GML: should use srsDimension as global same to srsName?

[Kanahiro]

Thesedays I've tried to use CityGML formatted data with GDAL/OGR and found some interesting issues.

• srsDimension is not always defined in a level of each features. Sometimes defined once on envelope.
  • You can see in testdata: https://github.com/OSGeo/gdal/blob/master/autotest/ogr/data/gml/citygml_compound_crs.gml .
  • However, ogr respects srsDimension which is defined only in a level of each feature. One in envelope seems to be not used.
• srsName is also defined once in envelope and ogr uses this as global SRS.

By these, when parsing GML which has 3d-vertices but srsDimension is not defined in each feature, ogr sometimes reads 3D-vertex as 2D incorrectly.
In above testdata, features are Solid type and this type seems to be treated as 3D (code). Therefore, parsing looks working well on autotest but this issue is not tested.

sampledata: 533946_urf_6668_yoto_op.gml.zip from https://www.geospatial.jp/ckan/dataset/plateau-tokyo23ku, CC-BY 4.0

discussion

• Can we use srsDimension in envelop as global same to srsName?

[jratike80]

I am not sure how the CityGML standard should be interpreted but doesn't this mean that all geometries should have 3d vertices and the extent is meaningful?

Spatial properties of all CityGML feature types are represented using the geometry classes defined in ISO 19107. Spatial representations can have 0-, 1-, 2-, or 3-dimensional extents depending on the respective feature type and Levels of Detail (LOD; the LOD concept is discussed in Clause 6.4.4 and Clause 7.2.5). With only a few exceptions, all geometries must use 3D coordinate values.

[Kanahiro]

Thank you for very important comment. That sentences seems to be in https://docs.ogc.org/is/20-010/20-010.html#toc23 .

With only a few exceptions, all geometries must use 3D coordinate values.

For me this could be read as CityGML coordinates are in 3D except for few exceptions but ogr parses GML in 2D as default (you can change this behavior by setting environment variable GML_SRS_DIMENSION_IF_MISSING) . Although this is a different issue to should use srsDimension as global, it is important to take this into consider.

1. Should we use srsDimension in envelope as global, same to srsName?
2. in parsing CityGML, should dimension be treated in 3D as default? (current default is 2D)

2 should be discussed in other issue, I've posted.

[rouault]

dealt in #6998