[gdal-dev] GML driver - gml:xlink resolving - just embedding?

[Johannes Echterhoff]

Even,
Thanks – also for the additional background info!
Johannes

Von: Even Rouault <even.rouault at spatialys.com>
Gesendet: Dienstag, 15. November 2022 13:27
An: Johannes Echterhoff <echterhoff at interactive-instruments.de>; gdal-dev at lists.osgeo.org
Betreff: Re: [gdal-dev] GML driver - gml:xlink resolving - just embedding?


Johannes,

Does resolving of xlink:href by the GML driver (when GML_SKIP_RESOLVE_ELEMS is used, i.e., set to NONE or HUGE) simply mean that the referenced resource is copied into the XML element that contains the xlink:href XML attribute? Tests seem to support this, but I’d really like to double-check on this list.
Yes, that's my recollection of how it works.


My somewhat naive assumption was that if the reference pointed to another GML object, then that would be recognized somehow (though the exact “how” is probably the issue; document-internal reference via @gml:id could work, but not an external reference), and duplication of objects avoided. The resolving also seems to remove the xlink:href attributes for resolved references, and the gml:id attributes on the elements that are copied into the main GML file. That, in fact, creates duplicate objects in my test case.

Complex GML is a pain to deal with and the "classic" GML driver is close its maximum potential. I believe the GML_SKIP_RESOLVE_ELEMS feature was mostly used to deal with geometry elements (like part of a boundary being used in different geometries, in particular use of GML topology elements), for which the current copy&paste strategy is fine to reconstruct a simple feature geometry. When xlink:href points at objects/features, then the ideal strategy is indeed much harder to define.

For anything advanced, using GMLAS (https://gdal.org/drivers/vector/gmlas.html#vector-gmlas) is suggested. I won't say recommended to avoid people complaining that  it goes to the other extreme of returning an output that is (supposed to be) fully lossless regarding the original content but hard to comprehend.

Even

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