[Qgis-user] Polygons with heights

[Chris Rogers]

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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