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, so many like SRTM and ASTER are really DSMs. Those created photogrammetrically were probably DTMs.
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|
|DEM--digital elevation model||Generic term, without implying what is show. There is not widespread agreement about these terms, and in particular USGS now uses DEM for what is here called a DTM. This usage follows the Maune book, and most European usage.|
|DSM—digital surface model||First return
|DTM—digital terrain model||
Yes, if point cloud has ground classification
|CHM—canopy height model||
|DoD: DEM of difference|
|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.
|For the DSM,
each point in the point cloud will be placed at
the nearest grid node, and the highest value
This is elevation coloring; the holes will make reflectance maps problematical. It might be a good idea to use a larger grid spacing, or try to interpolate across the voids.
|For the NVS, which might approximate a DTM. each point in the point cloud will be placed at the nearest grid node, and the lowest value retained. You need a grid size that is large enough that there are multiple returns in each grid cell, and one of them must have gotten to the ground.|
||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 6/27/2018