Elevation data

Elevation data

Types of data available

An important distinction needs to be made between the different types of elevation data you might see:

Digital surface model (DSM) : The height above sea level of the tops of things. So, for example, it incudes the tops of buildings, trees, etc..

Digital terrain model (DTM) : The height above sea level of the bare earth. These are generally derived by applying specialist processing to a DSM to remove buildings, trees, etc..

Both DSMs and DTMs will generally come in a raster image format, often with additional geospatial meta data. The value of each pixel is the height at that point.

As well as the elevations, further processing of DTM data is often done to derive quantities that are useful in various models and to generate a "prettier" visual layer. The definitions taken from the Environment Agency data portal are:

Aspect : Aspect identifies the downslope direction of the maximum rate of change in value from each cell to its neighbors. Aspect can be thought of as the slope direction.

Hillshade : A hillshade is a grayscale 3D representation of the surface, with the sun's relative position taken into account for shading the image. This function uses the altitude and azimuth properties to specify the sun's position.

Slope : Slope represents the rate of change of elevation for each pixel cell. Values are represented in degrees.

TSR : The Tinted Shaded Relief (TSR) is a color 3D representation of the terrain, created by merging the images from the Elevation-coded and Hillshade methods.

Measurement techniques

DSM and DTM data can be derived from point data collected by several different measurement techniques. Roughly ordered by decreasing accuracy/precision and increasing coverage/density the main ones are:

Spot heights : Manual measurements taken at relatively few specific points. This is the gold standard for accuracy of height but the data is usually sparse.

Light detection and ranging (LIDAR) from an aircraft : This is done by firing laser pulses from an aircraft and recording the return time. This gives fairly accurate heights and can give high point densities but usually has less coverage due to time and cost.

Light detection and ranging (LIDAR) from an satellite : Same as above but with a lower accuracies and densities but much better, often global, coverage.

Using the data

Ideally you want to use a DTM that is stored in a geo-tagged file format so that the location of each pixel is easy to calculate. It is important that the data is the raw elevations and not a visually pretty version like TSR as the pixel values in a TSR image don't correspond to elevation.