| Overview | Details | Data Sources | Links | References |
| Contour Maps | • | DEMs | • | Creating DEMs | • | Uses of DEMs | • | Other Products | • | LIDAR | • | DEM or DTM? |
Traditionally topography has been mapped by ground surveys. Firstly, a set of horizontal and vertical control positions or reference points, whose exact position and height are known, must be established. Topographic detail or elevation of intervening points are then built upon this framework by appropriate measurement of distances and angles.
The usual method of representing hills, mountains, depressions, etc. on a map is with contours. A contour is a line connecting points of equal elevation. The vertical distance between the contours is called the contour interval. Generally this interval depends on the scale of the map. On the 1:50,000 scale Ordnance Survey Discovery Series maps, this interval is 10m. Therefore, any elevation changes less than this are not represented. Spot elevations are usually given on maps for important points such as mountain peaks. Contours are a very clear way of representing height, but it can take some experience to become adept in quickly interpreting the different physical features from them.
| This image is an extract from an Ordnance Survey Ireland Discovery series topographic map. Note that the contours are shown at intervals of 10 m. |  |
Digital Elevation Models
A Digital Elevation Model (DEM) is a grid of data points, giving the height above sea level for each grid position. The grid is usually regular, which means that each point is separated by the same distance from its neighbouring point. This separation distance, known as spatial resolution, may be of the order of centimeters or meters or kilometers. This depends on how the original data were collected and also on what further processing may have been carried out on the data.
| This image shows an OSI DEM of the same area as above. The spacing between grid cells is 50m; each cell shows the average height in a 50m by 50m area. The differences in height are indicated by different grey shades. The area of these maps is approximately 1.75km by 1.25km. |  |
Methods of Creating DEMs
There are many ways of creating Digital Elevation Models. Elevations may be collected using traditional surveying methods or by using Global Positioning Systems (GPS). Using this latter method, it is possible to select the density of measurement points. Very few measurements are required in flat terrain, while many measurements may be taken in areas with highly varying topography. The points are then interpolated onto a
FOCUS:
Creating a Global DEM
 An artist's representation of the SRTM antennae in operation. Until recently, there were large parts of the Earth’s surface whose heights were not accurately known. In those countries which had good height information, different data collection methods and map projection systems were used, therefore it was often difficult to compare data across borders. In order to address these problems, in 2000, the space shuttle Endeavour flew the eleven day Shuttle Radar Topography Mission (SRTM) during which it collected imagery over all the Earth from 60° north to 60° south, in order to generate a Digital Elevation Model for the whole globe. Using Synthetic Aperture Radar (SAR) technology, the shuttle was able to gather data during hours of darkness and in cloudy conditions. Over the last six years NASA and other agencies have been processing this data in order to generate a homogenous and seamless global product. |
DEMs may also be generated from contours on pre-existing topographic maps. This could be a low cost solution for accessing DEM information, however, it is a very labour intensive process.
More recent methods of generating DEMs utilise imagery taken from aircraft or satellites. A pair of images taken from slightly different angles may be combined using stereographic techniques to generate a DEM. Using images from the SPOT series of satellites, DEMs with a horizontal grid spacing of 10 - 20 m and a height accuracy of approximately 20 m may be generated. The ASTER instrument on the TERRA satellite is also being used to generate stereo DEMs with a grid spacing of 30 m and a height accuracy of approximately 20 m. As both of these sensors work with images in the visible and infrared part of the spectrum, the presence of clouds will inhibit the generation of elevation points.
Another technique involves the use of Synthetic Aperture Radar (SAR) images. These images are collected by satellites irrespective of cloud conditions or darkness. Two or more images of the same area, acquired from slightly different positions, are combined using either stereographic techniques or in a process known as interferometery. In general, interferometric DEMs have a better height accuracy when compared with radar stereography. However a drawback of interferometry is that the method does not work over forested terrain.
 Click on the image to see an animation which shows a Landsat satellite image of Mexico City draped over a DEM. Generated by the United States Geological Survey. |
Quicktime Movie (4MB: Download may be slow.) |
Satellite radar altimeters are also used for generating DEMs. An altimeter measures the time it takes a radar pulse emmited by a sensor to be reflected back from the Earth's surface. From this time the distance of the object from the sensor and hence its height above an Earth reference surface can be determined. The spatial resolution of these DEMs is usually of the order of kilometers. DEMs of the Antarctic have been generated in this way.
Some Uses of DEMs
DEMs can be used for the geometric and radiometric correction of satellite images. For example, sunlight reflected to a satellite sensor from a forest on a mountain slope is different to that from a forest located on a flat surface. This occurs because of the different angular relationship between the sun, sensor and surface. A DEM can provide the angles required for each picture element (pixel) in order to make an appropriate radiometric correction. In addition, when projecting a satellite image into a map co-ordinate system (e.g. Irish National Grid), the DEM can take geometric shifts, due to changes in elevation, into account. In this way a more accurate positioning or rectification of the image is achieved.
DEMs can also be used for generating three dimensional terrain views. Such visualisations can be used to examine the visual impact of new constructions. This is especially true if the visualisations are animated, so that the viewer can fly-through the area virtually. Of course three dimensional views and fly-throughs are used by the military in flight simulators to aid combat planning and training.
High resolution, high accuracy DEMs are used in hydrological modelling and flood prediction, as variables such as slope, runoff rate and water depth can be easily calculated. They are also valuable in studying landslides, either for modelling scenarios or for looking at terrain changes before and after landslide or subsidence events.
High resolution DEMs are also used in the study of coastal erosion. Erosion or accretion rates can be calculated if elevation information is collected on a regular basis.
The location of transmission equipment can be optimised by the incorporation of DEMs in telecommunications modelling.
The global GTOPO30 DEM, with a ground spacing of 1km, is available free of charge from the US Geological Survey. This DEM is useful for regional scale studies or basic radiometric and geometric correction of some satellite imagery, but is not appropriate for detailed work.
Deriving Other Products from DEMs
| To generate
a hill-shaded version of a DEM, the user specifies an elevation
and location (e.g south east) for the sun position. This is then
used to calculate how shadows would fall and the output has shading
applied appropriately. This provides an image, which looks more
3-dimensional than the basic DEM. Slope and aspect can also be calculated and contour maps may also be derived. |
|
LIDAR for DEM Generation
LIDAR (Light Detection and Ranging) is an appropriate technique when very high resolution elevation data are required. Ground spacing is usually one to two metres, whereas the height accuracy is around 20 – 30 cm. LIDAR surveys are carried out by aircraft. The technique is quite simple. The instrument sends out a pulse of infrared laser energy. This pulse is reflected from the objects below the aircraft and part of the energy is returned to the sensor. Knowing the height and position of the aircraft above the surface and the time it took the pulse to be returned, the elevation can be determined.
A LIDAR survey was carried out in 2001 for the Department of Communications, Marine and Natural Resources (DCMNR), on the coastline between Carnsore Point in Co. Wexford and Dalkey Head in Co. Dublin, as part of a coastal erosion project.
The Airborne Topographic Mapper (ATM) system of NOAA/NASA/USGS, flown on board a DeHavilland Twin Otter aircraft travelling at 60 m/s scanning a swath of 300m, captured this image of a coastal area in the state of Washington. Note the small elevation changes that can be detected.
DEM or DTM?
A DEM is a Digital Elevation Model whereas a DTM is a Digital Terrain Model. The difference is that a DEM includes the elevation of objects on the Earth’s surface, such as trees and buildings. A DTM on the other hand is the elevation of a bare Earth, so it doesn’t take into account the height of trees, buildings, etc. However, to confuse matters, people often use DEM and DTM interchangeably!