3D rendering of a DEM of Tithonium Chasma on Mars

A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. It is also widely known as a digital terrain model (DTM). A DEM can be represented as a raster (a grid of squares) or as a triangular irregular network. DEMs are commonly built using remote sensing techniques, however, 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. Image File history File links Download high-resolution version (1274x972, 522 KB) 3D view of elevation model of Tithonium Chasma From http://astrogeology. ... Image File history File links Download high-resolution version (1274x972, 522 KB) 3D view of elevation model of Tithonium Chasma From http://astrogeology. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... For other uses, see Digital (disambiguation). ... An open surface with X-, Y-, and Z-contours shown. ... For discussion of land surfaces themselves, see Terrain. ... Imagine the smiley face in the top left corner as an RGB bitmap image. ... For other uses, see Square. ... A triangulated irregular network (TIN) is a digital data structure used in a geographic information system (GIS) for the representation of a surface. ... For the purported psychic ability to sense remotely, see Remote viewing right Synthetic aperture radar image of Death Valley colored using polarimetry In the broadest sense, remote sensing is the short or large-scale acquisition of information of an object or phenomenon, by the use of either recording or real... Surveyor at work with a leveling instrument. ... GIS redirects here. ... Terrain or relief is an essential aspect of physical geography, and as such its portrayal presents a central problem in cartography, and more recently GIS and 3D Visualization. ...

Production

Digital elevation models may be prepared in a number of ways, but they are frequently obtained by remote sensing rather than direct survey. One powerful technique for generating digital elevation models is interferometric synthetic aperture radar; two passes of a radar satellite (such as RADARSAT-1) suffice to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters. One also obtains an image of the surface cover. For the purported psychic ability to sense remotely, see Remote viewing right Synthetic aperture radar image of Death Valley colored using polarimetry In the broadest sense, remote sensing is the short or large-scale acquisition of information of an object or phenomenon, by the use of either recording or real... Surveyor at work with a leveling instrument. ... For the town in Russia, see Insar. ... RADARSAT-1 is Canadas first commercial Earth observation satellite. ...

Another powerful technique for generating a Digital Elevation Model is using the Digital image correlation method. It implies two optical images acquired with different angles taken from the same pass of an airplane or an Earth Observation satellite (such as the HRS intrument of SPOT5). SPOT (Satellite Pour lObservation de la Terre) refers to a series of commercial earth observation satellites launched by the French Space Agency CNES (Centre National dEtudes Spatiales). ...

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 the contour 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. In the mathematical subfield of numerical analysis, interpolation is a method of constructing new data points from a discrete set of known data points. ...

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;

Uses

Common uses of DEMs include:

• extracting terrain parameters
• modeling water flow or mass movement (for example avalanches)
• creation of relief maps
• rendering of 3D visualizations.
• 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 analyses in geomorphology and physical geography

The toe of an avalanche in Alaskas Kenai Fjords. ... Terrain or relief is an essential aspect of physical geography, and as such its portrayal presents a central problem in cartography, and more recently GIS and 3D Visualization. ... Visualization can refer to: Graphic Visualization as in any technique for creating images, diagrams, or animations to communicate any message. ... A raised-relief map or terrain model is a three dimensional representation, usually of terrain. ... The Georgian terrace of Royal Crescent (Bath, England) from a hot air balloon Intersection of E42 and E451 from an aircraft soon after takeoff from Frankfurt International Airport Moreton Island in Queensland, Australia Aerial photography is the taking of photographs of the ground while not supported by a ground-based... Satellite imagery consists of photographs of Earth or other planets made from artificial satellites. ... Gravity is a force of attraction that acts between bodies that have mass. ... Gravimetry is the measurement of a gravitational field. ... Definition Physical geodesy is the study of the physical properties of the gravity field of the Earth, the geopotential, with a view to their application in geodesy. ... Surface of the Earth Geomorphology is the study of landforms, including their origin and evolution, and the processes that shape them. ... True-color image of the Earths surface and atmosphere Physical geography (also know as geosystems or physiography) is a subfield of geography that focuses on the systematic study of patterns and processes within the hydrosphere, biosphere, atmosphere, and lithosphere. ...

Differences between DEMs and DTMs

A digital elevation model — also sometimes called a digital surface model (DSM) — generally refers to a representation of the earth's surface (or subset of this), including features such as vegetation, buildings, bridges, etc. The DEM often comprises much of the raw dataset, which may have been acquired through techniques such as photogrammetry, LiDAR, IfSAR, land surveying, etc. A digital terrain model (DTM) on the other hand is (generally) a filtered version of this surface, and hence can be a post-processed version of what was originally a DEM.^[1] The DTM provides a so-called bare-earth model, devoid of landscape features. While a DEM may be useful for landscape modelling, city modelling and visualisation applications, a DTM is often required for flood or drainage modelling, land-use studies, geological applications, and much more.^[2]

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 Shuttle Radar Topography Mission (SRTM) is also freely available for most of the globe and represents elevation at a 3 arc-second resolution (around 90 m). It has also been developed at 1 arc-second (30 m) resolution, but this has only been declassified for United States territory. The limitation with both 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. Submarine elevation (known as bathymetry) data is generated using ship mounted depth soundings. The SRTM30Plus dataset (used in NASA World Wind) attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model.^[3] GTOPO30 is a digital elevation model for the world, developed by USGS. It has a 30-arc second resolution, and is split into 33 tiles stored in the USGS DEM file format. ... The Shuttle Radar Topography Mission (SRTM) is a research effort that obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth to date. ... Bathymetry is the underwater equivalent to topography. ... Animation showing atmosphere and shading effects in v1. ...

The most usual grid (raster) is between 50 and 500 meters. In gravimetry e.g., the primary grid may be 50 m, but is switched to 100 or 500 meters in distances of about 5 or 10 kilometers. Suppose the smiley face in the top left corner is an RGB bitmap image. ...

Many national mapping agencies produce their own DEMs, often of a higher resolution and quality, but frequently these have to be purchased, and the cost is usually prohibitive to all except public authorities and large corporations.

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).^[4] Adjectives: Martian Atmosphere Surface pressure: 0. ... The Mars Global Surveyor (MGS) was a US spacecraft developed by NASA and the Jet Propulsion Laboratory and launched November 1996. ...

United States

The US Geological Survey produces the National Elevation Dataset, a seamless DEM for the contiguous United States, Hawaii and Puerto Rico based on 7.5' topographic mapping. As of the beginning of 2006, this replaces the earlier DEM tiled format (one DEM per USGS topographic map).^[5][6] The United States Geological Survey (USGS) is a scientific agency of the United States government. ... // Topographic maps are a variety of maps characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. ...

References

• Paula Messina. Terrain Analysis Home Page. Spatial Analysis and Remote Sensing Lab at Hunter College. Retrieved on 2007-02-16.
• Wilson, J.P.; Gallant, J.C. (2000). "Chapter 1", in Wilson, J.P., and Gallant, J.C. (Eds.): Terrain Analysis: Principles and Applications, 1–27. ISBN 0471321885. Retrieved on 2007-02-16. 

1. ^ Digital Elevation Models. Infoterra.
2. ^ "Appendix A – Glossary and Acronyms", Severn Tidal Tributaries Catchment Flood Management Plan – Scoping Stage. UK: Environment Agency. 
3. ^ see Martin Gamache's paper on free sources of global data, http://www.terrainmap.com/downloads/Gamache_final_web.pdf
4. ^ A basic guide for using Digital Elevation Models with Terragen
5. ^ http://eros.usgs.gov/DEMNotice.html
6. ^ see Herbert Glarner's paper on using USGS data, http://herbert.gandraxa.com/herbert/dem.asp

Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 47th day of the year in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 47th day of the year in the Gregorian calendar. ... (see also the List of environmental organizations) The Environment Agency (Welsh: Asiantaeth yr Amgylchedd) of England and Wales was created by the Environment Act 1995, along with the Scottish Environment Protection Agency. ...

DEM file formats

• USGS DEM
• SDTS DEM
• DTED
• DIMAP

The USGS DEM standard is a geospatial file format developed by the United States Geological Survey for storing a raster-based digital elevation model. ... Spatial Data Transfer Standard, or SDTS, is a standard used to describe earth-referenced spatial data. ... Digital Terrain Elevation Data [DTED] http://www. ...

External links

• Geo-Spatial Data Acquisition Homepage
• GTOPO30 Homepage
• GTOPO30 FTP Server
• SRTM Homepage
• SRTM30 Plus Homepage
• Terrainmap Homepage
• More information about available DEM data
• More information about DEM by Spot Image