A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. It is also widely known as a digital terrain model (DTM). An accurate DTM can be used for various types of mapping applications, to produce contours, and for orthorectification of aerial photography.
The above sample contour map depicts a simple hill and provides the following information.
- The elevation of land is typically its height above sea level. The numbers written on contour lines indicate the elevation of the lines. Topographic maps would be very cluttered if all contour lines were labeled, so only the heavier lines show labels.
- The elevation of unlabeled contour lines can be determined using the contour interval at the bottom of the map. The contour interval tells the vertical distance between neighboring lines. By counting the number of contours from a labeled line, and multiplying by the contour interval, you can calculate the elevation of any contour line.
- For points located between contour lines, you can estimate the elevation by examining the distance to the two closest contours.