Digital Elevation Models

(DEM)

Prof. Dr. Abudeif A. Bakheit

Geology Department. – Faculty of Science

Assiut University

•Digital Elevation Models (DEM’s) store continuously varying variables such as elevation, groundwater depth or soil thickness.

•Digital Terrain Models (DTM’s) are digital representations of altitude and are frequently used in hydrological, erosion and engineering geological studies.

Digital Elevation Models are made via the following techniques:

1-Photogrammetrical techniques.

2- Point interpolation techniques.

3- Interpolation of contour lines digitized from existing maps.

1-Photogrammetrical techniques:

• These methods use stereoscopic aerial photographs or satellite images to sample a large number of points, with X, Y and Z values, by means of advanced photogrammetrical equipment.

2- Point interpolation techniques:

• When point data is available for an area, obtained via ground surveys using theodolites and/or Global Positioning Systems (GPS), point interpolation can be used to generate a DEM.

3- Interpolation of contour lines digitized from existing maps:

•First the segment map (contour Map) is converted to a raster, using a georeference.

•A linear interpolation is made between the pixels with known altitude values, to obtain the elevations of the undefined values in between the rasterized contour lines.

•The output of the contour interpolation is a raster map in which each pixel in the map has a value.

Properties of “DEM” data:

•DEMs are always in the form of raster maps, with a value domain.

•Each pixel in the raster map contains the altitude of the center of the pixel.

• When the pixel size is chosen too large, ridges and small streams may be missed.

Properties of “DEM” data:

•The accuracy of a DEM depends very much on the detail of the contour lines, that were used for the interpolation.

•It depends also on the scale of the original topographic map from which the contour lines were digitized.

•The larger the scale of the map, and the smaller the contour interval, the more accurate the DEM will be.

Digital Elevation Models applications:

•They form one of the most frequently used spatial data sources in GIS projects.

•They are also the basis for a large number of derivative information.

Application Areas of DEMs:

•Slope steepness maps, showing the steepness of slopes in degrees, percentages, or radians for each location (pixel).

•Slope direction maps (also called slope aspect maps), showing the orientation or compass direction of slopes (between 0°-360°).

•Slope convexity/concavity maps, showing the change of slope angles within a short distance. From these maps you can see if slopes are straight, concave or convex in form.

Application Areas of DEMs:

•Hill shading maps (or shadow maps), showing the terrain under an artificial illumination, with bright sides and shadows.

•Hill shading is used to portray relief difference and terrain morphology in hilly and mountainous areas.

•The illumination source is usually chosen at an angle of 45° above the horizon in the northwest.

Application Areas of DEMs:

•Three dimensional views of the terrain from a user defined position above the terrain.

•Cross-sections indicating the altitude of the terrain along a line and represented in a graph (distance against altitude).

•Volume maps (or cut-and-fill maps), generated by overlaying two DEMs from different periods. This allows you to quantify the changes in elevation that took place as a result of slope flattening, road construction, landslides etc.

Map projections

1- The Geographic Projections

2-The Universal Transverse Mercator (UTM) Projection

•The UTM system divides the surface of the Earth between 80° S latitude and 84° N latitude into 60 zones. Each 6° of longitude in width and centered over a meridian of longitude.

•Zones are numbered from 1 to 60. Zone 1 is bounded by longitude 180° to 174° W. Zone numbering increases in an easterly direction.

•The UTM system segments each longitude zone into 20 latitude zones. Each latitude zone is 8 degrees high, and is lettered starting from "C" at 80° S, increasing up the English alphabet until "X", omitting the letters "I" and "O“.

•The last latitude zone, "X", is extended an extra 4 degrees, so it ends at 84° N latitude, thus covering the northern most land on the earth.

•Latitude zones "A" and "B" do exist, as do zones "Y" and Z". They cover the western and eastern sides of the Antarctic and Arctic regions respectively.

Practical Session:The practical session includes the following;

•Loading of the 3DEM software and the SRTM DEM data from the internet.

• Using 3DEM program in DEM data visualization

Loading of DEM data from the internet:•The following site contains the SRTM DEM data

http://e0srp01u.ecs.nasa.gov/srtm/version2/SRTM3/Africa/

SRTM Information:11-day mission, February 2000, collected 80% of the Earth’s land surface (60ºN to 56ºS latitude)

Products:• 30-meter product available for USA areas• 90-meter product for USA and non-USA areas• Ortho rectified radar images

Absolute vertical accuracy specification:16 meters

•The compressed downloaded data are extracted using Winrar. The extracted data can be visualized using the 3DEM software.

•The 3DEM software can be downloaded free from the internet.

•The 3DEM software can be used to;

- Visualize the DEM data

- Achieve some operations on the DEM data