Definition: Reference to an external information system containing any piece of information related to the spatial object.
Description: EXAMPLE 1: Reference to another spatial data set containing another view on buildings; the externalReference may be used for instance to ensure consistency between 2D and 3D representations of the same buildingsEXAMPLE 2: Reference to cadastral or dwelling register. The reference to this register may enable to find legal information related to the building, such as the owner(s) or valuation criteria (e.g. type of heating, toilet, kitchen)EXAMPLE 3: Reference to the system recording the building permits. The reference to the building permits may be used to find detailed information about the building physical and temporal aspects.