Digital Elevation Model

Height on a certain point of the earth surface is represented by an elevation above certain datum such as mean sea level. This elevation varies as one tend to move from one point to another on surface of earth. The earth surface is highly undulate which cause these variations in the heights. Elevation of earth surface thus becomes continuously varying. This height information can be represented digitally in vector formats or raster formats.

Representation of elevation

• Vector formats:
  The vector formats of height representation can be observed in topographic maps in form of points or lines. The point in such maps are referred to as spot heights which provides height information of certain location either building or a hill top. The lines in topographic maps that reveal height information are known as contours. Contours are the lines formed from connections of points having same height. Besides one of important vector format for terrain representation is triangulated irregular network (TIN).  
• Triangulated Irregular Network:
  TIN is formed from a set of triangles that are non-overlapping whose vertices carry the elevation information. TIN can be formed by the process of triangulation where most commonly used one is Delaunay triangulation. A Delaunay triangulation is a set of linked but non-overlapping triangles. One condition in this triangulation is that the circumscribing circle of each triangle should not include any other points. During formation TIN each of the triangles tend to become as nearly equilateral as possible. The interpolation of TIN produces DEM which is grid based surface model.
• Raster formats:
  The raster formats of height representation can be obtained from satellite imageries. This can also be formed by interpolation of contours and spot heights. The raster format is a continuous surface formed by grids or cells. Each cell value has the height information of that particular location. So a single elevation represents the entire area of the cell and this necessitates the requirement of finer spatial resolution of cell for greater accuracy.  Digital Elevation Model (DEM) that we use is in this format.

DEM

A digital elevation model (DEM) is a 3 dimensional representation of a terrain’ surface created from a terrain’s elevation data. The elevation can be measured from any reference datum. Topography of the earth’s surface is expressed in form of DEM. Hence the DEM can give basic information of topography such as elevation, slope and aspect of a terrain. Besides, features like drainage basins and stream network can also be produced from DEM. DEMs are widely used in hydrologic and geologic analyses, hazard monitoring, natural resources exploration, agricultural management etc.

DEM is often used as a generic term for elevation models only representing height information without any further definition about the surface. Thus, DEM can be further differentiated into digital surface model (DSM) and digital terrain model (DTM).

DSM

DSM stands for digital surface model. It counts height of the surface of the bare earth as well as the features lying above it like trees, buildings, and other structures. DSM used for landscape modeling, city modeling and visualization applications.

DTM

DTM stands for digital terrain model. It is bare earth model. DTM represents earth’s surface without vegetation, buildings, etc. DTM is used for flood modeling, watershed modeling, geological applications, and so on. DBTM (or Digital Basement Terrain Model) is the digital representation of the basement surface.

Techniques to generate DEM

Following is the list of techniques used to create DEM.

1. DEM can be produced from already existing data like topographic maps that contains height information in the form of contours, or spot height. Geo-referencing such maps and digitization of contours and spot height convert existing data into digital format which can be converted into DEM by interpolation methods.
2. Field measurements of heights with the help of total station theodolites and GPS gives you direct measurement of height of height information for existing terrains which can be imported into GIS software and converted into DEM.
3. Aerial and satellite images are another sources for DEM. Photogrammetry use stereo pair of images that are taken either from atmosphere or space to produce DEM. Stereo pairs of SPOT satellites had been used to produce DEM since its launch. ASTER DEM was also produced form stereo pairs of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument of the Terra Satellite. ALOS World 3D is produced from Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM), which was an optical sensor on board the Advanced Land Observing Satellite “ALOS”
4. Light detection and ranging (LiDAR) is another promising technique for generation of DEM. Differences in laser return times and wavelengths in LiDAR is used to make digital 3D representations of the target. Sometimes called as 3D laser scanners, LiDAR are used to produce very high resolution DEM used for a particular location or structures.
5. Radar (stands for radio detection and ranging) locate the position by transmitting, recording, and processing the radio signals. The generation of DEM is one of numerous applications of radar.
6. Interferometric Synthetic Aperture Radar (InSAR) produces DEM by use of double pass of radar satellite for a particular location. Most of the DEM data available today are products of InSAR microwave remote sensing. Shuttle Radar Topography Mission (SRTM) was equipped with two antennas to generate interferometric data. SRTM DEM is widely used worldwide.
7. The laser altimeters are also used to model the surfaces of moon and mars. Mars Orbiter Laser Altimeter (MOLA) is one of instrument in Mars Global Surveyor (MGS) that had been used to create DTM for mars. Similarly, Lunar Orbiter Laser Altimeter (LOLA) is an instrument embedded in Lunar Reconnaissance Orbiter (LRO) had been used to produce the DTM for lunar surfaces.

Links to download DEM

Here are some links to access freely available DEM sources. You may need to register to download these data-sets.

• SRTM DEM (30 m of USGS can be downloaded from Earth Explorer),
• ASTER DEM (30 m can be downloaded from Earth Explorer),
• ALOS World 3D (30 m DEM provided by JAXA, Japan),
• MOLA DEM (surface terrain model for Mars),
• Open Topography (LiDAR data, many other resources and softwares),
•  ALOS PALSAR RTC  (A 12.5 m DEM provided by Alaska Satellite Facility)
• TanDEM-X (a 90 m DEM provided by DLR, Germany)