City2IFC Mindmap - dd-bim/City2BIM GitHub Wiki

Implementation of a stand-alone tool (Revit independent) for converting CityGML (LOD1 / LOD2) to IFC

Preparations made:

• Separation of import logic and Revit write logic (see independent projects: City2BIM, City2IFC, City2RVT)
• Import logic: see and USE City2BIM logic for import nearly the same way as City2RVT does use each City2BIM.CityGml_Bldg with its geometry and semantic as data for IFC export

• use XBIM-library for writing IFC
• look at previous projects which used XBim (IfcHouse, GeoRefChecker, ... does)
• IfcHouse from E.R. (see git lab repro)

• always use the IFC documentation to know how the entity is structured which you want to use

• IFC doc overview: https://technical.buildingsmart.org/standards/ifc/ifc-schema-specifications/

  • At the index site you can quickly search for the entity you need

  • At each site of the entity I recommend to look at the "Attribute inheritance" (must be unfold) to see ALL attributes of the entity

  • Also use the described concepts in the docu (chapter 4) to understand how IFC works and to know what you have to implement for fulfilling standard (or rather MVD´s)

• Keep in mind: there are two possible ways to transfer geometry (at least in the Revit plugin)
• Decide which way should be transfered (or maybe both ways)
• For solid geometry I recommend the following IFC entities:
  • For each solid (which represents the geometry of an GmlBuilding or GmlBuildingPart) create an IFCBUILDINGELEMENTPROXY element
  • Attributes:
    1. GlobalId: assign via XBim
    2. OwnerHistory: assign information you want to transfer via XBim (e.g. see: https://github.com/dd-bim/IfcGeoRef/blob/master/IfcGeoRefChecker/IfcGeoRefChecker_v3/IfcGeoRefChecker_v3/IO/IfcWriter.cs
    3. Name: assign something specific (maybe BuildingId or BuildingPartId from CitGml)
    4. Decription: assign something specific or leave it empty
    5. ObjectType: dependent of attribute 9: I recommend to use some unambiguous label like "CityGML-Building(part)"
    6. ObjectPlacement: VERY IMPORTANT concept, for this see IfcHouse and inform yourself about coordinate systems in IFC, relative transformations look at other IFC-files, look into IfcHouse, talk to Enrico, look into LoGeoRef concept simple solution would be: ask for project base point in tool gui and use this coordinate for acquiring global georef regarding LoGeoRef concept, for each Proxy element calculate the difference to the project base point and store them into LocalPlacement of Proxy
    7. Representation: also VERY IMPORTANT, decide whcih geometry type you want to implement, I recommend BRep because the coordinates of the planes of the solid are stored in the CityGml_Bldg.Solid object, ATTENTION: if you use the relative transformations (you should do that for good BIM, see attribute 6) the coordinates must be relative to the specified local placement, talk to E. R: if you have problems to understand
    8. Tag: assign something specific or leave it empty
    9. PredefinedType: you have to use the enum specified in the IFC docu, I recommend "USERDEFINED", in this case don't forget to set attribute 5

For surface transfer an idea would be to transfer the surfaces to its representing IFC types, that would be IfcRoof, IfcWall, IfcSlab, (perhaps IfcVirtualElement), they than could be aggregated into IfcBuilding, this approach could be more difficult if you want to fulfill each implementation rule, there is reasearch necessary regarding concepts which have to implmented, e.g. a wall must have a relation to a material, also the relative transformations could get more complicated. However you can also add semantic to them (also the semantic for surfaces, e.g. "Dachneigung") via PropertySets (see next paragraph)

Use the possiblities of IFC to store user-defined semantic attributes: using PropertySets: IFCPROPERTY in IFCPROPERTYSET via IFCRELDEFINESBYPROPERTIES

Bind the properties for each Building object to the IfcBuildingElementProxy element via mentioned relation Decide for a name for the property set There is a lot of literature for acquirimng properties to objects (no big deal) Examples are also at IfcGeoRefChecker (for the map conversion attributes for Ifc2x3 data) and also in the web, e.g.: https://docs.xbim.net/examples/basic-model-operations.html see under Create