Digital elevation models

Digital elevation models are a useful way of storing and visualising elevation data. Like satellite imagery they consist of a raster, but unlike most of the imagery we have worked with, they have only one band. Every pixel contains solely an elevation value – usually the number of metres above sea level. The pixels are an average over the area that they cover – so if the DEM has a resolution of 100m, the value will be the average elevation within this 100 by 100m area.

_{DEM pixels are average elevation values calculated across the area the cover. Data courtesy of USGS.}

DEMs do not have to come from satellite data – they are also commonly derived from LiDAR systems mounted on aircraft, from measurements taken by GPS, or calculated from contour lines from topographical maps created using more traditional survey methods. Increasingly people also produce them using drone photography.

There are two main types of DEM – digital surface models (DSMs) include buildings and vegetation, while digital terrain models (DTMs) consist of just the Earth’s surface, with any natural or anthropogenic objects removed. We will be working only with DTMs for the rest of this step – so when we say ‘DEM’ bear in mind that we will always be talking about a digital terrain model!

_{Digital Surface Model (top) and Digital Terrain Model (bottom) of the wider landscape around Stonehenge. Notice how trees and buildings are not visible in the DTM. Data: UK Open Government License v3.0.}

Thanks to GIS software there is lots we can do with DEM data. We can calculate the slope (and the direction of the slope) of the terrain, create 3D models for satellite imagery or other data, create hydrological models showing where streams and rivers are likely to be, work-out which areas are visible from particular points in the landscape, and lots more! It is easy to see why DEMs are so important for landscape archaeologists – it is possible to find out so much about a site from a single dataset.

We will be downloading our own DEM data and doing some of these things next!