Products
DTM — Digital Terrain Model
Bare-earth 3D model with trees, buildings and other surface features removed or interpolated through.
A DTM is a raster where every pixel value is the elevation of the bare ground — vegetation, buildings, and other above-ground features have been removed or interpolated through. More processing and usually more source data than a DSM, because we are inferring what is under the canopy.
When to use
Hydrology, flood modelling, grading, earthworks, anything where you care about the ground itself rather than what sits on top of it.
Pricing
Priced separately from imagery, case-by-case. More expensive than a DSM because of the additional canopy-penetration processing required.