Restrict Xlinks for GML encoding

[clausnagel]

From a developer's point of view, I think XLinks are really one of the things that cause a lot of pain when processing (City)GML datasets. And what comes on top: polygons describing, for instance, the outer hull of a building can be spread over various feature types: the <Building> itself, <_BoundarySurface>s with <Opening>s of the building, <_BoundarySurface>s of building installations, and nested levels of building parts having their own boundary surfaces and building installations. And there might be a lot of Xlinks involved. CityGML 3.0 will add more feature types such as storeys or building units that may carry polygons that contribute to the outer building hull.

As a result, simply collecting all polygons of the building hull just to visualize the building can become a tricky task.

So, for CityGML 3.0, I would love to see more restrictions here. For example, all polygons describing the outer building hull should only be allowed to be modelled for the <Building> feature type (e.g. as gml:Solid). Every other feature type semantically classifying one or more of these polygons e.g. as <WallSurface> or as <Window> should not hold the geometry but should be forced to use a reference to the polygons of the building. This way, it would be easier for developers because it is clear where to look for geometry.