving Complete and Near-Lossless Conversion from IFC to CityGML ^{ †}

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*Computing for a better tomorrow: Proceedings of the 36th eCAADe conference*; Lodz, Poland, September 2018.

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## Abstract

**:**

## 1. Introduction

## 2. State of the Art: IFC → CityGML Conversion

## 3. A Triple Graph Grammar Approach

#### 3.1. TGG Introduction, Formal Concepts and Related Work

#### 3.2. Application of the TGG Approach in Context of the IFC2CityGML Conversion

- The directed edges of the IFC graph have circle end marks, inspired by Express-G [40]. Note that Express-G is used to document the IFC schema and there is no dedicated graphical representation for instance graphs, thus we are using the schema notation.
- The directed edges of the CityGML graph have arrow end marks, same as associations in UML class and object diagrams [41].
- The undirected edges of the correlation graph are represented as dashed lines without arrow end marks.

- to transform a model of one type into the another,
- to compute the correspondence between two existing models, or
- to maintain the consistency between models of the respective types.

#### 3.3. Direct Specification of Parsing and Transformation Rules for IFC → CityGML Conversion

- There are only two correlation graph edges, one in the set ${E}_{CON}$ and one in the set ${E}_{CON}^{+}$.
- The edge $e\in {E}_{CON}$ connects nodes ${n}_{1}\in {E}_{IFC}$ and ${n}_{2}\in {E}_{GML}$.
- The edge $e\in {E}_{CON}^{+}$ connects nodes ${n}_{3}\in {E}_{IFC}$ and ${n}_{4}\in {E}_{GML}^{+}$.
- a path exists (taking edges as undirected) from ${n}_{1}$ to ${n}_{3}$ via edges in ${E}_{IFC}$ and nodes in ${N}_{IFC}$.
- a path exists (taking edges as undirected) from ${n}_{2}$ to ${n}_{4}$ via edges in ${E}_{GML}^{+}$ and nodes in ${N}_{GML}^{+}$.

#### 3.4. Implementation of Rule-Based Conversion, Rule Set Specification and Application to Sample Data

#### 3.5. Advantages and Limitations of the Approach

## 4. Development of an ADE

#### 4.1. Overview and Related Work

#### 4.2. Arguments for an ADE and Shortcomings

- It is the preferred method of extending the CityGML data model (the other one being the Generics mechanism, which allows generic attributes and objects, but is not without shortcomings [46]);
- Thanks to the flexible extension of the data model, ADE allows for a (in practice) nearly lossless and strict conversion from IFC to CityGML;
- ADE supports overcoming the lower structural level than IFC, enabling introduction of new concepts not available in CityGML; and
- ADE enables storing only the data we are interested in for particular use cases and stakeholders (i.e., it allows being “selectively strict and lossless”).

#### 4.3. Towards an ADE for IFC within This Project

- Doing so from scratch developing a ”brand new” ADE.
- Modifying and/or extending an existing ADE (see for example the work of Kumar et al. [52] demonstrating that it is possible to customise an existing ADE to match particularities of a national context). Since ADEs are released as UML/XSD they can be further customised/extended and improved by others.

- Obsolescence: related work (such as the GeoBIM extension as one of the most prominent works in this field was published in 2011 [2]) might not be up-to-date anymore.
- Adapting to the Singapore context: development of a new ADE is not only beneficial to suit various geographical and architectural properties, but also to match properties of input IFC models that are specific to Singapore Government agencies.
- Fitting use cases: adaptation for use cases of interest to the project stakeholders. For example, the use case on planning vegetating roof surfaces (i.e., green roofs) would require information on the current status of the roofs.

- Identification of the relevant features and concepts to be translated from IFC to suit the geospatial context (see [2] for inspiration).
- Harmonisation with the local context (e.g., taking into account the local architecture and other aspects particular to a given geographic setting).
- Harmonisation with the national standardisation efforts (e.g., include attributes found in official standards).
- Identification of features relevant for a particular use case and stakeholder.

- Developing a single all-in ADE that is generic and applicable to a variety of use cases (similar to [2]).
- Developing custom (tailored) ADEs for each stakeholder that would focus on only one stakeholder and one use case at a time, e.g., one that suits the energy use case. This is not an entirely new idea, as some researchers develop such “mini-ADEs” or “light-ADEs” [54,55], which are specialised focusing on a narrow subject.

## 5. Discussion and Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**The project in a nutshell: from native BIM (Building Information Model) to the integration of CityGML models in Virtual Singapore.

**Figure 2.**A flowchart of the project: from use cases to native BIM requirements and from a native BIM model to a CityGML model and its uses.

**Figure 3.**Triple graph consisting of an IFC (Industry Foundation Classes) graph (left, edges with circle end marks), a CityGML graph (right, edges with arrow and marks), and a correlation graph (dashed edges).

**Figure 4.**A grammar to create the IFC–CityGML triple graph shown in Figure 3. For each rule, the left-hand-side (indicated in grey) specifies the correlated IFC and CityGML nodes that must exist before rule application; the right-hand-side of the rule (indicated in black with a plus sign) adds correlated IFC and CityGML nodes and their connections into the existing graph triple.

**Figure 5.**IFC-local rule B’ (

**left**) and IFC → CityGML transformation rule B” (

**right**) derived by splitting triple graph grammar rule B from Figure 4.

**Figure 6.**Example of a triple graph rule transforming an interior wall surface from IFC into CityGML. The left-hand-side of the rule (indicated in grey) specifies four IFC nodes, their mutual edges, and a correlated CityGML node; the right-hand-side of the rule (indicated in black with a plus sign) adds a CityGML node and a correlation edge between an existing IFC node and the new CityGML node.

**Figure 7.**Example conversion results: Revit advanced tutorial office building (

**left**) and handcrafted two-storey residential building (

**right**). Conversion includes spaces, walls, slabs and roofs, but roofs are omitted for illustrative reasons.

**Figure 8.**Excerpt from a simple ADE extending the CityGML data model and supporting the conservation of potentially useful information from architectural models and other sources. Such an enrichment of the data model may benefit the usability of the data in certain applications, e.g., pertaining to the local geographical context.

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**MDPI and ACS Style**

Stouffs, R.; Tauscher, H.; Biljecki, F.
Achieving Complete and Near-Lossless Conversion from IFC to CityGML . *ISPRS Int. J. Geo-Inf.* **2018**, *7*, 355.
https://doi.org/10.3390/ijgi7090355

**AMA Style**

Stouffs R, Tauscher H, Biljecki F.
Achieving Complete and Near-Lossless Conversion from IFC to CityGML . *ISPRS International Journal of Geo-Information*. 2018; 7(9):355.
https://doi.org/10.3390/ijgi7090355

**Chicago/Turabian Style**

Stouffs, Rudi, Helga Tauscher, and Filip Biljecki.
2018. "Achieving Complete and Near-Lossless Conversion from IFC to CityGML " *ISPRS International Journal of Geo-Information* 7, no. 9: 355.
https://doi.org/10.3390/ijgi7090355