# Processing BIM and GIS Models in Practice: Experiences and Recommendations from a GeoBIM Project in The Netherlands

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## Abstract

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## 1. Introduction

- with contextual GIS information, BIM methodologies can be better applied to infrastructural works;
- more detailed 3D city models can be built by reusing BIM data;
- smart city concepts can perform integrated reasoning on terrain, buildings and city infrastructure;
- and spatial analyses can support multiple levels of detail and the complete life cycles of objects.

## 2. Background

#### 2.1. Open Standards in GIS and BIM

#### 2.1.1. CityGML

**LOD0**- is non-volumetric and is an horizontal footprint and/or roof surface representation for buildings;
**LOD1**- is a block-shaped model of a building (with an horizontal roof);
**LOD2**- adds a generalised roof and installations such as balconies;
**LOD3**- adds, among others, windows, doors, and a full architectural exterior;
**LOD4**- models the interior of the building, potentially with pieces of furniture (CityGML does not mandate which indoor features need to be modelled, in practice resulting in models with a different granularity [10,11]) (LOD4 will be removed in CityGML 3.0. Instead, indoor and outdoor features will each be modelled at LOD0–3.).

#### 2.1.2. IFC

`.ifc`).

**Primitive instancing:**an object is represented based on a set number of predefined parameters. IFC uses this paradigm to define various forms of 2D profiles (Figure 2), as well as volumetric objects such as spheres, cones and pyramids.**CSG and Boolean operations:**an object is represented as a tree of Boolean set operations (union, intersection and difference) of volumetric objects (see Requicha [15] for more details). Half-spaces are often used to cut out the undesired parts of surfaces or volumes.**Sweep volumes:**a solid can also be defined by a 2D profile (a circle, a rectangle or an arbitrary polygon with or without holes) and a curve [16] along which the surface is extruded.**B-rep:**an object is represented by its bounding surfaces, either triangulated meshes, polygonal meshes or topological arrangements of free-form surfaces.

#### 2.2. Previous GIS-BIM Integration Efforts

## 3. Automatically Processing Complex Architectural Models in IFC

#### 3.1. Initial Methodology

`Polyhedron_3`.

`Nef_polyhedron_3`and then performing Boolean set union, intersection or difference operations.

`Polyhedron_3`to a

`Nef_polyhedron_3`, and before it is triangulated for the generation of a file used for visualisation (the simple OBJ format). Among others, tests were made for:

- combinatorial validity (2-manifoldness),
- surfaces that enclose a space,
- crashes/failures of CGAL’s triangulation algorithm (e.g., when a surface self-intersects),
- self-intersections,
- CGAL crashes/failures when converting to a
`Nef_polyhedron_3`.

`IfcFace`and

`IfcFaceSurface`. The latter has an associated elementary surface that describes the face geometry, which can be non-planar, while the former is always planar and its surface is given implicitly as the plane through the points of the associated polygonal loops. The polygonal boundary loops are given in three-dimensional coordinates and not in parametric coordinates of the corresponding surface. Hence, there can be issues with vertices not in agreement with the underlying surface. Building geometries are often polyhedral and support for curved geometries is limited in importing and exporting applications. For that reason only the planar class is encountered in the set of models assessed in this paper.

`IfcGeometricRepresentationContext`. However, CGAL Nef polyhedra pose stricter requirements on the planarity of faces. Therefore faces of every object are triangulated whenever a Nef polyhedron is created. This ensured that the conversion from a CGAL

`Polyhedron_3`to a

`Nef_polyhedron_3`is able to compute a plane passing through every face. Another possibility would have been to compute the best fitting plane per face and then snap a vertex to the intersection of the planes of its incident faces [33].

`Polyhedron_3`from such a mesh stored in a Nef polyhedron. While workarounds around these CGAL problems were found (and partly implemented), these involve very complex code, are too slow for practical use or do not cover all the many cases commonly present in IFC files.

#### 3.2. Final Methodology

`IfcBuildingElement`were analysed, which according to the IFC standard define a ‘major functional part of a building’, those being:

`IfcBeam`,

`IfcBuildingElementComponent`,

`IfcBuildingElementProxy`,

`IfcChimney`,

`IfcColumn`,

`IfcCovering`,

`IfcCurtainWall`,

`IfcDoor`,

`IfcFooting`,

`IfcMember`,

`IfcPile`,

`IfcPlate`,

`IfcRailing`,

`IfcRamp`,

`IfcRampFlight`,

`IfcRoof`,

`IfcShadingDevice`,

`IfcSlab`,

`IfcStair`,

`IfcStairFlight`,

`IfcWall`,

`IfcWindow`. Thus, all geometries in every IFC file were extracted to an easily-parsable format while preserving the most relevant semantic information present in the original input file. This allowed us to overcome a shortcoming of the methodology of Donkers et al. [27].

#### 3.3. A Proposed Set of IFC Modelling Guidelines

**Georeferencing**. IFC files should contain their precise real-world location using the latitude, longitude and altitude values in the`IfcSite`taking into account the offset given by the WorldCoordinateSystem of the`IfcGeometricRepresentationContext`for the 3D model and the 2D plan (if used). Due to practical difficulties, it cannot be expected that these values match the reality with the Precision given in the`IfcGeometricRepresentationContext`, but the values should be easy to set to within a few meters of the real location. In addition, if the y-axis of the WorldCoordinateSystem in the`IfcGeometricRepresentationContext`does not match the true North direction, the TrueNorth attribute should be set as well. See Section 5 for more details on how these values are stored.**Valid volumetric objects**. Use volumetric objects as much as possible (e.g., sweeps and geometric primitives), test that they conform to their entity definition and take care to make sure these are watertight when using boundary representations.**No intersections**. There should be no self-intersections or intersections between objects, much like the BIM Basic IDM disallows intersections between objects by stating: ‘There are no duplicates or intersections permitted. Make sure this is checked in IFC.’ Among others, the Solibri Model Checker (https://www.solibri.com/faq/using-the-space-validation-rule-to-ensure-model-accuracy/) and the simplebim Space Boundary add-on (http://datacubist.com/support/addon-spaceboundary.html) are able to detect overlapping objects.**Forming enclosed spaces that are also modelled as IfcSpaces**. The necessary objects so as to form enclosed spaces should be modelled, and these objects should fit properly with each other and without gaps between them. In addition, enclosed spaces should also be modelled explicitly and with their precise geometry as`IfcSpaces`. Simpler applications that make straightforward use of these`IfcSpaces`can then avoid more complex geometry processing.**Using specific entities**. Always use the most specific entity type possible, avoiding generic types such as`IfcBuildingElementProxy`, use these specific entity types consistently across all objects of a model, and ensure that all related geometries of an object are marked as such. Additionally, when conversions to CityGML are concerned, it is worth focusing on entities that have a direct mapping to CityGML classes, such as`IfcSlab`.

## 4. Integrating Subsurface Information with IFC

#### 4.1. Aim and Scope of This Subproject

#### 4.2. The Solution

`IfcSpace`class and which can easily represent the voxel structure of GeoTOP.

`IfcSpace`. These are the GeoTOP strat and lithok classes (Figure 11).

#### 4.3. Recommendations

- Further investigation of practices of the BIM practitioners is needed regarding subsurface issues. To obtain initial understanding of how they use (or would like to use) subsurface data, a small questionnaire was set up, which was distributed as part of the BIM-loket newsletter. The responses show that BIM practitioners are interested in subsurface data, but currently have difficulties using it. Also, it must be said that it was very hard to get the BIM community involved. In this project, an attempt was made to organise a session for BIM vendors, but there was little to no interest.
- It is necessary to develop more tools that are able to handle other types of subsurface data with more/different coverage.
- Discussions should be started between BIM and GIS practitioners for better handling of georeferencing/alignment practices and the establishment of guidelines to support this.
- It is desirable to investigate possible approaches to allow the automatic integration/alignments of the models in either BIM or GIS systems.
- Finally, the issues of integrating rough and fuzzy subsurface data with detailed objects represented in BIM that have a much higher accuracy should also be investigated. Most BIM sites fall within a complete voxel or a few voxels only, so it is worth considering what is the value of conclusions drawn from this data integration which may have severe impact on a risk analysis? This requires the close involvement of geological data experts.

## 5. Georeferencing IFC Models

`IfcSite`, which defines an area where construction works are undertaken, and optionally allows for storage of the real-world location of a project using the RefLatitude, RefLongitude and RefElevation attributes. The latitude and longitude are defined as angles with degrees, minutes, seconds, and optionally millionths of seconds with respect to the world geodetic system WGS84 (EPSG:4326). Positive values represent locations north of the equator, west of the geodetic zero meridian (nominally the Greenwich prime meridian) in IFC2x3, or east of the zero meridian IFC4. Note that this difference in the two IFC versions is risks confusing users and implementers in practice. Negative values represent locations south of the equator, east of the zero meridian in IFC2x3, or west of the zero meridian in IFC4. These angles are expressed according to the type

`IfcCompoundPlaneAngleMeasure`and all components (i.e., degrees, minutes, seconds and millionth-seconds of arc) should have the same sign. According to the IFC standard, the geographic reference given might be the exact location of the origin of the local placement of the

`IfcSite`or it might be an approximate position for informational purposes only. The elevation is defined according to the datum elevation relative to sea level.

`IfcSite`contains a few other attributes that allow for an approximation of the real-world location to be given. The LandTitleNumber can store the designation of the site within a regional system (e.g., a cadastral record ID), and the SiteAddress can store the postal address of the site.

`IfcGeometricRepresentationContext`is used to define the coordinate space of an IFC model in 3D and optionally the 2D plan of such a model. This entity can be used to offset the project coordinate system from the global point of origin using the WorldCoordinateSystem attribute, it defines the Precision under which two given points are still assumed to be identical, and it defines the direction of the TrueNorth relative to the underlying coordinate system. The latter attribute defaults to the positive direction of the y-axis of the WorldCoordinateSystem.

`IfcSite`with an optional offset and true North direction given by the

`IfcGeometricRepresentationContext`, should make it possible to precisely georeference an IFC model. In fact, it seems that most IFC files do fill in the requisite values in the

`IfcSite`. However, these values are almost always set to zero, to a default or wrong location, or to a very rough approximation of the real location (e.g., a point in the same city). This is unfortunately compounded by the mismatched definitions of the positive direction for the longitude in IFC2x3 and IFC4.

#### 5.1. Adding Georeferencing Information in Revit 2018

`IfcSite`as well. Meanwhile, the survey point (Figure 15c) stores the geodetic data related to the project, which is assumed to be imported from other sources provided by the surveyors of the project (e.g., some shared CAD files of the construction site). When such source files are missing, all the values are set to 0 by default, including the angle to true North, thus assuming that it coincides with the y-axis of the project.

#### 5.2. Example: Correction of the Georeferencing of the Witte de Withstraat Model Using Revit

`IfcSite`attributes (Figure 16b), but the elevation is kept at $0.0$ (which is not really a problem for this model in The Netherlands).

#### 5.3. IfcLocator: An Open-Source Web Service for Georeferencing IFC Models

**IfcLocator**(https://github.com/tudelft3d/IfcLocator) was developed, a supporting tool that allows the user to view and check the location of the project stored in an IFC file and to correct that information when necessary. The tool is based on Cesium (https://cesium.com/), which is an open source Javascript library for 3D mapping, including an intuitive 3D viewer of the globe. The choice of a web-based Javascript open source code was motivated by the high flexibility offered by such approach, which is cross-platform. Furthermore, the users can perform all the operation locally, which means they do not have to send their model to a remote server.

## 6. Conclusions

- many more geometry types are usually natively supported in BIM software than in GIS software;
- geometric errors (e.g., self-intersections) are often found in places that are inconspicuous or invisible from the outside;
- data is often only used for visualisation purposes, which does not require geometric and topological correctness;
- or the errors only show after the implicit and parametrised types have been converted into explicit geometries (which is during the conversion to IFC).

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**A building represented in LOD0 to LOD4 (image from Biljecki et al. [12]).

**Figure 2.**IFC defines various types of parametric curved profiles such as (

**a**) those based on the characters U, L, Z, C and T and (

**b**) those based on trapezia, (rounded) rectangles, circles with/without holes and ellipses. Note the various types of tapered and curved parts of the profiles. These are most commonly used in extrusions such as those shown here.

**Figure 3.**Test IFC files from the City of The Hague. (

**a**) CUVO Ockenburghstraat KOW; (

**b**) Rabarberstraat 144; (

**c**) Witte de Withstraat.

**Figure 5.**Test IFC files from the City of The Hague after processing with initial methodology. (

**a**) CUVO Ockenburghstraat KOW; (

**b**) Rabarberstraat 144; (

**c**) Witte de Withstraat.

**Figure 6.**Some of the errors present in the test IFC files. (

**a**) A prismatic polyhedron with an obvious self-intersection. The self-intersecting top and bottom faces of the polyhedron are not shown. (

**b**) A self-intersecting representation of a beam. (

**c**) A topological manifold that contains non-obvious geometric intersections. The bottom of the polyhedron, seemingly composed of three rectangular faces, actually has only two rectangular faces that overlap along the middle third.

**Figure 7.**Distributions of maximal vertex distance from underlying surface. To assess the distance from implicitly defined planes, a surface normal is found using Newell’s algorithm (see e.g., [34], p. 15). Arbitrary precision floating point arithmetic is used during computation. (

**a**) shows that all distances are within the geometric precision defined in the file. As shown in (

**b**) this is not always the case.

**Figure 9.**GeoTOP data is encoded based on a voxel model with a resolution of 100 × 100 × 0.5 m. Each voxel in the model contains information on the lithostratigraphy and lithological classes, including the probability of occurrence for each lithological class (the values are based on interpolations of data from boreholes).

**Figure 11.**Produced voxels from the GeoTOP underneath the Faculty of Architecture of the TU Delft (visualised on BIM Vision software).

**Figure 12.**Attempting to put a BIM model of a bridge (provided by VolkerInfra for testing) and the generated GeoTOP voxel model.

**Figure 15.**Setting the Project Base Point and Survey Point of the Witte_de_Withstraat model. (

**a**) Making the base and survey points visible in Revit; (

**b**) Project Base Point; (

**c**) Survey Point.

**Figure 16.**Correcting the location of the Witte de Withstraat model. (

**a**) Setting in correct location in the Internet Mapping Service tool; (

**b**) The new coordinates in the IFC file.

**Figure 17.**A screenshot from Google Maps is used to identify the orientation of the building with respect to the true North. The green circles indicate the project base and survey points.

**Figure 18.**Detection and correction of an IFC file’s TrueNorth. (

**a**) Finding of the angle between the true north (green line) and the project north (orange). (

**b**) Correction of the Project Base Point’s True North attribute.

**Figure 19.**(

**a**) Superposed floor plan and image oriented with respect to the true North; (

**b**) Superposed floor plan and image oriented with respect to the project North; (

**c**) New

**IfcDirection**of the true north; (

**d**) New Survey Point of the project.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Arroyo Ohori, K.; Diakité, A.; Krijnen, T.; Ledoux, H.; Stoter, J. Processing BIM and GIS Models in Practice: Experiences and Recommendations from a GeoBIM Project in The Netherlands. *ISPRS Int. J. Geo-Inf.* **2018**, *7*, 311.
https://doi.org/10.3390/ijgi7080311

**AMA Style**

Arroyo Ohori K, Diakité A, Krijnen T, Ledoux H, Stoter J. Processing BIM and GIS Models in Practice: Experiences and Recommendations from a GeoBIM Project in The Netherlands. *ISPRS International Journal of Geo-Information*. 2018; 7(8):311.
https://doi.org/10.3390/ijgi7080311

**Chicago/Turabian Style**

Arroyo Ohori, Ken, Abdoulaye Diakité, Thomas Krijnen, Hugo Ledoux, and Jantien Stoter. 2018. "Processing BIM and GIS Models in Practice: Experiences and Recommendations from a GeoBIM Project in The Netherlands" *ISPRS International Journal of Geo-Information* 7, no. 8: 311.
https://doi.org/10.3390/ijgi7080311