How We Create DEM From Topographic Maps
In short, the creation of DEMs involves capturing certain features from topographic maps, and converting this information into a raster grid of elevation values. It is these elevation values, one per pixel, that are then used to portray elevation.
1. The process starts with the source topographic map(s). Certain features are digitized, or digitally captured, from the maps. The following features are captured through digitizing:
- All elevation contour lines
- All spot heights/benchmark heights
- All coastlines
- All water boundaries and drains
- For a DEM where multiple adjacent map sheets are used as source materials, estimated elevation points are inserted along map edges. This allows the DEM to cover the entire map area
- Some source maps have areas that are void of elevation/topography information, or where this information is obscured. In these cases, estimated elevation values are inserted along the boundaries of the void areas, ensuring that the DEM will extend to the edge of these boundaries
2. Next, all features are aligned along source map edges to ensure a seamless coverage of elevation values over the entire map area. The result is an integrated DEM.
3. After the maps have been vectorized and edge-matched for a particular area, a DEM is produced. The original digitized features represent a form of topology that is converted, through terrain analyzing software, to an elevation grid. The conversion is done through the application of a Triangulated Irregular Network. The TIN employs linear interpolation of the existing features to create individual elevation values. Additionally, to more accurately represent ridges and valleys, an inference algorithm is used to estimate the breaklines and valleys from the last contour and local slope trend.
