What is a Digital Terrain Model (DTM)?
When you refer to the USGS LiDAR Base Specification, a digital terrain model (DTM) actually has two definitions depending on where you live.
- In some countries, a DTM is actually synonymous with a DEM. This means that a DTM is simply an elevation surface representing the bare earth referenced to a common vertical datum.
- In the United States and other countries, a DTM has a slight different meaning. A DTM is a vector data set composed of regularly spaced points and natural features such as ridges and breaklines. A DTM augments a DEM by including linear features of the bare-earth terrain.
DTMs are typically created through stereo photogrammetry like in the example above. For example, contour lines are in purple. The DTM points are regularly-spaced and characterize the shape of the bare-earth terrain.
In the image above, you can see how the DTM is not continuous and that it’s not a surface model. From these regularly-space and contour lines, you can interpolate a DTM into a DEM. A DTM represents distinctive terrain features much better because of its 3D breaklines and regularly spaced 3D mass points.
Information sourced from: https://gisgeography.com/dem-dsm-dtm-differences/