A digital terrain model (DTM) is an elevation dataset representing the bare earth — the ground surface with buildings, vegetation and other objects removed. It answers the question of what the land itself looks like beneath whatever sits on it.
How it differs from a DSM
A digital surface model records the top of everything present, including canopy and rooftops. A DTM removes those and models the terrain underneath. Both fall under the broader heading of digital elevation model, which is why that term should always be clarified before use.
Where DTMs are the right choice
Bare-earth models are used for flood modelling and drainage, civil engineering and earthworks, slope stability and landslide risk, agricultural planning, and archaeological survey — where stripping tree cover can reveal earthworks invisible from the air.
How the ground is separated from the objects
DTMs are usually derived from LiDAR by filtering point returns to classify which struck the ground and which struck something above it. That classification is an interpretation, not a measurement, so DTM quality depends heavily on how well the filtering handles dense vegetation, steep slopes and built-up areas.
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