Introduction to Unit 3: Digital Elevation Models

Digital Elevation Models (DEMs) are digital representations of relief. This unit covers the generation, evaluation and simple application of DEMs, with the aim of making the user aware of the choices and their implications available at each of the stages. The unit is broken up into the following sections:

1. Into the third dimension – digital elevation models
2. Data sources for elevation
3. Interpolating elevation data
4. Making calculations using elevation
5. Visualisation using elevation
6. Truly 3-dimensional data

 
The first section ‘Into the third dimension – digital elevation models’ describes how elevation data are actually stored within a GIS. The second and third sections explain how DEMs are created. ‘Data sources for elevation’ describes the ways in which the elevation of the Earth’s surface can be measured. ‘Interpolating elevation data’ explains how elevation values in between measurements (such as between contours) can best be estimated using GIS. This process of estimating the values of a property in between measurement points is known as spatial interpolation.

The fourth and fifth sections look at some of the ways that calculations can be made from DEMs to help manage environmental resources. ‘Making calculations using elevation’ describes how slope, aspect and type of landform can be estimated from a DEM. The fifth section looks at how DEMs can be used to assess the visual impact of an activity on the landscape, such as the felling of a timber stand or the building of a new visitor centre in a national park.

Raper (1989) describes three types of spatial data:

• 2-dimensional data: Most GIS data fall into this category. The location of all such data are described in terms of x and y co-ordinates on a horizontal plane. Examples would be the boundary of a forest stand, a polyline depicting a river, or points indicating the locations of bird’s nets.
• 2.5-dimensional data: These are digital map layers depicting the elevation of the Earth’s surface. DEMs are therefore 2.5-dimensional data. Because 2.5 dimensional data depict a surface, in most cases (except for overhanging cliffs), there is usually only one height value stored for each location on an x, y plane.
• 3-dimensional data: These are data with an x, y, and a z co-ordinate. Examples of 3-dimensional data might be tracks (polylines) depicting the movements of whales within an ocean in terms of latitude, longitude and depth, or a 3-dimensional polygon, indicating the extent of water-bearing rocks beneath the earth’s surface.

 
Most of this unit is therefore concerned with 2.5-dimensional data. The final section, however, is concerned with the third category of spatial data – truly 3-dimensional data. It looks at how hydrogeological data can be stored within a GIS.

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