The contour lines are the set of lines connecting all points at the same elevation in a model used to represent the relief on a large scale.

The DTM is a homogeneous and regular point grid indicating the height of the ground level in order to model its surface. The DTM 1m is achieved by interpolating in Lambert 2008 source data in Lambert 72 and at a 1m-resolution from the Flemish and Brussels regions, and by adding Lambert 2008 data at 1m-resolution from the Walloon Region. The DTM 5m has an additional source, namely drawn structure lines and points adapted during systematic and continuous update by photogrammetric surveys. The DTM 20m is obtained by resampling of the DTM 1m.