The effects of population growth, urbanization and climate change are motivating us to contemplate serious changes in how we build and maintain buildings and infrastructure. This change will require new tools. There are important industry drivers that suggest that AEC (architecture, engineering and construction) and geospatial interoperability is one of them. A specific challenge currently facing the AEC and geospatial industries is integrating building information models (BIM) and geospatial infrastructure models for full life-cycle project workflows. Autodesk and ESRI have announced a partnership to further the integration of AEC and geospatial models and data which is already showing tangible results, but in a proprietary environment. A recent paper by the Integrated Digital Built Environment (IDBE) joint working group working group, a collaborative effort by buildingSMART International (bSI) and the Open Geospatial Consortium (OGC) to further open standards for AEC/geospatial interoperability, has identified the conceptual and other incompatibilities between the geospatial and AEC modeling of built infrastructure and suggested a way forward toward addressing incompatibilities.
Clashing tectonic plates
The BIM+geospatial problem is the latest symptom of a broader problem. I began working in the utility and telecom sector in 1993, specifically in developing software for records management (called network documentation outside of North America), and at that time the magnitude of the AEC+geospatial cultural mismatch problem was just beginning to be appreciated by utilities and telecoms. The Between The Poles blog is nearly 15 years old and one of the persistent themes from the very beginning in 2006 was the challenge of integrating CAD and GIS data in a common workflow (some examples; 2006, 2007, 2008 ). In utilities and telecoms planners tend to use GIS tools, engineers and designers CAD tools, construction folks in the field paper CAD drawings, and asset managers GIS integrated with their FM tools. Fashioning AEC and geospatial data into an efficient data flow from planning through design and construction to operations and maintenance continues to be a challenge.
There are parallels between what happened in the 1990s in unsuccessfully addressing CAD+GIS interoperability and the current challenge of BIM+geospatial interoperability. But there are also important differences which provide grounds for optimism that the availability of open standards for both BIM and geospatial, a vibrant open source geospatial community, and a new willingness on the part of major software players in the BIM and geospatial industries will make it possible to successfully address the latest interoperability challenge facing the AEC (architecture, engineering and construction) and geospatial industries.
Building information modeling (BIM) has been applied to design-build construction projects for many years. A growing number of countries are mandating BIM for public projects. While the UK government has said that "...we know that the largest prize for BIM lies in the operational stages of the project life-cycle", until recently there has not been hard data to support this conjecture. Similarly there has been only anecdotal support for an integrated BIM and geospatial approach for design, build, operate and maintain projects. Now we are beginning to see data from real world projects that offer evidence for the benefits of an integrated BIM+geospatial full lifecycle approach for construction projects.
Research in the Netherlands at the Delft University of Technology delved into interoperable digital building and infrastructure model standards developed to serve the AEC (architecture, engineering and construction) and geospatial sectors. Specifically, the research was focused on translating the geometry and topology of 3D digital BIM models represented in buildingSMART's Industry Foundation Classes (IFC) standard into 3D forms compatible with the Open Geospatial Consortium's CityGML standard. The researchers found is that there is more than one way to transform a BIM model to CityGML. While it was possible to transform most BIM models, it is not possible to convert all BIM models to CityGML. The findings of this research has reemphasized the challenge of integrating AEC and geospatial data, the first major industrial symptom of which appeared over 25 years ago in the form of 2D integration of CAD and GIS.
Comparison of IFC, CityGML, and LandInfra
The IDBE working group is a joint collaboration between buildingSMART and the OGC. Among the first products of IDBE is a paper Built environment data standards and their integration: an analysis of IFC, CityGML and LandInfra that described the incompatibilities and similarities among three open standards used in the AEC and geospatial worlds.
The three standards are IFC (Industry Foundation Classes), developed by bSI, is used predominantly for exchange of rich, fine-scale building and infrastructure data in the AEC industry, CityGML, an OGC standard, for capturing large scale urban data including buildings and transportation infrastructure originally for simulations, and LandInfra, jointly developed by bSI and OGC, for modelling land and civil engineering infrastructure facilities and designed as a successor to LandXML.
An important difference among the standards is how real-world objects are conceptualized. IFC represents a building as an aggregation of building elements such as walls, doors, and windows, CityGML represents internal building volumes as rooms, and LandInfra represents buildings as features of a facility with only an outer shape and footprint.
Another important difference is coordinate systems. IFC uses a rectilinear coordinate system to represent structures in engineering precision over relatively small, flat surfaces. CityGML and LandInfra model structures that need to take into account the Earth's curvature and therefore use an appropriate geographic projection system.
The standards use different geometric representations of objects. CAD and BIM design tools use splines, nurbs, spirals and many other parametrized forms. Geospatial applications generally use points and multi-point lines and polygons.
Topological relationships are also handled differently. In CityGML, topologies such as containment or spatial adjacency are represented implicitly in the boundaries between different geometric objects. IFC defines objects and their geometries and separately and explicitly identifies topological relationships between them.
Level of Detail and Level of Development (LOD) are handled differently. CityGML uses LOD to progressively define more detailed representations of a structure. For example, for a house; LOD 0 is a building footprint; LOD1, a 3D volume; LOD 2 includes a 3D roof geometry, LOD 3, windows and doors, and LOD 4 detailed structures and furniture. Levels of Development in BIM refer to levels of progress through a construction project and does not correspond to LODs in CityGML. There is no concept of LOD in LandInfra.
Conclusion
The consequence of these differences among the standards is that software interoperability and data integration are difficult. Three approaches to resolving this interoperability challenge have been proposed.
- Schema mapping: This approach attempts to map each object in one standard to be mapped onto its nearest match in another standard. The TU Delft research described earlier investigated this approach and concluded that unambiguous mappings between CityGM and IFC was not possible in all cases.
- Federation: Another approach is to support multiple schemas in one representation. For example, a Revit BIM model can be displayed in a geospatial environment.This limits the generality of some operations such as spatial search, spatial analytics, and CAD and BIM operations.
- Link referencing: A third approach is a linked federation using embedding links in the primary model to components from instances of secondary schemas. The feasibility of such an approach was confirmed in OGC Testbed 4 (2007), in which web services were used for 'merging' CAD, GIS and BIM data.
Next steps
The IDBE has made several recommendations for moving forward to resolve the latest AEC/geospatial interoperability challenge in an open, standards-based way.
- Identify and prioritize uses cases.
- Compile a shared vocabulary and a shared resource for identifying synonyms across different domains. An example of the latter is the CB-NL developed in the Netherland for mapping synonyms across the construction life-cycle.
- Develop a best practice document for coordinate systems for 3D georeferencing.
- Devise a system of common unique identifiers (Oids) for real-world, physical objects that remain
constant and unique. - Collaborative approach for harmonizing conceptual models.
I would add that owners of buildings and infrastructure, recognizing that an important benefit of open interoperability standards is increased competition among software vendors, should consider ways that open standards are incorporated into their procurement requirements. Government procurement was a major driver that motivated virtually all BIM tool vendors to incorporate IFC support into their products.
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