Method

The methods used in the development of the IFC for GIS project are presented below.

The aim of the IFC for GIS project was to use the Industry Foundation Classes (IFC) model as the specification for the exchange of limited but meaningful information between GIS and AEC/FM systems and vice versa. The intent was to use entities that are already established within the Coordination and Code Checking views of the IFC schema so as to be able to reuse as far as possible the tools, techniques and capabilities already developed by vendors at the AEC/FM side of the exchange

The IFG project set out to define a bridge between AEC/FM systems and GIS systems. It did not seek to create a complete geographical information inside IFC. Instead, it recognized the existence of other competent models for this purpose, notable the model underlying the Geographic Markup Language (GML) produced by the Open GIS Consortium (OGC). However, by providing the bridge, IFC facilitates the mapping of information between AEC/FM and GIS.

The approach to creating the bridge has been to create an ‘overlap’ between such models through the provision of entities whose occurrence and location in coordinate space can be recognized in both the AEC/FM and GIS world. Practically, this means understanding AEC/FM location information in the context of a map projection at a reasonable accuracy. Broadly, the area of overlap is seen as a one kilometre square centred upon a particular building of interest. Within this range, the maximum error at the limits of the square due to the projection of the curvature of the ellipsoid onto a Cartesian plane is estimated at four millimetres.

Technical completion of the project followed a well defined approach that considers 'as-is' current capabilities, 'to-be' future expectations and the gap between them.

As-Is

This activity focussed on the IFC2x2 schema, the intent being to define those entities that support, or might support GIS applications. 'As-is' analysis did not consider on whether the capabilities are complete; simply that they exist in some form.

Gap Analysis

The 'gap analysis' focussed on the separation between capabilities that needed to be added to the IFC 2x edition 2 (IFC 2x2) model as entities (which therefore required specific model extensions) and those that could be added as property sets (which enable easier and more dynamic extension).

The approach adopted was that as used for the 'Migration study to OOCAD' carried out for the Building Construction Authority of Singapore as a preliminary action to their development of automated building regulations.

The results of the gap analysis provided a list of:

• capabilities that were fully covered by the IFC 2x2 model
• capabilities that could be supported by the IFC 2x2 model with the addition of property sets
• capabilities that required the provision of additional GIS capability to the existing IFC 2x2 model.

To-Be

This activity focussed on the development of an edition of the IFC which incorporated new capabilities required to support the inclusion of GIS information. The result of this work is the IFC 2x3g schema (the 'g' suffix identifying that this extension is developed to support a geographic capability)

Use Cases

A key tool in the development of the IFC for GIS project has been the specification of requirements in the form of 'Use Cases'. A use case is defined as 'a sequence of interactions between one or more actors and the system for describing the functional requirements of the system from the user’s point of view'.

Use cases were developed in a series of workshops with interested users from government and industry and were documented as narrative text including identifying name, purpose, description, actors involved, input data requirements, output data requirements, control data requirements. The complete set of use cases is available.

Mapping Specifications

To demonstrate the facility of the extended IFC schema resulting from the project, a number of trial conversions have been carried out between IFC and the GML schema (by the Open Geospatial Consortium). Illustrations of these and the actual mappings are included.