If elevation parameter is used then 3D vector features follow the elevation model by using individual elevation values for the vertices. Height for vertices is interpolated from elevation raster map using given interpolation method.
For conversion of 2D points or lines to 3D can be used v.to.3d or v.drape. In opposite to v.extrude, these modules do not extrude vector features, they defines z-coordinate for the features from given parameters or by sampling elevation raster map values. It means that no feature type conversion is applied, points remain still points in the output vector map. Same applies for the lines.
v.extrude modifies only features geometry. Feature categories remain untouched and attribute data is copied from input vector map to the output.
By default, all features (including features without category) from input vector map are processed (layer=-1). Feature selection can be applied by layer, cats or where parameter.
v.extrude input=houses output=houses3D height=5 type=area
v.extrude input=houses output=houses3D elevation=dem height_column=height type=area
v.extrude input=geodetic_pts output=points3D height=200 type=point
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