[Qgis-user] Polygons with heights

[Karl Magnus Jönsson]

Hi Chris!
It is a bit hard to understand what you have done just from the images. What have you done different in the two attempts? Why are they different? And why can’t you use the brown one if it is working correctly? Does the pink layer allow z-geometries?

Med vänliga hälsningar

Karl-Magnus Jönsson

Från: QGIS-User <qgis-user-bounces at lists.osgeo.org> För Chris Rogers via QGIS-User
Skickat: den 15 februari 2026 14:04
Till: qgis-user at lists.osgeo.org
Ämne: [Qgis-user] Polygons with heights

I'm trying to make a road bridge over a railway by making a gentle embanked slope up to the bridge, then a slope down. So I make the polygon (with z values) to represent the plan shape (see first attachment). The attachment shows two attempts, one brown, one pink. When I now do a 3d view with Qgis2threejs, we see the second attachment. The brown bridge is correctly represented, but the pink bridge has the wrong shape.

Why are they different, how do we get the brown shape rather than the pink one?

I next want to raise the bridge to the raster layer. I managed to do this with the brown shape by firstly raising the polygon to the raster (I forget exactly how this was done, but I think I used the drape function, then raised some of the vertices by the required amount). Draping the pink one does not raise the z-values.  I also tried the Sample raster values function, but this crashed the QGIS session.

What I'm trying to do looks to be quite a simple and natural thing - maybe there is a clean way to do it in QGIS, but I've not been able to find it ...

Thanks!
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