1. Digital elevation model

2. Digital elevation model
A digital elevation model (DEM) is a digital model or 3D representation of a terrain's surface — commonly for a planet (including Earth), moon, or asteroid — created from terrain elevation data.
3D rendering of a DEM of  Tithonium Chasma on Mars

3. Terminology
There is no universal usage of the terms digital elevation model (DEM), digital terrain model (DTM) and digital surface model (DSM) in scientific literature.
In most cases the term digital surface model represents the earth's surface and includes all objects on it.
In contrast to a DSM, the digital terrain model (DTM) represents the bare ground surface without any objects like plants and buildings (see the figure on the right).

4. Terminology continue..
All datasets which are captured with satellites, airplanes or other flying platforms are originally DSMs (like SRTM or the ASTER GDEM).
In the following the term DEM is used as a generic term for DSMs and DTMs.

5. Types of DEM
A DEM can be represented as a raster (a grid of squares, also known as a heightmap when representing elevation) or as a vector-based triangular irregular network (TIN).
The TIN DEM dataset is also referred to as a primary (measured) DEM, whereas the Raster DEM is referred to as a secondary (computed) DEM. 
Heightmap of Earth's surface (including water and ice) in equirectangular projection, normalized as 8-bit grayscale, where lighter values indicate higher elevation

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The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR, land surveying, etc..
DEMs are commonly built using data collected using remote sensing techniques, but they may also be built from land surveying.
DEMs are used often in geographic information systems, and are the most common basis for digitally produced relief maps.
While a DSM may be useful for landscape modeling, city modeling and visualization applications, a DTM is often required for flood or drainage modeling, land-use studies, geological applications, and other applications.

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One powerful technique for generating digital elevation models is interferometric synthetic aperture radar where two passes of a radar satellite (such as RADARSAT-1 or TerraSAR-X or Cosmo SkyMed), or a single pass if the satellite is equipped with two antennas (like the SRTM instrumentation), collect sufficient data to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters.
Other kinds of stereoscopic pairs can be employed using the digital image correlation method, where two optical images are acquired with different angles taken from the same pass of an airplane or an Earth Observation Satellite (such as the HRS instrument of SPOT5 or the VNIR band of ASTER).

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The SPOT 1 satellite (1986) provided the first usable elevation data for a sizeable portion of the planet's landmass, using two-pass stereoscopic correlation.
Later, further data were provided by the European Remote-Sensing Satellite (ERS, 1991) using the same method, the Shuttle Radar Topography Mission (SRTM, 2000) using single-pass SAR and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER, 2000) instrumentation on the Terra satellite using double-pass stereo pairs.

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The HRS instrument on SPOT 5 has acquired over 100 million square kilometers of stereo pairs.
Older methods of generating DEMs often involve interpolating digital contour maps that may have been produced by direct survey of the land surface.
This method is still used in mountain areas, where interferometry is not always satisfactory.
Note that contour line data or any other sampled elevation datasets (by GPS or ground survey) are not DEMs, but may be considered digital terrain models.
A DEM implies that elevation is available continuously at each location in the study area.

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The quality of a DEM is a measure of how accurate elevation is at each pixel (absolute accuracy) and how accurately is the morphology presented (relative accuracy).
Several factors play an important role for quality of DEM-derived products:
Terrain roughness;
Sampling density (elevation data collection method);
Grid resolution or pixel size;
Interpolation algorithm;
Vertical resolution;
Terrain analysis algorithm;
Reference 3D products include quality masks that give information on the coastline, lake, snow, clouds, correlation etc.

11. Methods for obtaining elevation data used to create DEMs
Lidar
Stereo photogrammetry from aerial surveys
Structure from motion / Multi-view stereo applied to aerial photography
Block adjustment from optical satellite imagery
Interferometry from radar data
Real Time Kinematic GPS
Topographic maps
Theodolite or total station
Doppler radar
Focus variation
Inertial surveys
Surveying and mapping drones
Range imaging
Gatewing X100 unmanned aerial vehicle

12. Uses
Digital Elevation Model - Red Rocks Amphitheater, Colorado obtained using an UAV (DroneMapper)
Bezmiechowa airfield 3D Digital Surface Model obtained using Pteryx UAV flying 200 m above hilltop

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Digital Surface Model of motorwayinterchange construction site. Note that tunnels are closed.
Example DEM flown with the Gatewing X100 in Assenede
Digital Terrain Model Generator + Textures(Maps) + Vectors

14. Uses continue.. Common uses of DEMs include:
Extracting terrain parameters for geomorphology
Modeling water flow for hydrology or mass movement (for example avalanches and landslides)
Creation of relief maps
Rendering of 3D visualizations.
3D flight planning and TERCOM

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Creation of physical models (including raised relief maps)
Rectification of aerial photography or satellite imagery
Reduction (terrain correction) of gravity measurements (gravimetry, physical geodesy)
Terrain analysis in geomorphology and physical geography
Geographic Information Systems (GIS)
Engineering and infrastructure design
Satellite navigation (for example GPS and GLONASS)

16. Uses Continue.. Line-of-sight analysis Base mapping Flight simulation
Precision farming and forestry
Surface analysis
Intelligent transportation systems (ITS)
Auto safety / Advanced Driver Assistance Systems (ADAS)
Archaeology

17. Sources
A free DEM of the whole world called GTOPO30 (30 arcsecond resolution, approx. 1 km) is available, but its quality is variable and in some areas it is very poor.
A much higher quality DEM from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument of the Terra satellite is also freely available for 99% of the globe, and represents elevation at 30 meter resolution.

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A similarly high resolution was previously only available for the United States territory under the Shuttle Radar Topography Mission (SRTM) data, while most of the rest of the planet was only covered in a 3 arc-second resolution (around 90 meters).
The limitation with the GTOPO30 and SRTM datasets is that they cover continental landmasses only, and SRTM does not cover the polar regions and has mountain and desert no data (void) areas.

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SRTM data, being derived from radar, represents the elevation of the first-reflected surface — quite often tree tops.
So, the data are not necessarily representative of the ground surface, but the top of whatever is first encountered by the radar.
Submarine elevation (known as bathymetry) data is generated using ship-mounted depth soundings.

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The SRTM30Plus dataset (used in NASA World Wind) attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model. 
Another global model is Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) with 7.5 arc second resolution.

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It is based on SRTM data and combines other data outside SRTM coverage.
A novel global DEM of postings lower than 12 m and a height accuracy of less than 2 m is expected from the TanDEM-X satellite mission which started in July 2010.
The most common grid (raster) spacing is between 50 and 500 meters.

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Free DEMs are also available for Mars: the MEGDR, or Mission Experiment Gridded Data Record, from the Mars Global Surveyor's Mars Orbiter Laser Altimeter (MOLA) instrument; and NASA's Mars Digital Terrain Model (DTM).
OpenTopography is a community access resource of high-resolution, Earth science-oriented, topography data, and related tools and resources that warehouses a great deal of DEM data. 

23. THANKS…