Essay: Integration of BIM and 3D GIS: adding on 3D mesh modes in GIS and Topographic feature in BIM

Abstract :

BIM-3DGIS integration is an important issue for the Geo-science industry recently. There are three types of integration approaches currently by the latest technologies, such as (1) GIS platform based (add-ons with BIM features), (2) BIM platform based (add-on with GIS features), and (3) creating a new BIM-GIS application. For the first approach, it has been solved by importing the “.ifc” format in the GIS. In this essay, some others alternative methods, and approaches will be introduced to aid the integrations.

Approach 1: 3D GIS platform based (add-ons with BIM) by 3D mesh models

3D mesh models are an alternative method for the 3D GIS-BIM integrations, to strike a balance between the detainees and overall coverage of the building information in the 3D GIS platform. Since the mesh model has a high detailedness and its shapes can cover a large area (in the GIS). Compared with the “.ifc” traditional integrations approaches, although the mesh models are not designated to be the building infrastructures’ models, still, the 3D mesh models coding and their language format are also XML and object-based, such as the “.3ds” Max file, it used in the interior design a lot for the detailed modeling, it is in binary file format and written with detailed information about the meshed object, like the vertices list, face materials, and mapping coordinate list are included in the “.3ds” file. The “.3ds” file is popular in the 3D programming industry as well, the meshes in the models are made of triangular, which is similar to the land surveying triangulation in the GIS, but in the meshed models, the triangulation is in detailed building and infrastructure. Still, there are some limitations of the use of the “.3ds” file, as the number of the vertices in the “.3ds” files are limited to 65536 per mesh, which means the highly detailed models can only cover a limited area; moreover, the accurate vertex normal cannot be stored in the file itself, instead, the smoothing groups are used that helps the receiving program can reconstruct a representation of vertex normal, which implies the files requires high efficient computers to run with the mesh file “.3ds”.

The texture, materials, and colors of the .3ds are better than the .obj, instead, the .obj can work with a larger area with detailedness as the .obj have three files spread, Figure 2 shows a 3D mesh model in which each of the mesh models contains three files, the PNG files visuals color into the building surface, the MTL file is used to draw the materials and the .obj is the 3D object.

Figure 2 : a 3D mesh models file in USB drive

Reason for the Approach 1 : integrate by the mesh models

Integrating the BIM with 3D GIS, importing and exporting of models is the key point that influences the quality of LODs. Meanwhile, the detailed and high-quality BIM models transfer to the GIS-based platform may be a challenge, rather maintaining the high quality or increasing the speed and reducing the size of data should be well controlled in the integrations. Therefore, the meshed BIM models can be a good option for integrations. As the 3D mesh models can provide various detainees, from LOD1-LOD4 in GIS, such as the “.3ds” file with higher quality and details and the “.obj” file with lower quality but larger coverage area.

Moreover, mesh models can be the interchanges of from BIM to various GIS media, since importing 3D mesh models in ArcGIS is faster and easier than the .ifc, compared with the mesh models file type and “.ifc” files, mesh models are usually grouped in one single 3D object in the GIS, beside the “.ifc” files may contain lots of layers and 3D object in the GIS platform (content), therefore the mesh models are more manageable than the “.ifc” contains lots of layers files. Furthermore, the mesh models have higher visualization of the building layout than the “.ifc” files, such as the “.obj” geometry file with “.mtl” material format and “.jpg” photo format. The “.mtl” can aid the materials determinations in the GIS platform and the “.jpg” is useful to enhance the overall cityscape, as the “.jpg” file is usually from the aerial (UVA) photos that show the materials and building outline in-depth in the GIS, the Appendix 1 shows the procedures of the aerial photo to mesh models conversion.

Problems may be found in the Approach 1 : integrate by the mesh models

There are some limitations for the mesh models by both “.3ds” and “.obj”, the relations between LODs (detailedness) and file, importing and transferring 3D object issues, and the technologies limitations.

For the relation between LODs and files, the higher the LODs, the higher the size of the file. Moreover, for the importing and transferring 3D object issue, the “.3ds” file types is a complex issue for the ArcGIS importing 3D files, as itself is based on the binary structure, the file structure and uncommon for the non-Autodesk applications, therefore “.obj” is better for the importing and transferring throughout the BIM-GIS platform. But still, the wavefront, light-wave, and geometry-based technologies (attached in the Appendix 3) lead some of the 3D object inner side of the building infrastructures cannot be covered (shown in Figure 3) *it only happens in the LandsD .obj surveying models, it is fine when the models is export from the BIM software (shown in Appendix 6).

Figure 3 : “.obj” models samples shows that the limitation of the wave-front technologies

Approach 2: BIM platform based (add-ons with GIS) by adding Topographic layers “Open BIM Site”

Open BIM Site is an application that based on BIM platform that currently integrated 3D GIS into BIM. For the add on GIS features, the topographic features and coordinate system were the major concern for the integration process, since the BIM models usually not concerning the topographic and geological profiles of the site, beside GIS system usually primarily focusing on the topographic and geological profiles; also, the BIM primarily used the local coordinate systems, and GIS used the regional and UTM (worldwide) coordinate system. These differences can be solved by adding on the GIS topological models, such as the DTM (digital terrain modelling) in the BIM apps, such as the “Open BIM site” created the GIS topological surface layer for the building models to stick with the reality terrain (shown in Figure 4). The process of the Open BIM site was illustrated the interchanges and modeling technologies in the Flowchart 2.

Reason for the Approach 2 : BIM based with topographic layers

BIM based platform instead of GIS is because of the detailedness and LODs concerning, since BIM originally has a higher level of details than the GIS based platform, such as the BIM has more detailed layer and textures in the building and infrastructures. The topographic layer can enhance the visualization and BIM modeling designs in the construction field, such that the BIM models can on top of the current topography surface layers (demonstration in the Appendix 4). Moreover, the topographic layers can be transfer by the .asc (digital terrain models) files, the advantages of the .asc and DTMs files would be small file size, since the files only contain the numerical terrain dates, the interchange and reconstruct of the terrain can be done in both BIM and GIS platforms.

Problems may be found in the Approach 2 : BIM based with topographic layers

The coordinate system transformation would be the problems that need to be solved in the conventions, since the GIS is regional coordinate system (like Hong Kong 1980 grid) and the BIM is localized coordinate system (based on the size and shape of the construction site. Therefore, to do the interchange and conversion the BIM site grid reference should be georeferenced into the regional coordinate system, marked down the BIM site grid reference corelated with the regional system would be a method to solve this issue. Also, the Open BIM site used to aerial photo provide by the Google Earth (shown in Figure 5) to solve the Georeferencing issue.