Editing Digital elevation model (section)

==Sources==

=== Global ===
Released at the beginning of 2022, [https://www.fathom.global/academic-papers/a-30-m-global-map-of-elevation-with-forests-and-buildings-removed/ FABDEM] offers a bare earth simulation of the Earth's surface at 30 arc-second resolution. Adapted from GLO-30, the data removes all forests and buildings. The data is free to download non-commercially and through the [https://www.fathom.global/product/fabdem/ developer's website] at a cost commercially.

An alternative free global DEM is called [[GTOPO30]] (30 [[arcsecond]] [[Angular resolution|resolution]], c. 1&nbsp;[[km]]  along the equator) 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. 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&nbsp;arc-second resolution (around 90&nbsp;meters  along the equator). 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&mdash;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 sounding]]s. When land topography and bathymetry is combined, a truly [[global relief model]]  is obtained.   The SRTM30Plus dataset (used in [[NASA World Wind]]) attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model.<ref>{{cite web|url=http://www.terrainmap.com/downloads/Gamache_final_web.pdf|title=Martin Gamache's paper on free sources of global data}}</ref>  The Earth2014 global topography and relief model<ref name=HirtRexer2015>{{cite journal|last1=Hirt|first1=C.|last2=Rexer|first2=M.|title=Earth2014: 1 arc-min shape, topography, bedrock and ice-sheet models - available as gridded data and degree-10,800 spherical harmonics|journal=International Journal of Applied Earth Observation and Geoinformation|volume=39|pages=103–112|url=http://ddfe.curtin.edu.au/models/Earth2014/Hirt_Rexer2015_Earth2014.pdf|access-date=February 20, 2016|doi=10.1016/j.jag.2015.03.001|year=2015|bibcode=2015IJAEO..39..103H|hdl=20.500.11937/25468|hdl-access=free}}</ref> provides layered topography grids at 1 arc-minute resolution. Other than SRTM30plus, Earth2014 provides information on ice-sheet heights and bedrock (that is, topography below the ice) over Antarctica and Greenland. Another global model is Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) with 7.5 arc second resolution. It is based on SRTM data and combines other data outside SRTM coverage. A novel global DEM of postings lower than 12&nbsp;m and a height accuracy of less than 2&nbsp;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&nbsp;meters. In gravimetry e.g., the primary grid may be 50&nbsp;m, but is switched to 100 or 500&nbsp;meters in distances of about 5 or 10 kilometers.

Since 2002, the HRS instrument on SPOT 5 has acquired over 100 million square kilometers of stereo pairs used to produce a DTED2  format DEM (with a 30-meter posting) DEM format DTED2 over 50 million km<sup>2</sup>.<ref>{{cite web|url=http://www.astrium-geo.com/en/66-geo-elevation-and-dem|title=GEO Elevation Services : Airbus Defence and Space|website=www.astrium-geo.com|access-date=2012-01-11|archive-date=2014-06-26|archive-url=https://web.archive.org/web/20140626025812/http://www.astrium-geo.com/en/66-geo-elevation-and-dem|url-status=dead}}</ref> The radar satellite [[RADARSAT-2]] has been used by [[MacDonald, Dettwiler and Associates Ltd.]] to provide DEMs for commercial and military customers.<ref>{{cite web|url=http://gs.mdacorporation.com/Mapping/DigitalElevationModels.aspx|title=International - Geospatial|website=gs.mdacorporation.com|access-date=2012-02-02|archive-date=2016-03-04|archive-url=https://web.archive.org/web/20160304121452/http://gs.mdacorporation.com/Mapping/DigitalElevationModels.aspx|url-status=dead}}</ref>

In 2014, acquisitions from radar satellites TerraSAR-X and TanDEM-X will be available in the form of a uniform global coverage with a resolution of 12&nbsp;meters.<ref>{{cite web|url=http://www.astrium-geo.com/terrasar-x/|title=TerraSAR-X : Airbus Defence and Space|website=www.astrium-geo.com|access-date=2012-01-11|archive-date=2014-08-12|archive-url=https://web.archive.org/web/20140812160313/http://www.astrium-geo.com/terrasar-x/|url-status=dead}}</ref>

ALOS provides since 2016 a global 1-arc second DSM free of charge,<ref>{{cite web|url=http://www.eorc.jaxa.jp/ALOS/en/aw3d30/|title=ALOS World 3D - 30m|website=www.eorc.jaxa.jp|access-date=2017-09-09|archive-date=2020-05-04|archive-url=https://web.archive.org/web/20200504132807/https://www.eorc.jaxa.jp/ALOS/en/aw3d30/|url-status=dead}}</ref> and a commercial 5 meter DSM/DTM.<ref>{{cite web|url=http://www.aw3d.jp/en/products/|title=ALOS World 3D|website=www.aw3d.jp}}</ref>

=== Local ===
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 dataset]] programs.

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).<ref>{{cite web|url=http://www2.cs.uh.edu/~somalley/DemTutorial/#DEM|title=A basic guide for using Digital Elevation Models with Terragen|url-status=dead|archive-url=https://web.archive.org/web/20070519135415/http://www2.cs.uh.edu/~somalley/DemTutorial/#DEM|archive-date=2007-05-19}}</ref>

=== Websites ===
OpenTopography<ref name="OpenTopography">{{cite web|url=http://www.opentopography.org/|title=OpenTopography|website=www.opentopography.org}}</ref> is a web based community resource for access to high-resolution, Earth science-oriented, topography data (lidar and DEM data), and processing tools running on commodity and high performance compute system along with educational resources.<ref name="opentopography.org">{{cite web|url=http://www.opentopography.org/about/|title=About OpenTopography}}</ref> OpenTopography is based at the San Diego Supercomputer Center<ref>{{Cite web|url=http://www.sdsc.edu|title=San Diego Supercomputer Center|website=www.sdsc.edu|access-date=2018-08-16}}</ref> at the University of California San Diego and is operated in collaboration with colleagues in the School of Earth and Space Exploration at Arizona State University and UNAVCO.<ref>{{Cite web|url=http://www.unavco.org|title=Home {{!}} UNAVCO|website=www.unavco.org|language=en-US|access-date=2018-08-16}}</ref> Core operational support for OpenTopography comes from the National Science Foundation, Division of Earth Sciences.

The OpenDemSearcher is a Mapclient with a visualization of regions with free available middle and high resolution DEMs.<ref>[http://www.opendem.info/opendemsearcher.html  OpenDemSearcher]</ref>
[[File:Moon_elevation.stl|thumb|[[STL (file format)|STL 3D model]] of the [[Moon]] with 10× elevation exaggeration rendered with data from the [[Lunar Orbiter Laser Altimeter]] of the [[Lunar Reconnaissance Orbiter]] ]]