A
digital elevation model is a digital model or 3-D representation of a terrain's surface — commonly for a planet (including Earth), moon, or asteroid — created from terrain
elevationThe elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....
data.
There is no common usage of the terms
digital elevation model (DEM),
digital terrain model (DTM) and
digital surface model (DSM) in scientific literature. In the 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 represents the bare ground surface without any objects like plants and buildings (see Figure on the right).
The term Digital Elevation Model 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 is also representing other morphological elements. There are also definitions which equalise the terms DEM and DSM.
In the Web definitions can be found which define the DEM as a digital regularly spaced GRID and a DTM as a real three-dimensional model (TIN).
Most of the data providers (USGS,
ERSDACASTER is a Japanese sensor which is one of five remote sensory devices on board the Terra satellite launched into Earth orbit by NASA in 1999...
, CGIAR ) 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 GDEMASTER is a Japanese sensor which is one of five remote sensory devices on board the Terra satellite launched into Earth orbit by NASA in 1999...
). It is possible to compute a DTM from high resolution DSM datasets with complex algorithms (Li et al, 2005).
In the following the term DEM is used as a generic term for DSMs and DTMs.
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 as a primary (measured) DEM, whereas the Raster DEM is referred as a secondary (computed) DEM.
DEMs are commonly built 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.
The DEM could be acquired through techniques such as
photogrammetryPhotogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....
,
LiDARLIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
,
IfSARIfsar is a village in the municipality of Čajniče, Bosnia and Herzegovina.-References:...
, land surveying, etc. (Li et al. 2005). 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 much more.
Production
Mappers may prepare digital elevation models in a number of ways, but they frequently use
remote sensingRemote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
rather than direct
surveySee Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
data. One powerful technique for generating digital elevation models is
interferometric synthetic aperture radarInterferometric synthetic aperture radar, also abbreviated InSAR or IfSAR, is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar images to generate maps of surface deformation or digital elevation, using differences in the phase of...
: two passes of a radar satellite (such as
RADARSAT-1Radarsat-1 is Canada's first commercial Earth observation satellite.-Mission:It was launched at 14h22 UTC on November 4, 1995 from Vandenberg AFB in California, into a sun-synchronous orbit above the Earth with an altitude of 798 kilometers and inclination of 98.6 degrees...
or
TerraSAR-XTerraSAR-X, a German Earth observation satellite, is a joint venture being carried out under a public-private-partnership between the German Aerospace Center DLR and EADS Astrium GmbH; the exclusive commercial exploitation rights are held by the geo-information service provider Infoterra GmbH....
), or a single pass if the satellite is equipped with two antennas (like the
SRTMThe Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...
instrumentation), suffice to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters . Alternatively, other kinds of stereoscopic pairs can be employed using the
digital image correlationDigital Image Correlation and Tracking is an optical method that employs tracking & image registration techniques for accurate 2D and 3D measurements of changes in images. This is often used to measure deformation , displacement, and strain, but it is widely applied in many areas of science and...
method, where two optical images acquired with different angles taken from the same pass of an airplane or an
Earth Observation SatelliteEarth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc....
(such as the HRS instrument of
SPOT5SPOT is a high-resolution, optical imaging Earth observation satellite system operating from space. It is run by Spot Image based in Toulouse, France...
or the
VNIRVNIR is an initialism for visible and near-infrared, a portion of the electromagnetic spectrum having wavelengths between approximately 400 and 1400 nanometers ...
band of
ASTERASTER is a Japanese sensor which is one of five remote sensory devices on board the Terra satellite launched into Earth orbit by NASA in 1999...
).
In 1986, the
SPOT 1 satelliteSPOT is a high-resolution, optical imaging Earth observation satellite system operating from space. It is run by Spot Image based in Toulouse, France...
provided the first usable elevation data for a sizeable portion of the planet's landmass, using two-passes stereoscopic correlation. Later, further data were provided by the
European Remote-Sensing SatelliteEuropean remote sensing satellite was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.-Instruments:...
(ERS) using the same method, the
Shuttle Radar Topography MissionThe Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...
using single-pass SAR and the
ASTERASTER is a Japanese sensor which is one of five remote sensory devices on board the Terra satellite launched into Earth orbit by NASA in 1999...
instrumentation on the Terra satellite using double-pass stereo pairs.
Older methods of generating DEMs often involve
interpolatingIn the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
digital contour maps that may have been produced by direct survey of the land surface; this method is still used in
mountainImage:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...
areas, where
interferometryInterferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
is not always satisfactory. Note that the
contour lineA contour line of a function of two variables is a curve along which the function has a constant value. In cartography, a contour line joins points of equal elevation above a given level, such as mean sea level...
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 pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
size;
- interpolation algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
;
- vertical resolution;
- terrain analysis algorithm;
Methods for obtaining elevation data used to create DEMs
- LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
- Stereo photogrammetry
Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....
from aerial surveys
- Real Time Kinematic
Real Time Kinematic satellite navigation is a technique used in land survey and in hydrographic survey based on the use of carrier phase measurements of the GPS, GLONASS and/or Galileo signals where a single reference station provides the real-time corrections, providing up to centimetre-level...
GPS
- Topographic map
A topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. Traditional definitions require a topographic map to show both natural and man-made features...
s
- Theodolite
A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology...
or total stationA total station is an electronic/optical instrument used in modern surveying. The total station is an electronic theodolite integrated with an electronic distance meter to read slope distances from the instrument to a particular point....
- Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...
- Focus variation
Focus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field.-Algorithm:The algorithm work as follows:# at first images with difference focus are captured...
- Inertial surveys
Uses
Common uses of DEMs include:
- Extracting terrain parameters
- Modeling water flow or mass movement (for example avalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...
s and landslideA landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
s)
- Creation of relief maps
- Rendering of 3D visualizations.
- 3d flight planning
- Creation of physical models (including raised-relief map
A raised-relief map or terrain model is a three-dimensional representation, usually of terrain. When representing terrain, the elevation dimension is usually exaggerated by a factor between five and ten; this facilitates the visual recognition of terrain features.-History:In his 1665 paper for the...
s)
- Rectification of aerial photography
Aerial photography is the taking of photographs of the ground from an elevated position. The term usually refers to images in which the camera is not supported by a ground-based structure. Cameras may be hand held or mounted, and photographs may be taken by a photographer, triggered remotely or...
or satellite imagerySatellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
.
- Reduction (terrain correction) of gravity measurements (gravimetry
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of gravitational field or the properties of matter responsible for its creation are of interest...
, physical geodesyPhysical 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.-Measurement procedure:...
).
- Terrain analyses in geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
and physical geographyPhysical geography is one of the two major subfields of geography. Physical geography is that branch of natural science which deals with the study of processes and patterns in the natural environment like the atmosphere, biosphere and geosphere, as opposed to the cultural or built environment, the...
- Geographic Information System
A geographic information system, geographical information science, or geospatial information studies is a system designed to capture, store, manipulate, analyze, manage, and present all types of geographically referenced data...
s (GIS)
- Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
and infrastructureInfrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...
design
- Global positioning systems (GPS)
- Line-of-sight analysis
- Base mapping
- Flight simulation
- Precision farming and forestry
Forestry is the interdisciplinary profession embracing the science, art, and craft of creating, managing, using, and conserving forests and associated resources in a sustainable manner to meet desired goals, needs, and values for human benefit. Forestry is practiced in plantations and natural stands...
- Surface analysis
- Intelligent transportation systems (ITS)
- Auto safety / Advanced Driver Assistance Systems
Advanced Driver Assistance Systems, or ADAS, are systems to help the driver in its driving process. When designed with a safe Human-Machine Interface it should increase car safety and more generally road safety.Examples of such a system are:...
(ADAS)
Sources
A free DEM of the whole world called
GTOPO30GTOPO30 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.-See also:* Shuttle Radar Topography Mission...
(30 arcsecond
resolutionAngular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
, approx. 1
kmKM, Km, or km may stand for:*Kilometre *Kernel methods*Kettle Moraine High School*Khmer language *Kuomintang , a centre-right political party in the Republic of China on Taiwan...
) 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 RadiometerASTER is a Japanese sensor which is one of five remote sensory devices on board the Terra satellite launched into Earth orbit by NASA in 1999...
(ASTER) instrument of the Terra satellite is also freely available for 99% of the globe, and represents elevation at a 30 meter resolution. A similarly high resolution was previously only available for the
United States territoryUnited States territory is any extent of region under the jurisdiction of the federal government of the United States, including all waters including all U.S. Naval carriers. The United States has traditionally proclaimed the sovereign rights for exploring, exploiting, conserving, and managing its...
under the
Shuttle Radar Topography MissionThe Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...
(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. 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
bathymetryBathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
) data is generated using ship-mounted
depth soundingDepth sounding refers to a historical nautical term for measuring depth; it is often referred to simply as sounding. Sounding is finding the depth of a given point in a body of water. Sounding data is used in bathymetry to make maps of the floor of a body of water. Soundings were traditionally...
s. The SRTM30Plus dataset (used in
NASA World WindWorld Wind is an open-source virtual globe developed by NASA and the open source community for use on personal computers. Old versions need Microsoft Windows but the more recent Java version, , is cross platform and provides a suite of . The World Wind Java version was awarded in November 2009...
) attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model. A novel global DEM of postings lower than 12m and a height accuracy of less than 2m is expected being generated by the
TanDEM-XTanDEM-X is the name of TerraSAR-X's twin satellite, a German Earth observation satellite using SAR - a modern radar imaging technology. It is a second, almost identical spacecraft to TerraSAR-X...
satellite mission which started in July 2010.
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.
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. DEMs are often a product of
National LIDAR DatasetA National LIDAR Dataset refers to a high-resolution LIDAR dataset comprising most – and ideally all - of a nation’s terrain. Datasets of this type typically meet specified quality standards and are publicly available for free in one or more uniform formats from government or academic sources...
programs.
Free DEMs are also available for
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
: the MEGDR, or Mission Experiment Gridded Data Record, from the
Mars Global SurveyorThe Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...
's Mars Orbiter Laser Altimeter (MOLA) instrument; and NASA's Mars Digital Terrain Model (DTM).
United States
The US Geological Survey produces the
National Elevation DatasetThe National Elevation Dataset consists of high precision ground surface elevation data for the United States. It is maintained by the USGS and all the data are in public domain-Source:...
, 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 mapA topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. Traditional definitions require a topographic map to show both natural and man-made features...
).
See also
- TerraSAR-X
TerraSAR-X, a German Earth observation satellite, is a joint venture being carried out under a public-private-partnership between the German Aerospace Center DLR and EADS Astrium GmbH; the exclusive commercial exploitation rights are held by the geo-information service provider Infoterra GmbH....
:a German Earth observation satellite
- TanDEM-X
TanDEM-X is the name of TerraSAR-X's twin satellite, a German Earth observation satellite using SAR - a modern radar imaging technology. It is a second, almost identical spacecraft to TerraSAR-X...
: generation of a world-wide, consistent, timely, high-precision Digital Elevation Model
DEM file formats
- USGS DEM
The USGS DEM standard is a geospatial file format developed by the United States Geological Survey for storing a raster-based digital elevation model. It is an open standard, and is used throughout the world...
- SDTS
Spatial Data Transfer Standard, or SDTS, is a standard used to describe earth-referenced spatial data. It was designed to easily transfer and use spatial data on different computer platforms.-External links:* ** *...
DEM
- DTED
DTED is a standard of digital datasets which consists of a matrix of terrain elevation values. This standard was originally developed in the 1970s to support aircraft radar simulation and prediction....
- DIMAP
External links