What should a bare earth digital terrain model (DTM) portray?

National mapping agencies in North American and western Europe have released free lidar point clouds with densities of 2-23 points/m², and derived terrain grids. Geomorphometric processing uses a bare earth digital terrain model (DTM), which can be acquired from mapping agencies or created from the point cloud to better control its characteristics. Free software provides tools for noise removal, ground classification, surface generation, void filling, surface smoothing, and hydraulic conditioning. Tests with three ground classification algorithms, and four surface generation algorithms show that they produced very similar results. The main issues for geomorphometric operations on DTMs involve whether the highest and lowest ground points should be in the DTM if they are not on a grid node, how water, buildings, and roads should be treated, if using a DTM of lower resolution will effectively filter out noise and allow much faster processing, and if lower resolution DTMs should be created directly from the point cloud or by processing a higher resolution DTM.