I have digitized an LULC layer from satellite imagery, now I want to extract the elevation data from it. I have obtained DEM images from SRTM amd ASTER. How do I proceed?
Since I think you deal with polygons, you could first generate centroids* (or a representative point for each polygon) for those polygons and use the points to pick elevation data from the DEM.
Finally, join your polygon and point layer and copy the elevation data to your polygon layer.
Not sure if that's the best way to get it done, though. I didn't actually try it, but if you are lost on any step, tell me and I'll elaborate a bit more on that.
- Be aware that in some cases a centroid could lay outside of the base polygon, other algorithms ensure points lay inside polygons. I'd use the ST_PointOnSurface function from PostGIS.
On the other hand, if you actually need Z coordinates assigned to each vertex of your polygons, then that could be a good idea for a QGIS plugin.