Editing Digital elevation model (section)

==Production==
Mappers may prepare digital elevation models in a number of ways, but they frequently use [[remote sensing]] rather than direct [[Surveying|survey]] data.

Older methods of generating DEMs often involve [[interpolation|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.

===Satellite mapping===
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 [[Shuttle Radar Topography Mission|SRTM]] instrumentation), collect sufficient data to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters.<ref>{{cite web|url=http://www.intelligence-airbusds.com/worlddem/|title=WorldDEM(TM): Airbus Defence and Space|website=www.intelligence-airbusds.com|access-date=2018-01-05|archive-date=2018-06-04|archive-url=https://web.archive.org/web/20180604103046/http://www.intelligence-airbusds.com/worlddem/|url-status=dead}}</ref> 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 [[SPOT (satellites)|SPOT5]] or the [[VNIR]] band of [[Advanced Spaceborne Thermal Emission and Reflection Radiometer|ASTER]]).<ref name=Nikolakopoulous>{{cite journal|last1=Nikolakopoulos |first1=K. G. |last2=Kamaratakis |first2=E. K |last3=Chrysoulakis |first3=N. |date=10 November 2006 |title=SRTM vs ASTER elevation products. Comparison for two regions in Crete, Greece |journal=International Journal of Remote Sensing |volume=27 |issue=21 |pages=4819–4838 |issn=0143-1161 |url=http://www.iacm.forth.gr/_docs/pubs/4/Nikolakopoulos_et_al_2006.pdf |access-date=June 22, 2010 |doi=10.1080/01431160600835853 |bibcode=2006IJRS...27.4819N |s2cid=1939968 |url-status=dead |archive-url=https://web.archive.org/web/20110721081314/http://www.iacm.forth.gr/_docs/pubs/4/Nikolakopoulos_et_al_2006.pdf |archive-date=July 21, 2011 }}</ref>

The [[SPOT (satellites)|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.<ref name=Nikolakopoulous/>

The HRS instrument on SPOT 5 has acquired over 100 million square kilometers of stereo pairs.

===Planetary mapping===
[[image:PIA02040 Martian hemispheres by MOLA.jpg|upright=1.2|thumb|MOLA digital elevation model showing the two hemispheres of Mars. This image appeared on the cover of ''Science'' magazine in May 1999.]]
A tool of increasing value in [[planetary science]] has been use of orbital altimetry used to make digital elevation map of planets.  A primary tool for this is [[Lidar|laser altimetry]] but radar altimetry is also used.<ref>{{Citation|last1=Hargitai|first1=Henrik|title=Methods in Planetary Topographic Mapping: A Review|date=2019|work=Planetary Cartography and GIS|pages=147–174|editor-last=Hargitai|editor-first=Henrik|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-62849-3_6|isbn=978-3-319-62848-6|last2=Willner|first2=Konrad|last3=Buchroithner|first3=Manfred|series=Lecture Notes in Geoinformation and Cartography |s2cid=133855780}}</ref> Planetary digital elevation maps made using laser altimetry include the [[Mars Orbiter Laser Altimeter]] (MOLA) mapping of Mars,<ref name=" Banerdt">Bruce Banerdt, [https://mars.nasa.gov/MPF/martianchronicle/martianchron3/marschro35.html Orbital Laser Altimeter], ''The Martian Chronicle, Volume 1'', No. 3, NASA.  Retrieved 11 March 2019.</ref> the [[Lunar Orbital Laser Altimeter]] (LOLA)<ref>NASA, [https://lola.gsfc.nasa.gov LOLA].  Retrieved 11 March 2019.</ref> and Lunar Altimeter (LALT) mapping of the Moon, and the Mercury Laser Altimeter (MLA) mapping of Mercury.<ref>John F. Cavanaugh, ''et al.,'' "[http://www-geodyn.mit.edu/cavanaugh.mla.ssr07.pdf The Mercury Laser Altimeter Instrument for the MESSENGER Mission]", ''Space Sci Rev'', DOI 10.1007/s11214-007-9273-4, 24 August 2007.  Retrieved 11 March 2019.</ref> In planetary mapping, each planetary body has a unique reference surface.<ref>{{Citation|last1=Hargitai|first1=Henrik|title=Fundamental Frameworks in Planetary Mapping: A Review|date=2019|work=Planetary Cartography and GIS|pages=75–101|editor-last=Hargitai|editor-first=Henrik|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-62849-3_4|isbn=978-3-319-62848-6|last2=Willner|first2=Konrad|last3=Hare|first3=Trent|series=Lecture Notes in Geoinformation and Cartography |s2cid=133867607}}</ref> [[New Horizons|New Horizons']] Long Range Reconnaissance Imager used stereo photogrammetry to produce partial surface elevation maps of [[Pluto]] and [[486958 Arrokoth]].<ref>{{cite web |title=Astropedia - Pluto New Horizons LORRI - MVIC Global DEM 300m |url=https://astrogeology.usgs.gov/search/map/pluto_new_horizons_lorri_mvic_global_dem_300m |website=astrogeology.usgs.gov}}</ref><ref>{{cite journal |last1=Schenk |first1=Paul |last2=Singer |first2=Kelsi |last3=Beyer |first3=Ross |last4=Beddingfield |first4=Chloe |last5=Robbins |first5=Stuart J. |last6=McKinnon |first6=William B. |last7=Lauer |first7=Tod R. |last8=Verbiscer |first8=Anne J. |last9=Keane |first9=James. T. |last10=Dhingra |first10=Rajani D. |last11=Moore |first11=Jeffrey |last12=Parker |first12=Joel W. |last13=Olkin |first13=Cathy |last14=Spencer |first14=John |last15=Weaver |first15=Hal |last16=Stern |first16=S. Alan |title=Origins of pits and troughs and degradation on a small primitive planetesimal in the Kuiper Belt: high-resolution topography of (486958) Arrokoth (aka 2014 MU69) from New Horizons |journal=Icarus |date=1 March 2021 |volume=356 |pages=113834 |doi=10.1016/j.icarus.2020.113834 |issn=0019-1035}}</ref>

===Methods for obtaining elevation data used to create DEMs===

[[File:GatewingX100.jpg|thumb|Gatewing X100 [[unmanned aerial vehicle]]]]

* [[Lidar]]<ref name="Campbell"/>
* [[Radar]]
* [[Stereo photogrammetry]] from [[aerial surveys]]
** [[Structure from motion]] / Multi-view stereo applied to aerial photography<ref>{{Cite journal | doi=10.1029/2011JF002289| title=Straightforward reconstruction of 3D surfaces and topography with a camera: Accuracy and geoscience application| journal=Journal of Geophysical Research: Earth Surface| volume=117| year=2012| last1=James| first1=M. R.| last2=Robson| first2=S.| issue=F3| pages=n/a| bibcode=2012JGRF..117.3017J| url=https://eprints.lancs.ac.uk/id/eprint/56018/1/James_and_Robson_2012_SfM_MVS.pdf| doi-access=free}}</ref>
* Block adjustment from optical satellite imagery
* Interferometry from radar data
* [[Real Time Kinematic]] [[GPS]]
* [[Topographic map]]s
* [[Theodolite]] or [[total station]]
* [[Doppler radar]]
* [[Focus variation]]
* Inertial surveys
* Surveying and mapping [[UAV|drones]]
* [[Range imaging]]

===Accuracy===

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). Quality assessment of DEM can be performed by comparison of DEMs from different sources.<ref>{{cite journal |last1=Szypuła |first1=Bartłomiej |title=Quality assessment of DEM derived from topographic maps for geomorphometric purposes |journal=Open Geosciences |date=1 January 2019 |volume=11 |issue=1 |pages=843–865 |doi=10.1515/geo-2019-0066 |bibcode=2019OGeo...11...66S |url=https://doi.org/10.1515/geo-2019-0066 |language=en |issn=2391-5447|hdl=20.500.12128/11742 |s2cid=208868204 |hdl-access=free }}</ref> 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.