[Qgis-developer] Why is it not possible to extract the z value of a wkb geometry with PyQGIS while it is possible with ogr or Shapely ?

[Martin Laloux]

Hello,
Why PyQGIS (which use the GEOS library) can not extract the z coordinate of
a 3D object while it recognizes that the WKB geometry is 3D  ?

For example, with a 3D point shapefile:
mylayer = qgis.utils.iface.activeLayer()
*mylayer.geometryType() == QGis.Point
True*
select the first object
sel = mylayer.selectedFeatures()[0]
geom = sel.geometry()
*geom.wkbType() == QGis.WKBPoint
False*
*geom.wkbType() == QGis.WKBPoint25D*
*True*
but no z in
*geom.asPoint()
(202586,89818.4)*
dir(geom.asPoint())
['__class__', ..., *'x', 'y*']
str(geom.asPoint().wellKnownText())
'POINT(202585.879480043950024992 89818.376070138576324098)'

*But if I use ogr or Shapely (also using the GEOS library) to treat the
resulting geometry,** no problem with the z value** :*

*With ogr:*
from osgeo import ogr
*wkb = geom.asWkb()*
geom_ogr = ogr.CreateGeometryFromWkb(wkb)
*geom_ogr.GetGeometryType() == ogr.wkbPoint25D
True*
and:
dir(geom_ogr)
['AddGeometry',..., *'GetX', 'GetY', 'GetZ'*, 'Intersect',...]
*geom_ogr.GetX(),geom_ogr.GetY(),geom_ogr.GetZ()
(202585.87948004395, 89818.376070138576, 168.35000610351562)*
geom_ogr.ExportToWkt()
'POINT (202585.879480043950025 89818.376070138576324 168.350006103515625)'

*With Shapely*:
from shapely.wkb import loads
wkb_shap = loads(wkb)
*wkb_shap.has_z
True*
and
dir(wkb_shap)
['__array_interface__',...,'*coords*',...,  'wkt', '*x'*, 'xy', '*y*', '*z*
']
*list(wkb_shap.coords)
[(202585.87948004395, 89818.376070138576, 168.35000610351562)]
wkb_shap.x,wkb_shap.y,wkb_shap.z
(202585.87948004395, 89818.376070138576, 168.35000610351562)*
# but no z in WKT.
wkb_shap.wkt
'POINT (202585.8794800439500250 89818.3760701385763241)'

Therefore, the z value is  present in the geometry obtained with PyQGIS and
it should be possible to to extract the z value. But it is beyond my
skills because
the qgis module is compiled (qgis.so)

Thanks in advance
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