I have a points layer with a Z (elevation) field and a terrain model (points layer or polygons layer -triangles- or eventually raster). I'm looking for a way to select the entities of my points layer that are at least 10m under the terrain model. Any way to do this in QGIS 1.8 ?
If your terrain model is a raster or polygon there is a very straightforward solution. Get the Point Sampling Tool Plugin and use this to get the DTM height from the ratser and add it as a new attribute to your points data. Then use a selection query to select the points where your original point height field is more than 10m less than the new raster height attribute.
If your DTM is in a point format you will have problems without converting to something else (a raster may be preferable) because you know nothing about the space in between