In photogrammetry modeling, Digital Terrain Models (DTM) and Digital Surface Models (DSM) are both types of 3D models that represent the Earth's surface, but they differ in what features they include:

1. Digital Surface Model (DSM)
- What It Represents: A DSM represents the **top surface** of the landscape, including all natural and man-made features such as trees, buildings, vegetation, and infrastructure.
- Usage: DSMs are useful in applications where understanding surface heights and obstructions is critical, such as in urban planning, line-of-sight analysis, and forestry. DSMs are often used in visual simulations like flight path analysis or telecommunication planning (e.g., determining the position of cellular towers).
- Features Included: Trees, buildings, vegetation, and other objects on top of the earth’s surface are captured.

2. Digital Terrain Model (DTM)
- What It Represents: A DTM, on the other hand, represents the **bare-earth terrain** without any surface objects. It focuses on the underlying topography, eliminating trees, buildings, and other obstacles to provide a clearer view of the ground itself.
- Usage: DTMs are often used in applications like hydrological modeling, flood risk analysis, or road and terrain planning where the natural ground surface is of interest.
- Features Included: Only the earth’s surface, excluding all surface features like trees and buildings.

Key Differences:
- Representation: DSM shows everything on the surface (including buildings, trees, etc.), while DTM focuses only on the bare ground.
- Applications: DSM is used for surface analysis including obstacles, whereas DTM is used for terrain-focused tasks like water flow and topography studies.

Example in Photogrammetry:
- In aerial photogrammetry, a DSM would give you the height of tree canopies and building rooftops, while a DTM would give you the elevation of the ground beneath those trees or buildings.

In summary, DSM gives a "top view" of the surface, while DTM gives a "bare-earth" view, stripped of any surface features.

We can see an example on Strayos where viewing the DSM, we see two truck appear on the model. 

Now, looking at the DTM, we do not see these features in the model. 

The DSM and DTM models can be downloaded as contour files in their respective coordinate planes and contour intervals in the download menu.