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fag general ifc spec

General understanding of the IFC Specification

 

  1. The notation of the IFC Schema is EXPRESS, what does EXPRESS stand for and where do I get more information?
  2. How does the different versions of the IFC model relate to each other?
  3. I heard about ifcXML, what is the difference between IFC and ifcXML?
  4. What are the advantages between IFC and ifcXML?

 


 

Q:    The notation of the IFC Schema is EXPRESS, what does EXPRESS stand for and where do I get more information?

A:    EXPRESS is a data definition standard developed to enable a formal definition of industrial data. It allows to formally validating a population of data types (the entities and attributes). It is an international standard, ISO10303-11, and used in several standards for the definition and exchange of product data, such as STEP, CIS/2 or IFC.
ISO documents are copyrighted and published by the member organisations of ISO. An overview of EXPRESS can be found at http://en.wikipedia.org/wiki/ISO_10303-11.


Q:     How does the different versions of the IFC model relate to each other?

A:    There have been five principal releases of the IFC over since 1996; these had been IFC1.5.1, IFC2.0, IFC2x, IFC2x2 and IFC2x3. Since the release of IFC2x (published in 2000), each release has left the core (or ‘platform’) of the IFC specification unchanged, and has added to it. This platform guarantee has meant that most vendors have had no difficulty in upgrading their application to IFC2x2, IFC2x3 and hopefully in future to IFC2x4. Since the publication of IFC2x new releases where mainly driven by adding new concepts to the IFC specification in order to capture more exchange use cases and to improve existing definitions reflecting the lessons learnt from implementation and usage. Since IFC2x a new major release has been published every 3 years. See also the following overview.


Q:     I heard about ifcXML, what is the difference between IFC and ifcXML?

A:    Content wise there is no difference. ifcXML represents the same content of the IFC specification as IFC EXPRESS does. The ifcXML XSD schema is generated from the master EXPRESS representation by a language binding described by ISO10303-28ed2. ifcXML files are XML document files that validate against the ifcXML XSD. An IFC model can be written and read from a data file in these ways:

  • Usually a *.ifc file contains the model represented in ‘STEP physical file format’ STF
  • Alternatively exactly the same information can be represented as XML using the ifcXML schema definition.  The file extension is *.ifcxml.
  • A current proposal within the Implementation Support Group would allow to read/write a zipped *.ifc or *.ifcxml file directly from a file with the extension *.ifczip



Q:     What are the advantages between IFC and ifcXML?

A:    ifcXML files are larger than the equivalent *.ifc (SPF) files, they are usually between 4-8 times larger. However the tools and toolkits to read, transform and write XML are freely available. Many enterprise and desktop systems can handle XML. Therefore ifcXML has been added as a valid representation of the IFC specification. The ifcXML XSD releases are published here. More information on ifcXML development and implementation is available in the ifcXML Implementation Guide.