Drop M/Z values algorithm doesn’t remove Z values in KML

[diegoalarc]

What is the bug or the crash?

When I try to remove the Z values from a KML, the algorithm converts them to 0. But if I try to convert them to geojson, these values are removed.

Steps to reproduce the issue

1. Go to 'Processing Toolbox'.
2. Click on 'Drop M/Z values'.
3. Run the algo using a file with Z values and save it as KML
4. Open the file as text.
5. See error

Versions

version 3.30.3

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[nicogodet]

I believe this is by design.
KML always contains X, Y Z (https://docs.ogc.org/is/12-007r2/12-007r2.html)

[diegoalarc]

Were you able to find a paragraph where in this doc they mention that it's always 3D? thanks in advance.

[nicogodet]

§6.2

The KML encoding of every kml:Location and coordinate tuple uses geodetic longitude, geodetic latitude, and altitude (in that order) as defined in Annex B.

[nicogodet]

But https://docs.ogc.org/is/12-007r2/12-007r2.html#1212 says that altitude is optional.

[diegoalarc]

But, shouldn’t the "Drop M/Z values" drop the Z value anyway?

[agiudiceandrea]

@nicogodet the issue also occur if the input layer of the "Drop M/Z values" processing algorithm is e.g. a plain Point (without Z and M) layer and the output layer is a KML layer.

[agiudiceandrea]

The issue is not related to the "Drop M/Z values" processing algorithm itself. In fact the issue also occur with any other processing algorithm when the output layer is a KML layer: e.g. using the Dissolve algorithm with a plain Point (without Z and M) input Layer, a MultiPointZ KML layer is created instead of a MultiPoint KML layer.
It seems to me the issue is due to the fact that GDAL has 2 KML drivers (the KML driver and the LIBKML driver) and that the processing algorithms use the LIBKML driver instead of the KML driver (used when a layer is exported via Export->Save Features As...).
@diegoalarc, to avoid the issue, you can disable the LIBKML driver in Settings->Options->Data Sources->GDAL->Vector Drivers.
Probably @rouault knows if this issue can be fixed in GDAL and or in QGIS.

[rouault]

Probably @rouault knows if this issue can be fixed in GDAL and or in QGIS.

With a 2D geometry, GDAL calls libkml with just latitute, and longitude, but my quick scan at libkml itself is that it always add a Z=0 is not explicitly provided. So I don't think this can be fixed in GDAL. QGIS could potentially auto-switch to the KML driver for that algorithm.

[agiudiceandrea]

QGIS could potentially auto-switch to the KML driver for that algorithm.

@rouault, it seems that all the processing algorithms in QGIS use the LIBKML driver so a KML layer created by any processing algorithm (with vector output) has an added Z coordinate.