A digital elevation model (DEM) is a digital file of terrain elevations for ground positions. It is a raster representing the elevation of the ground and objects, such as buildings and trees, with pixel values in the images.
In most cases, the best source of elevation for your project is a good DEM. You may want to use a DEM when you are orthorectifying an image to provide geometric correction for relief displacement, or when you do not have surveyed elevation measurements for your ground control points (GCPs) or tie points.
Ensure that the DEM will provide the level of accuracy that you require for your project, and that it includes the features that you are trying to analyze.
For example:
- When you want an orthorectified image that is accurate to 0.01 meters, a DEM with 1 kilometer resolution will probably not deliver the accuracy you need.
- If you want to analyze highway overpasses, a DEM that was smoothed and resampled to 30 meters will not provide the details that you need.
- If you want a smooth, low-resolution orthorectified image, a DEM containing fine features, such as buildings, may contain too much detail.
Besides providing a source of elevation, the DEM itself has many uses. For example:
- Geologists use DEMs to identify geological structures in topography.
- Mapping agencies use DEMs as the source of topographic information and contour lines for maps.
- Environmentalists use DEMs to identify risk areas and flow patterns.
- Disaster management agencies use DEMs to identify flood risk areas and to determine accessibility.
- Telecommunications companies use DEMs to identify regions of visibility for radio or cell towers. They can also use the texture of the DEM to predict how the terrain can effect signal strength and reflection.