Digital Elevation Models

A Digital Elevation Model is an important data layer in many applications. DEMs are widely used for topographic maps and based on a grid file of elevation it is possible to derive topographic contours and compute elevation slope and aspect.

Depending on the required resolution various existing DEM data sets can be used. The quality of a DEM is usually expressed by the grid cell size and the accuracy of the elevation. For most regional or global applications, data from the Shuttle Radar Topography Mission (SRTM) can be used. This data set has a grid cell size of 90 metres and an accuracy of 10-16 metres. This is an inexpensive solution as the data is public domain. However, some processing and quality control needs to be applied and GRAS has the necessary expertise to perform this.

Other solutions include DEMs produced by high-resolution satellites with stereo-looking capabilities. With the most recent satellites, grid sizes down to 2 metres can be produced based on very high resolution imagery such as WorldView, QuickBird and SPOT.  

In some applications, much finer resolutions and accuracies are required. In this case aerial radar or lidar campaigns can provide very high resolution DEMs with grid cell sizes down to centimetres. These services are available through GRAS's partner network. Another solution for small areas is to use differential GPS (DGPS) to collect points and interpolate these to a DEM.

Hydrological modelling
DEMs are crucial in hydrological modelling as the topography controls the flow of water. Due to the close collaboration with DHI, GRAS has extensive knowledge about DEM requirements for hydrological modelling. In a recent project with DHI, GRAS developed a new method for topographic model generation for the Okavango Delta in Botswana with an accuracy of app. 1 metre.

Other applications
Other important applications include the telecommunications industry where the placement of broadcast antennas can be optimised using a DEM and GIS analyses. DEMs are also vital in other engineering applications, e.g. for calculating the volume of proposed reservoirs, and determining landslide probability.

DEMs are also useful in environmental change applications. A DEM can enhance the understanding of change by analysing changes in relation to the topographic properties, for instance by analysing the relation between crop patterns and terrain exposure.