How to model Level of Detail (LoD)?

[FransKnibbe]

The Level of Detail (LoD) can be a useful aspect of geometry. CityGML defines five levels, and the technical university of Delft has proposed an improved LoD specification: https://3d.bk.tudelft.nl/lod/.

Has the cityrdf team already thought about how to model LoD? Will there be subclasses of GeoSPARQL Geometry for example? Or will LoD be modelled as skos:Concepts? Or as pecial properties? Will it be possible to use the refined LoD spec of the TU Delft?

I am asking this specifically because I am involved in a project where we want to specify LoD in Linked Data. It would be good to know how any modelling solution aligns with the results of the cityrdf work.

Greetings,
Frans

[nataschake]

Hi Frans!
CityGML v3 supports basic LODs via set of object properties like lodXSolid, lodXMultiCurve etc., where X is in 0,1,2,3,4,

core:lod3Solid a owl:ObjectProperty ;
    rdfs:label "lod3Solid"@en ;
    rdfs:domain core:AbstractSpace ;
    rdfs:range gmlowl:Solid ;
    rdfs:subPropertyOf geo:hasGeometry ;
    skos:definition "Relates to a 3D Solid geometry that represents the space in Level of Detail 3."@en .

Coming from the UML representation of CityGML v3, only the relations between classes (AbstractSpace and Solid, as you see in the example) convey the "meaning" of LOD level.
If you need to define LOD2.1, you might take the direction of extending the core module with definitions of necessary object properties, and define, e.g.

core:lod21Solid a owl:ObjectProperty ;
...your definitions goes here

HTH

[FransKnibbe]

Thank you Nataliya, your reply is very helpful! One further question though: are the LoD levels themselves modelled in RDF somewhere? For example, is it possible to get a description on "Level of Detail 3"?

[rob-metalinkage]

Please note the CityRDF draft implementation proposed requires a working group to be active to further test and adopt it. The CityGML SWG did propose to recharter but that activity hasnt happened. If there is interest and someone wants to put their hand up to chair such a group I am happy to help with making this happen.

Also, by publishing modules as "BuildingBlocks" [1] we can support publication of profiles - and also transformations between CityGML terms and related standards. An open question here is do we do this by class or package? Or package first and class later if finer grained profiling required?

[1] https://ogcincubator.github.io/bblocks-docs/

[nataschake]

One further question though: are the LoD levels themselves modelled in RDF somewhere? For example, is it possible to get a description on "Level of Detail 3"?

Classes conveying meaning of "being LOD level X" do not exist in CityGML, it is not necessary. All the surfaces, solids and curves generated for different LODs, are just geometrical surfaces and curves, defined in GML namespace http://www.opengis.net/ont/gml#.

A dataset can store separate geometries for different LODs under the same city object ID, letting software dynamically pick the right detail level.

[FransKnibbe]

@nataschake , I was thinking about the case when a data consumer encounters a spatial feature with a core:lod3Solid property, for example, in the wild. How does the data consumer know how to interpret "Level of Detail 3"?

[nataschake]

yes, this semantics remains outside.