By today's standards, the first DEM's were crude and generalized. They had large spacing between postings. It was also unclear how they accounted for vegetation and buildings.
With the advent of lidar (lasers collecting many thousands of elevations per second), the resolution and detail increased substantially. We can now create multiple surfaces, each with a specific name and properties.
Five grids for Fort Monroe, Virginia
|Grid||Synonyms||Comments||MICRODEM produces from point cloud, with drop in the bucket algorithm|
|DSM—digital surface model||First return
|DTM—digital terrain model||
||Yes, if point cloud has ground classification|
|CHM—canopy height model||
|DSM--First return DEM. This includes trees and buildings.||DTM--Bare earth DEM||CHM Difference grid, shown as Elevation map (red is tall).|
|DTM/DSM for the same area in Portsmouth, UK.
The DTM is very generalized, and does not provide a good visualization of what is there. It also has clear ghosts from the building removal process.
||Two lidar surveys a month apart (April 2010
and July 2010) in Pennsylvania, from
Note the spikes corresponding to tree trunks in the leaf on red DTM, which are missing in the green leaf off DTM.
The black leaf on DSM is generally a little higher than the blue leaf off DSM. Viewed in a reflectance map the two DSMs are very hard to differentiate.
Urban lidar displays
Last revision 11/23/2017