Definition In scientific literature there is no universal agreement about the usage of the terms: digital elevation model (DEM) digital terrain model (DTM) digital surface model (DSM) DEM is often used as synonimous of DTM, but in many cases it used as a generic term for both DTMs and DSMs. The most common representation of a DEM is a raster, where the DNs correspond to the average elevation value of the area framed by the cell.

Uses Terrain analysis in geomorphology and physical geography Modelling water flow for hydrology Modelling soil erosion or mass movement Creation of relief maps Rendering of 3D visualizations Rectification of aerial photography or satellite imagery Climatology Urban planning Logistics and communications Mining ...and many more other GIS applications

Sources of elevation data Stereo photogrammetry from aerial surveys traditional: manually by a trained photogrammetrist contours of topographic maps, to be interpolated modern: by automatic stereo-correlation Stereo-correlation from optical satellite imagery SPOT (off-track stereoscopy) Aster DEM (along-track stereoscopy) Interferometry from radar data European Remote Sensing Satellite ERS (multi-pass) Shuttle Radar Topography Mission SRTM (single-pass) Lidar interpolation of a matrix of xyz points obtained using a laser beam

Aspect The rate of change in the x direction for cell 'e' is : [dz/dx] = ((c + 2f + i) - (a + 2d + g)) / 8 The rate of change in the y direction for cell 'e' is : [dz/dy] = ((g + 2h + i) - (a + 2b + c)) / 8 Taking the rate of change in both the x and y direction for cell 'e', aspect is calculated using: aspect = 57.29578 * atan2 ([dz/dy], -[dz/dx]) The aspect value is converted to compass direction values (0–360 degrees, 0 and 360 indicate North) The aspect image is generaly classified into 8 classes, i.e. N, NE, E, SE, S, SW, W, NW NoData value is assigned to flat areas a b c d e f g h i atan2

Slope a b c d e f g h i Can be computed in different ways: like the aspect, it is usually calculated using a 3 x 3 matrix around each pixel Horn’s algorithm:

When deriving aspect and slope information from a DEM, if the quality is poor, artefacts become visible specially in level or flat areas. DEM aspect slope

Hillshading Each pixel of the DEM is “artificially illuminated”, with the virtual sun positioned at a given azimuth and elevation (90° - zenith angle). The max illumination is when the pixel slope is perpendicular to the sun direction, and the aspect coincides with the azimuth. Pixel in shadow (not reachable by direct illumination) are set to 0. Usually algorithms calculate only direct shadows, and do not determine whether a surface is shadowed by another one. As most software procedures assume that planar coordinates and elevation data are expressed with the same units (e.g. both easting, northing, and elevation are in meters), special care should be paid when using DEM in geographic coordinates.

Direction of Steepest Descent 30 30 80 74 63 69 67 56 60 52 48 80 74 63 69 67 56 60 52 48 Slope:

Flow direction grid 9

The problem of the sink A pixel is surrounded by points having a higher elevation. If it is a DEM artifact, increase elevation of the pixel until the pit drains to a neighbor, or “carve” one of the neighbors.

Flow Accumulation Grid. 3 2 11 1 15 5 24 3 2 2 11 1 1 15 2 5 24 1

Threshold Drainage Area 3 2 2 1 11 Example: 5 cells 1 15 2 5 1 24

Stream segments 3 2 11 1 15 5 24

Convert stream segments into vectors Drainage Lines are drawn through the centers of cells on the stream segment. Drainage Points are located at the centers of the outlet cells of the catchments.

Catchments For every stream segment, there is a corresponding catchment. Also the catchments can be converted to vectors (polygons).

Catchments Subwatersheds Catchments Watershed Catchments can be hyerarchically grouped into watersheds. Watersheds are defined by their outlet points.

Sources of global DEM data Aster Global Digital Elevation Model (GDEM) Version 1 (2009) 30m Version 2 (2011) reduction of spikes and wells improved elevation accuracy Improved definition of water bodies and coastlines Shuttle Radar Topographic Mission Version 1 (2000 ->) 90m (3 arc-seconds), 30m (1 arc-second) for the USA Version 2 (2005) well-defined water bodies and coastlines absence of spikes and wells missing data ('voids') are still present Version 3 (“SRTM Plus”, 2014) void-filled (mostly using GDEM) 1 arc-second already released for most of Africa, other areas in the future http://gdex.cr.usgs.gov/gdex/