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.
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.
DEM is often used as a generic term for DSMs and DTMs, only representing height information without any further definition about the surface. Other definitions equalise the terms DEM and DTM, or define the DEM as a subset of the DTM, which also represents other morphological elements. There are also definitions which equalise the terms DEM and DSM. On the Web definitions can be found which define DEM as a regularly spaced GRID and a DTM as a three-dimensional model (TIN). Most of the data providers (USGS, ERSDAC, CGIAR, Spot Image) use the term DEM as a generic term for DSMs and DTMs. All datasets which are captured with satellites, airplanes or other flying platforms are originally DSMs (like SRTM or the ASTER GDEM). It is possible to compute a DTM from high resolution DSM datasets with complex algorithms. In the following the term DEM is used as a generic term for DSMs and DTMs.
Types of DEM
Height map of Earth’s surface (including water and ice) in equirectangular projection, normalized as 8-bit grayscale, where lighter values indicate higher elevation.
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. 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.
Representation of Elevation Data
2. Raster (Example GRID and ASCII), which could be square, rectangular, hexagonal, triangular in shape) (GRID and ASCII stands for “Generic Region for Information Display” and “American Standard Code for Information Interchange” respectively)
2. Vector (Example TIN, which is triangular only and stands for “Triangulated Irregular Network”).
Mappers may prepare digital elevation models in a number of ways, but they frequently use remote sensing rather than direct survey data. 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).
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 inter-ferometry 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.
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 pixelsize;
- vertical resolution;
- terrain analysis algorithm;
- Reference 3D products include quality masks that give information on the coastline, lake, snow, clouds, correlation etc.
Methods for obtaining elevation data used to create DEMs
- Stereo photogrammetryfrom aerial surveys
- Structure from motion/ Multi-view stereo applied to aerial photography
- Block adjustment from optical satellite imagery
- Interferometry from radar data
- Real Time KinematicGPS
- Topographic maps
- Theodoliteor total station
- Doppler radar
- Surveying and mapping drones
Free Data sources
1. Space Shuttle Radar Topography Mission (SRTM)
This 1-arc second global digital elevation model has a spatial resolution of about 30 meters covering most of the world with absolute vertical height accuracy of less than 16m. SRTM DEM data is being housed on the USGS Earth Explorer server.
2. ASTER Global Digital Elevation Model
A joint operation between NASA and Japan was the birth of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). ASTER GDEM boasted a global resolution of 90 meters with a resolution of 30 meters in the United States. Despite its high spatial resolution and greater coverage (80% of the Earth), users were dissatisfied with it because of its artifacts, which often occurred in cloudy areas. You can download the ASTER DEM data for free from the “USGS Earth Explorer”.
3. JAXA’s Global ALOS 3D World
The ALOS World 3d is a 30-meter spatial resolution digital surface model (DSM) constructed by the Japan Aerospace Exploration Agency’s (JAXA). Recently, this DSM has been made available to the public. It is the most precise global-scale elevation data at this time using the Advanced Land Observing Satellite “DAICHI” (ALOS). The DSM was generated using stereo mapping (PRISM) for worldwide topographic data with its optical stereoscopic observation. In order to obtain this highly accurate DSM, you’ll have to register online through the “JAXA Global ALOS portal” to download it.
4. Indian Portal Bhuvan
CARTOSAT 1 and 2 derived stereo DEM, available for entire India freely. For specification and other details go through Bhuvan website at http://bhuvan.nrsc.gov.in/data/download/index.php
Uses of DEM
Digital Terrain Model Generator + Textures (Maps) + Vectors
Common uses of DEMs include:
- Extracting terrain parameters for geomorphology
- Modeling water flowfor hydrology or mass movement (for example avalanches and landslides)
- Modeling soils wetness with Cartographic Depth to Water Indexes (DTW-index)
- Creation of relief maps
- Rendering of 3D visualizations.
- 3D flight planningand TERCOM
- Creation of physical models (including raised relief maps)
- Rectification of aerial photographyor satellite imagery
- Reduction (terrain correction) of gravitymeasurements (gravimetry, physical geodesy)
- Terrain analysis in geomorphologyand physical geography
- Geographic Information Systems(GIS)
- Engineeringand infrastructure design
- Satellite navigation(for example GPS and GLONASS)
- Line-of-sight analysis
- Base mapping
- Flight simulation
- Precision farmingand forestry
- Surface analysis
- Intelligent transportation systems(ITS)
- Auto safety / Advanced Driver Assistance Systems(ADAS)
Source(s): Wikipedia, NPTEL, GIS tutorials