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Digital elevation model
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A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, moon, or asteroid. A "global DEM" refers to a discrete global grid. DEMs are used often in geographic information systems (GIS), and are the most common basis for digitally produced relief maps. A digital terrain model (DTM) represents specifically the ground surface while DEM and DSM may represent tree top canopy or building roofs.
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,[1] geological applications, and other applications,[2] and in planetary science.
Terminology
[edit]
There is no universal usage of the terms digital elevation model (DEM), digital terrain model (DTM) and digital surface model (DSM) in scientific literature. 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 (see the figure on the right).[3][4]
DEM is often used as a generic term for DSMs and DTMs,[5] only representing height information without any further definition about the surface.[6] Other definitions equalise the terms DEM and DTM,[7] equalise the terms DEM and DSM,[8] define the DEM as a subset of the DTM, which also represents other morphological elements,[9] or define a DEM as a rectangular grid and a DTM as a three-dimensional model (TIN).[10] Most of the data providers (USGS, ERSDAC, CGIAR, Spot Image) use the term DEM as a generic term for DSMs and DTMs. Some datasets such as SRTM or the ASTER GDEM are originally DSMs, although in forested areas, SRTM reaches into the tree canopy giving readings somewhere between a DSM and a DTM). DTMs are created from high resolution DSM datasets using complex algorithms to filter out buildings and other objects, a process known as "bare-earth extraction".[11][12] In the following, the term DEM is used as a generic term for DSMs and DTMs.
Types
[edit]
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).[13] 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.[14] The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR or InSAR, land surveying, etc. (Li et al. 2005).
DEMs are commonly built using data collected using remote sensing techniques, but they may also be built from land surveying.
Rendering
[edit]
The digital elevation model itself consists of a matrix of numbers, but the data from a DEM is often rendered in visual form to make it understandable to humans. This visualization may be in the form of a contoured topographic map, or could use shading and false color assignment (or "pseudo-color") to render elevations as colors (for example, using green for the lowest elevations, shading to red, with white for the highest elevation.).
Visualizations are sometimes also done as oblique views, reconstructing a synthetic visual image of the terrain as it would appear looking down at an angle. In these oblique visualizations, elevations are sometimes scaled using "vertical exaggeration" in order to make subtle elevation differences more noticeable.[15] Some scientists,[16] [17] however, object to vertical exaggeration as misleading the viewer about the true landscape.
Production
[edit]Mappers may prepare digital elevation models in a number of ways, but they frequently use remote sensing rather than direct survey data.
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 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
[edit]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.[18] 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).[19]
The 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.[19]
The HRS instrument on SPOT 5 has acquired over 100 million square kilometers of stereo pairs.
Planetary mapping
[edit]
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 laser altimetry but radar altimetry is also used.[20] Planetary digital elevation maps made using laser altimetry include the Mars Orbiter Laser Altimeter (MOLA) mapping of Mars,[21] the Lunar Orbital Laser Altimeter (LOLA)[22] and Lunar Altimeter (LALT) mapping of the Moon, and the Mercury Laser Altimeter (MLA) mapping of Mercury.[23] In planetary mapping, each planetary body has a unique reference surface.[24] New Horizons' Long Range Reconnaissance Imager used stereo photogrammetry to produce partial surface elevation maps of Pluto and 486958 Arrokoth.[25][26]
Methods for obtaining elevation data used to create DEMs
[edit]
- Lidar[27]
- Radar
- Stereo photogrammetry from aerial surveys
- Structure from motion / Multi-view stereo applied to aerial photography[28]
- Block adjustment from optical satellite imagery
- Interferometry from radar data
- Real Time Kinematic GPS
- Topographic maps
- Theodolite or total station
- Doppler radar
- Focus variation
- Inertial surveys
- Surveying and mapping drones
- Range imaging
Accuracy
[edit]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.[29] 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.
Uses
[edit]
Common uses of DEMs include:
- Extracting terrain parameters for geomorphology
- Modeling water flow for hydrology or mass movement (for example avalanches and landslides)
- Modeling soils wetness with Cartographic Depth to Water Indexes (DTW-index)[27]
- Creation of relief maps
- Rendering of 3D visualizations.
- 3D flight planning and TERCOM
- Creation of physical models (including raised relief maps and 3D printed terrain models)[30]
- Rectification of aerial photography or satellite imagery
- Reduction (terrain correction) of gravity measurements (gravimetry, physical geodesy)
- Terrain analysis in geomorphology and physical geography
- Geographic information systems (GIS)
- Engineering and infrastructure design
- Satellite navigation (for example GPS and GLONASS)
- Line-of-sight analysis
- Base mapping
- Flight simulation
- Train simulation
- Precision farming and forestry[31]
- Surface analysis
- Intelligent transportation systems (ITS)
- Auto safety / advanced driver-assistance systems (ADAS)
- Archaeology
Sources
[edit]Global
[edit]Released at the beginning of 2022, 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 developer's website at a cost commercially.
An alternative free global DEM is called GTOPO30 (30 arcsecond resolution, c. 1 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 arc-second resolution (around 90 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—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 soundings. 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.[32] The Earth2014 global topography and relief model[33] 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 m and a height accuracy of less than 2 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 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.
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 km2.[34] The radar satellite RADARSAT-2 has been used by MacDonald, Dettwiler and Associates Ltd. to provide DEMs for commercial and military customers.[35]
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 meters.[36]
ALOS provides since 2016 a global 1-arc second DSM free of charge,[37] and a commercial 5 meter DSM/DTM.[38]
Local
[edit]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).[39]
Websites
[edit]OpenTopography[40] 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.[41] OpenTopography is based at the San Diego Supercomputer Center[42] 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.[43] 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.[44]
See also
[edit]- Ground slope and aspect (ground spatial gradient)
- Digital outcrop model
- Global Relief Model
- Physical terrain model
- Terrain cartography
- Terrain rendering
DEM file formats
[edit]- Bathymetric Attributed Grid (BAG)
- DTED
- DIMAP Sentinel 1 ESA data base
- SDTS DEM
- USGS DEM
References
[edit]- ^ I. Balenovic, H. Marjanovic, D. Vuletic, etc. Quality assessment of high density digital surface model over different land cover classes. PERIODICUM BIOLOGORUM. VOL. 117, No 4, 459–470, 2015.
- ^ "Appendix A – Glossary and Acronyms" (PDF). Severn Tidal Tributaries Catchment Flood Management Plan – Scoping Stage. UK: Environment Agency. Archived from the original (PDF) on 2007-07-10.
- ^ "Intermap Digital Surface Model: accurate, seamless, wide-area surface models". Archived from the original on 2011-09-28.
- ^ Li, Z., Zhu, Q. and Gold, C. (2005), Digital terrain modeling: principles and methodology, CRC Press, Boca Raton, FL.
- ^ Hirt, C. (2014). "Digital Terrain Models" (PDF). Encyclopedia of Geodesy. pp. 1–6. doi:10.1007/978-3-319-02370-0_31-1. ISBN 978-3-319-01868-3. Retrieved October 14, 2024.
- ^ Peckham, Robert Joseph; Jordan, Gyozo (Eds.)(2007): Development and Applications in a Policy Support Environment Series: Lecture Notes in Geoinformation and Cartography. Heidelberg.
- ^ Podobnikar, Tomaz (2008). "Methods for visual quality assessment of a digital terrain model". S.A.P.I.EN.S. 1 (2).
- ^ Adrian W. Graham, Nicholas C. Kirkman, Peter M. Paul (2007): Mobile radio network design in the VHF and UHF bands: a practical approach. West Sussex.
- ^ "DIN Standard 18709-1". Archived from the original on 2011-01-11.
- ^ "Landslide Glossary USGS". Archived from the original on 2011-05-16.
- ^ Li, Z., Zhu, Q. and Gold, C. (2005), Digital terrain modeling: principles and methodology, CRC Press, Boca Raton, FL.
- ^ "Understanding Digital Surface Models, Digital Terrain Models and Digital Elevation Models: A Comprehensive Guide to Digital Models of the Earth's Surface". FlyGuys. March 2023. Retrieved 7 September 2023.
- ^ DeMers, Michael (2002). GIS Modeling in Raster. Wiley. ISBN 978-0-471-31965-8.
- ^ Ronald Toppe (1987): Terrain models — A tool for natural hazard Mapping Archived 2020-07-29 at the Wayback Machine. In: Avalanche Formation, Movement and Effects (Proceedings of the Davos Symposium, September 1986). IAHS Publ. no. 162,1987
- ^ Making 3D Terrain Maps, Shaded Relief. Retrieved 11 March 2019.
- ^ David Morrison, ""Flat-Venus Society" organizes", EOS, Volume 73, Issue 9, American Geophysical Union, 3 March 1992, p. 99. doi:10.1029/91EO00076. Retrieved 11 March 2019.
- ^ Robert Simmon. "Elegant Figures What Not To Do: Vertical Exaggeration," NASA Earth Observatory, November 5, 2010. Retrieved 11 March 2019.
- ^ "WorldDEM(TM): Airbus Defence and Space". www.intelligence-airbusds.com. Archived from the original on 2018-06-04. Retrieved 2018-01-05.
- ^ a b Nikolakopoulos, K. G.; Kamaratakis, E. K; Chrysoulakis, N. (10 November 2006). "SRTM vs ASTER elevation products. Comparison for two regions in Crete, Greece" (PDF). International Journal of Remote Sensing. 27 (21): 4819–4838. Bibcode:2006IJRS...27.4819N. doi:10.1080/01431160600835853. ISSN 0143-1161. S2CID 1939968. Archived from the original (PDF) on July 21, 2011. Retrieved June 22, 2010.
- ^ Hargitai, Henrik; Willner, Konrad; Buchroithner, Manfred (2019), Hargitai, Henrik (ed.), "Methods in Planetary Topographic Mapping: A Review", Planetary Cartography and GIS, Lecture Notes in Geoinformation and Cartography, Springer International Publishing, pp. 147–174, doi:10.1007/978-3-319-62849-3_6, ISBN 978-3-319-62848-6, S2CID 133855780
- ^ Bruce Banerdt, Orbital Laser Altimeter, The Martian Chronicle, Volume 1, No. 3, NASA. Retrieved 11 March 2019.
- ^ NASA, LOLA. Retrieved 11 March 2019.
- ^ John F. Cavanaugh, et al., "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.
- ^ Hargitai, Henrik; Willner, Konrad; Hare, Trent (2019), Hargitai, Henrik (ed.), "Fundamental Frameworks in Planetary Mapping: A Review", Planetary Cartography and GIS, Lecture Notes in Geoinformation and Cartography, Springer International Publishing, pp. 75–101, doi:10.1007/978-3-319-62849-3_4, ISBN 978-3-319-62848-6, S2CID 133867607
- ^ "Astropedia – Pluto New Horizons LORRI – MVIC Global DEM 300m". astrogeology.usgs.gov.
- ^ Schenk, Paul; Singer, Kelsi; Beyer, Ross; Beddingfield, Chloe; Robbins, Stuart J.; McKinnon, William B.; Lauer, Tod R.; Verbiscer, Anne J.; Keane, James. T.; Dhingra, Rajani D.; Moore, Jeffrey; Parker, Joel W.; Olkin, Cathy; Spencer, John; Weaver, Hal; Stern, S. Alan (1 March 2021). "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". Icarus. 356 113834. Bibcode:2021Icar..35613834S. doi:10.1016/j.icarus.2020.113834. ISSN 0019-1035.
- ^ a b Campbell, D. M. H.; White, B.; Arp, P. A. (2013-11-01). "Modeling and mapping soil resistance to penetration and rutting using LiDAR-derived digital elevation data". Journal of Soil and Water Conservation. 68 (6): 460–473. doi:10.2489/jswc.68.6.460. ISSN 0022-4561.
- ^ James, M. R.; Robson, S. (2012). "Straightforward reconstruction of 3D surfaces and topography with a camera: Accuracy and geoscience application" (PDF). Journal of Geophysical Research: Earth Surface. 117 (F3): n/a. Bibcode:2012JGRF..117.3017J. doi:10.1029/2011JF002289.
- ^ Szypuła, Bartłomiej (1 January 2019). "Quality assessment of DEM derived from topographic maps for geomorphometric purposes". Open Geosciences. 11 (1): 843–865. Bibcode:2019OGeo...11...66S. doi:10.1515/geo-2019-0066. hdl:20.500.12128/11742. ISSN 2391-5447. S2CID 208868204.
- ^ Adams, Aaron (2019). A Comparative Usability Assessment of Augmented Reality 3-D Printed Terrain Models and 2-D Topographic Maps. NMSU. Retrieved 11 March 2022.
{{cite book}}: CS1 maint: location missing publisher (link) - ^ "I. Balenović, A. Seletković, R. Pernar, A. Jazbec. Estimation of the mean tree height of forest stands by photogrammetric measurement using digital aerial images of high spatial resolution. ANNALS OF FOREST RESEARCH. 58(1), P. 125-143, 2015".
- ^ "Martin Gamache's paper on free sources of global data" (PDF).
- ^ Hirt, C.; Rexer, M. (2015). "Earth2014: 1 arc-min shape, topography, bedrock and ice-sheet models - available as gridded data and degree-10,800 spherical harmonics" (PDF). International Journal of Applied Earth Observation and Geoinformation. 39: 103–112. Bibcode:2015IJAEO..39..103H. doi:10.1016/j.jag.2015.03.001. hdl:20.500.11937/25468. Retrieved February 20, 2016.
- ^ "GEO Elevation Services : Airbus Defence and Space". www.astrium-geo.com. Archived from the original on 2014-06-26. Retrieved 2012-01-11.
- ^ "International – Geospatial". gs.mdacorporation.com. Archived from the original on 2016-03-04. Retrieved 2012-02-02.
- ^ "TerraSAR-X : Airbus Defence and Space". www.astrium-geo.com. Archived from the original on 2014-08-12. Retrieved 2012-01-11.
- ^ "ALOS World 3D – 30m". www.eorc.jaxa.jp. Archived from the original on 2020-05-04. Retrieved 2017-09-09.
- ^ "ALOS World 3D". www.aw3d.jp.
- ^ "A basic guide for using Digital Elevation Models with Terragen". Archived from the original on 2007-05-19.
- ^ "OpenTopography". www.opentopography.org.
- ^ "About OpenTopography".
- ^ "San Diego Supercomputer Center". www.sdsc.edu. Retrieved 2018-08-16.
- ^ "Home | UNAVCO". www.unavco.org. Retrieved 2018-08-16.
- ^ OpenDemSearcher
Further reading
[edit]- Wilson, J.P.; Gallant, J.C. (2000). "Chapter 1" (PDF). In Wilson, J.P.; Gallant, J.C. (eds.). Terrain Analysis: Principles and Applications. New York: Wiley. pp. 1–27. ISBN 978-0-471-32188-0. Retrieved 2007-02-16.
- Hirt, C.; Filmer, M.S.; Featherstone, W.E. (2010). "Comparison and validation of recent freely-available ASTER-GDEM ver1, SRTM ver4.1 and GEODATA DEM-9S ver3 digital elevation models over Australia". Australian Journal of Earth Sciences. 57 (3): 337–347. Bibcode:2010AuJES..57..337H. doi:10.1080/08120091003677553. hdl:20.500.11937/43846. S2CID 140651372. Retrieved May 5, 2012.
- Rexer, M.; Hirt, C. (2014). "Comparison of free high-resolution digital elevation data sets (ASTER GDEM2, SRTM v2.1/v4.1) and validation against accurate heights from the Australian National Gravity Database" (PDF). Australian Journal of Earth Sciences. 61 (2): 213–226. Bibcode:2014AuJES..61..213R. doi:10.1080/08120099.2014.884983. hdl:20.500.11937/38264. S2CID 3783826. Archived from the original (PDF) on June 7, 2016. Retrieved April 24, 2014.
External links
[edit]- DEM Quality Comparison
- Terrainmap.com
- Maps-for-free.com
- Geo-Spatial Data Acquisition Archived 2013-08-22 at the Wayback Machine
- Elevation Mapper, Create geo-referenced elevation maps
- Data products
- Satellite Geodesy by Scripps Institution of Oceanography
- Shuttle Radar Topography Mission by NASA/JPL
- Global 30 Arc-Second Elevation (GTOPO30) by the U.S. Geological Survey
- Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) by the U.S. Geological Survey
- Earth2014 by Technische Universität München
- Sonny's LiDAR Digital Terrain Models of Europe
Digital elevation model
View on Grokipediafrom Grokipedia
Terminology and Fundamentals
Definitions and Key Concepts
A digital elevation model (DEM) is a three-dimensional representation of a terrain's surface, typically depicting the bare earth topographic surface excluding vegetation, buildings, and other surface objects, stored as a raster grid of elevation values in digital form suitable for computer processing.[1][3][2] This grid-based structure consists of a regular lattice of discrete points, where each point is defined by horizontal coordinates (x, y) in a projected or geographic system and a corresponding vertical elevation (z), enabling the approximation of a continuous terrain surface.[2][8] The foundational concept of digital terrain representation originated in 1958 with the introduction of the digital terrain model (DTM) by C. L. Miller and R. A. LaFlamme, who described it as a statistical model of a continuous surface using arrays of xyz coordinates derived from photogrammetric data.[8] The term "digital elevation model" emerged in the 1970s as computing and geographic information systems (GIS) advanced, distinguishing simpler grid-based elevation datasets from more complex terrain models, and it became a standard for topographic mapping by agencies like the U.S. Geological Survey.[2][9] DEMs gained widespread adoption during this period for applications in GIS and remote sensing, providing essential data for terrain analysis and environmental modeling.[1] At its core, a DEM's mathematical representation models the terrain elevation $ z $ at a grid point $ (i,j) $ as $ z(i,j) = f(x_i, y_j) $, where $ f $ approximates the underlying continuous terrain function, and $ x_i $, $ y_j $ are the sampled horizontal positions.[2][8] Key components include spatial resolution, with horizontal resolution defining the grid spacing (e.g., 30 m or 1 arc-second) and vertical resolution specifying the precision of elevation values, often influencing the model's accuracy in capturing terrain features.[1][2] Coordinate systems are critical, typically employing projected grids like Universal Transverse Mercator (UTM) for horizontal positioning and vertical datums such as mean sea level for elevations, measured in units like meters.[2] Common data types for elevation values include integers for coarser resolutions or floating-point numbers for higher precision, stored in formats that support geospatial analysis.[2][1]Distinctions Between DEM, DSM, and DTM
A digital elevation model (DEM) serves as the overarching term for any raster-based representation of terrain elevations, typically in a georeferenced grid format. In contrast, a digital surface model (DSM) specifically captures the uppermost surface, encompassing not only the bare ground but also overlying features such as vegetation, buildings, and other anthropogenic or natural objects, effectively representing the lower boundary of the atmosphere. Meanwhile, a digital terrain model (DTM) focuses exclusively on the bare-earth surface, delineating the interface between the lithosphere and atmosphere by excluding all above-ground elements like trees and structures.[2] The terminology surrounding these models has evolved over time, with early literature from the 1980s frequently employing "DTM" to describe bare-earth representations, particularly in photogrammetric contexts, as seen in proceedings from the International Society for Photogrammetry and Remote Sensing (ISPRS). By the late 20th and early 21st centuries, "DEM" became the standardized generic term, as reflected in modern geographic information standards, while "DTM" retained specificity for ground-only models in some usages. For instance, the U.S. Geological Survey (USGS) produces bare-earth elevation data but designates it as a DEM rather than a DTM, highlighting regional variations in nomenclature. In certain contexts, DTMs may emphasize interpolated or vector-based ground surfaces derived from raw data, distinguishing them further from unprocessed DEMs.[1] These distinctions manifest clearly in practical scenarios; for example, in forested regions, a DSM will record higher elevations due to tree canopies, whereas a corresponding DTM or bare-earth DEM will reflect only the underlying soil surface after filtering out vegetation. Deriving a DEM from a DSM often involves subtracting estimated heights of surface features, such as through lidar-based classification algorithms, to isolate the ground level. Regarding advantages, DEMs and DTMs are preferred for hydrological modeling because they prevent artifacts like artificial drainage paths caused by buildings or dense foliage in DSMs. Conversely, DSMs excel in applications requiring visibility analysis, such as line-of-sight calculations, by incorporating real-world obstacles that a bare-earth model would overlook.[2][10]Types and Representations
Primary Types of Elevation Models
Digital elevation models (DEMs) are primarily categorized by their structural representations, which determine how elevation data is stored, processed, and analyzed for terrain applications. The most common types include raster-based models, which use a uniform grid structure; vector-based models such as Triangulated Irregular Networks (TINs); hybrid approaches that integrate elements of both; and other variants like contour-based or point cloud representations. These structures balance factors like data density, computational demands, and fidelity to terrain features, enabling tailored use in geospatial analysis.[11][12] Raster-based DEMs represent terrain as a regular grid of cells, where each cell stores an elevation value typically at its center, forming a continuous surface suitable for uniform spatial analysis. This grid structure facilitates straightforward arithmetic operations and integration with other raster datasets, making it ideal for large-scale coverage where consistent resolution is prioritized over variable detail. For instance, the Shuttle Radar Topography Mission (SRTM) dataset employs a raster format with 30-meter or 90-meter grid spacing to provide near-global elevation data, enabling efficient processing across vast areas. The simplicity of rasters supports rapid simulations, such as hydrological flow modeling, due to their alignment with pixel-based algorithms in geographic information systems (GIS).[13][14] Vector-based alternatives, particularly Triangulated Irregular Networks (TINs), model the terrain surface using a set of non-overlapping triangles formed via Delaunay triangulation from irregularly spaced elevation points. This approach allows variable resolution, with denser triangles in areas of high topographic variability and sparser ones in flatter regions, making TINs efficient for representing sparse or unevenly distributed data without redundant points. TINs excel in preserving linear features like ridges or valleys, which is advantageous for applications requiring precise surface interpolation over complex terrains. In hydrological modeling, TINs are preferred to maintain breaklines such as river channels, ensuring accurate flow path delineation with fewer data points than a comparable raster grid.[15][16][17] Hybrid models combine raster and TIN elements to achieve adaptive resolution, leveraging the uniformity of grids for broad coverage while incorporating TIN facets for enhanced detail in rugged or feature-rich areas. These models often start with a coarse raster base and overlay TIN refinements along critical boundaries, optimizing both storage and analytical precision for heterogeneous landscapes. Such integration is particularly useful in scenarios demanding scalable detail, like urban planning where flat expanses require less resolution than steep slopes.[18][12] Other variants include contour-based models, derived from isohypses (lines of equal elevation) that are interpolated to form a grid or surface, and point cloud representations, which capture raw 3D coordinates from sources like LiDAR before processing into a DEM. Contour-based approaches are effective for legacy topographic maps, where elevations are inferred between lines to generate a DEM, though they may introduce smoothing artifacts in undulating terrain. Point clouds, consisting of discrete elevation points, serve as pre-processed inputs for DEM creation, retaining high-fidelity details from airborne surveys but requiring interpolation to form a continuous model.[19][20] Selection of a primary type depends on analytical needs: raster models are favored for computational efficiency in large-scale simulations due to their grid-based uniformity and compatibility with parallel processing, while TINs offer storage savings in complex terrains by using fewer points to represent variability. For example, raster DEMs like SRTM support global environmental modeling, whereas TINs enhance hydrological applications by explicitly honoring breaklines in river networks.[21][13][16]Visualization and Rendering Techniques
Visualization and rendering techniques for digital elevation models (DEMs) enable the effective display of topographic data, facilitating interpretation of terrain features through simulated lighting, line-based representations, and three-dimensional views. These methods transform raw elevation grids into interpretable visuals, often emphasizing relief and orientation without altering the underlying data structure.[22] Hillshading simulates illumination on a terrain surface to highlight elevation variations, commonly employing Lambert's cosine law for diffuse reflection. Under this model, the intensity $ I $ at a point is computed as $ I = \cos(\theta) $, where $ \theta $ is the angle between the surface normal and the light source direction, often representing a virtual sun position; this is modulated by the elevation gradient derived from neighboring grid cells to accentuate slopes.[22] The technique assumes a Lambertian surface, producing grayscale images where brighter areas face the light and shadows reveal depressions, aiding in the perception of landform shapes. Seminal work by Horn formalized this approach in computer vision contexts, applying it to digital terrain models for efficient shading computation.[23] Contour generation creates isolines representing constant elevation levels by interpolating across the raster grid of a DEM. Algorithms such as Marching Squares traverse the grid cell by cell, identifying edge intersections where the elevation threshold is crossed and connecting them to form smooth contours; this method efficiently handles binary decisions at each cell's four vertices to output vector lines.[24] Widely adopted for topographic mapping, it supports adaptive smoothing to reduce jagged artifacts in variable-resolution data, ensuring contours align with natural terrain breaks.[25] Three-dimensional perspectives enhance DEM interpretation by extruding elevation data into immersive views, often draping orthorectified textures like satellite imagery over the surface for contextual realism. In software such as ArcGIS, this involves generating a triangulated irregular network (TIN) from the DEM and overlaying raster layers, allowing interactive rotation and zoom to reveal spatial relationships. Anaglyph stereo techniques further deepen perception by rendering left- and right-eye views in complementary colors (e.g., red-cyan), viewable with inexpensive glasses to simulate binocular depth from the monoscopic elevation data.[26] Slope and aspect maps derive from DEM gradients to visualize terrain steepness and orientation, typically colored for intuitive analysis. Slope angle $ \alpha $ is calculated as $ \tan(\alpha) = \sqrt{\left( \frac{dz}{dx} \right)^2 + \left( \frac{dz}{dy} \right)^2} $, where partial derivatives approximate the rise over run in x and y directions using finite differences across grid cells; values are often classified into categories (e.g., 0-5° in green, >30° in red) to map erosion potential or vegetation suitability. Aspect, the downhill-facing direction, is derived as the azimuth of the gradient vector, rendered in a circular color scheme (e.g., north in blue, south in red) to indicate exposure to sunlight or wind. These derivative visualizations prioritize categorical rendering over raw values for clarity in geomorphic studies.[27][28] Advanced rendering leverages graphics processing units (GPUs) for real-time display of large-scale DEMs in virtual globes like Google Earth, employing hardware tessellation to dynamically subdivide terrain meshes based on viewer proximity. This enables seamless zooming across planetary extents without preprocessing the entire dataset, using level-of-detail hierarchies to balance performance and fidelity. For subsurface or volumetric extensions of DEMs, such as geological strata, volume rendering techniques ray-march through voxel data, accumulating opacity and color along sight lines to reveal internal structures, often accelerated by GPU shaders for interactive exploration.[29][30] Common tools for these visualizations include open-source QGIS with plugins like the Relief Visualization Toolbox, which implements multidirectional hillshading and analytical shading, and MATLAB's Mapping Toolbox for scripted relief plotting. Outputs are standardized in formats such as GeoTIFF for shaded relief, preserving georeferencing and enabling layering in GIS workflows.[31][32]Generation Methods
Data Acquisition Techniques
Data acquisition techniques for digital elevation models (DEMs) involve collecting raw elevation measurements from various remote sensing and ground-based platforms, providing the foundational point clouds or profiles that are later processed into gridded models. These methods range from traditional stereoscopic analysis to advanced laser and radar systems, enabling coverage from local scales to global extents. Historical approaches, such as manual photogrammetry dating back to the early 20th century, have evolved into automated, high-resolution techniques that leverage airborne and spaceborne sensors.[33] Photogrammetry derives elevation data by analyzing parallax shifts in overlapping stereo aerial or satellite images, where height is computed from the geometric displacement between left and right perspectives in a stereopair. This method, pioneered in the 1930s for topographic mapping, initially relied on manual stereoplotters but now uses automated image matching algorithms to generate dense point clouds. Modern implementations often employ unmanned aerial vehicles (UAVs) for high-resolution surveys, achieving sub-meter vertical accuracy over targeted areas.[33][34] Light Detection and Ranging (LiDAR) acquires elevation data through airborne or terrestrial laser scanners that emit pulses and measure the round-trip travel time $ t $ to compute distance as $ \frac{c t}{2} $, where $ c $ is the speed of light. Discrete-return LiDAR records individual pulse echoes to distinguish ground from vegetation, while full-waveform systems capture the entire reflected signal for enhanced vegetation penetration and accuracy. This active sensing technique produces point densities exceeding 10 points per square meter, supporting DEMs with vertical accuracies of 10-15 cm in open terrain.[35][36] Radar interferometry, particularly Interferometric Synthetic Aperture Radar (InSAR), generates elevation from phase differences $ \Delta \phi $ between two or more synthetic aperture radar (SAR) images acquired from slightly offset satellite positions, related to height variation $ \Delta h $ approximately by the equationwhere $ B_\perp $ is the perpendicular baseline, $ \lambda $ is the radar wavelength, $ r $ is the slant range, and $ \theta $ is the incidence angle.[37] This satellite-based method provides wide-area coverage, with vertical resolutions of 1-5 meters, though it is sensitive to decorrelation in vegetated or changing terrains.[38] Satellite altimetry collects global elevation profiles using onboard radar or laser instruments, such as the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2), which employs a photon-counting laser altimeter with approximately 13-meter diameter footprints spaced about 0.7 meters apart along tracks. While offering high vertical precision of about 0.1 meters, its sparse sampling limits direct DEM generation to coarse resolutions around 100 meters without interpolation.[39][40] Ground surveys provide high-accuracy reference points for DEM initialization and validation, using Real-Time Kinematic Global Positioning System (RTK-GPS) to achieve centimeter-level vertical precision through carrier-phase corrections from base stations. Traditional differential leveling establishes benchmarks with sub-centimeter accuracy over short distances, serving as control for larger-scale acquisitions.[41][42] Emerging techniques include Structure-from-Motion (SfM), which reconstructs 3D elevation models from overlapping UAV photographs by estimating camera positions and scene geometry algorithmically, yielding DEMs with resolutions under 5 cm suitable for local monitoring. Recent advances include machine learning methods, such as diffusion models for high-resolution DEM generation from low-resolution inputs. Crowdsourced data from smartphone barometers and GPS tracks, sometimes integrated into platforms like OpenStreetMap, contribute opportunistic elevation points, though with variable accuracy due to sensor limitations.[43][44][45][46]
Processing and Interpolation Methods
Once raw elevation data, such as point clouds from LiDAR, are acquired, pre-processing is essential to prepare them for DEM creation by removing noise and classifying points to isolate terrain surfaces. Noise filtering often employs median filters, which replace each elevation value with the median of neighboring values within a defined window, effectively reducing outliers while preserving edges better than mean filters.[47] In LiDAR datasets, classification distinguishes ground points from vegetation or structures using algorithms like progressive morphological filtering or cloth simulation, enabling the extraction of bare-earth elevations.[48] Interpolation methods then generate continuous raster surfaces from these processed points, categorized as deterministic or stochastic. Deterministic approaches, such as bilinear and bicubic spline interpolation, produce smooth grids by fitting polynomials across neighboring cells; bilinear uses linear weighting in two dimensions for basic resampling, while bicubic incorporates higher-order terms for reduced aliasing in varied terrain.[49] A common exact method is Inverse Distance Weighting (IDW), where the interpolated elevation $ z $ at a point is computed as
with weights $ w_i = 1 / d_i^p $ based on distance $ d_i $ from known points and power parameter $ p $ (typically 2), emphasizing nearby samples.[50] Stochastic methods, including Kriging and radial basis functions, account for spatial autocorrelation and uncertainty; Kriging estimates variance through semivariograms to provide prediction errors alongside elevations, ideal for geostatistical analysis in heterogeneous landscapes.[51] Splines and radial basis functions model uncertainty by minimizing global error with flexible, radially symmetric kernels, supporting probabilistic outputs for risk assessment.[52]
Post-interpolation, DEM editing enforces hydrological consistency and structural accuracy. Hydrological correction involves filling sinks—artificial depressions from data errors—using algorithms like priority-flood to create depressionless surfaces that simulate realistic flow paths without altering broader topography.[53] Breakline enforcement incorporates linear features, such as cliffs, by constraining interpolation along these edges to maintain sharp discontinuities, often via constrained TINs or spline adjustments.[54]
Software tools facilitate these steps, with GDAL handling raster interpolation and editing through command-line utilities like gdal_grid for IDW or Kriging.[55] LAStools processes LiDAR-specific tasks, including ground classification and noise removal via lasground and lasnoise.[56] Historically, DEM processing in the 1970s relied on manual contour digitization and simple gridding, transitioning post-2000 to automated pipelines driven by LiDAR and global datasets.[57]
Resolution considerations during processing balance detail and computation; downsampling coarse data like 30 m SRTM to coarser grids reduces artifacts but loses fine features, while upsampling high-resolution 1 m LiDAR introduces smoothing trade-offs, often requiring adaptive methods to minimize distortion.[58][59]
Quality and Accuracy
Sources of Error in DEMs
Errors in digital elevation models (DEMs) arise from multiple stages of their creation and can significantly impact their reliability for various applications. Acquisition errors, inherent to the data collection process, include sensor noise in technologies like LiDAR, where pulse jitter can introduce vertical inaccuracies on the order of 10 cm.[60] Similarly, in interferometric synthetic aperture radar (InSAR), atmospheric refraction causes phase delays that propagate as elevation errors, primarily due to variations in the refractive index from water vapor and pressure.[61] Processing errors occur during data manipulation and can alter the represented terrain. Interpolation artifacts, for instance, often result in the smoothing of sharp features such as cliffs or ridges, reducing the fidelity of complex topography.[62] Datum inconsistencies, such as mismatches between ellipsoidal heights (e.g., WGS84) and orthometric heights (e.g., relative to the geoid), further introduce systematic offsets in elevation values. Environmental factors contribute to inaccuracies by obscuring or modifying the terrain surface captured in the data. In digital surface models (DSMs), vegetation occlusion can elevate measurements above the bare earth, leading to biased representations of underlying topography. Snow cover variability similarly affects seasonal DEMs, as transient accumulation masks true ground levels and varies with weather conditions. Urban clutter, including buildings and infrastructure, complicates bare-earth extraction in populated areas, often resulting in erroneous high points. Resolution limitations in DEM grids can cause aliasing effects, particularly in low-resolution models where steep slopes exceeding 45° are under-sampled, leading to misrepresented gradients and artificial flat areas.[62] Temporal errors stem from landscape dynamics and data age; for example, erosion or construction activities alter elevations between acquisition dates, while outdated surveys from the 1980s may no longer reflect current conditions due to natural or anthropogenic changes. Systematic biases further compound these issues through geometric transformations. Projection distortions in non-local coordinate grids can stretch or compress elevation data, especially over large extents where map projections deviate from the Earth's curvature.[62] Vertical datum shifts, such as those between NAVD88 and WGS84, typically amount to approximately 1 m differences depending on location, arising from variations in geoid undulation.[63] Historical examples illustrate the evolution of these challenges; early DEMs like Digital Terrain Elevation Data (DTED) Level 0, derived from analog photogrammetric methods in the 1970s–1990s, exhibited errors up to 100 m vertically due to limitations in manual contour digitization and coarse source materials.[64]Validation and Assessment Metrics
Validation of digital elevation models (DEMs) relies on quantitative and qualitative methods to quantify accuracy and reliability, often using independent reference data such as ground surveys or high-precision altimetry. These assessments help users determine the suitability of a DEM for specific applications by measuring deviations between predicted elevations and true values. Common approaches include direct comparisons with ground truth data and statistical evaluations that account for error distributions. One fundamental metric is the root mean square error (RMSE), calculated as RMSE = √[Σ (z_pred - z_true)^2 / n], where z_pred represents the elevation from the DEM, z_true is the surveyed ground truth elevation, and n is the number of validation points. This metric provides a measure of overall vertical accuracy, with lower values indicating better performance; for instance, the ASTER Global DEM version 3 achieves an RMSE of approximately 8.52 meters when validated against control points. RMSE is widely used because it penalizes larger errors more heavily and aligns with standards for elevation data assessment. Cross-validation techniques further evaluate interpolation accuracy in DEM generation, particularly for gridded models derived from sparse point data. K-fold cross-validation divides the dataset into k subsets, training the interpolation model on k-1 folds and testing on the held-out fold, repeating this process to estimate overall error; this method is effective for assessing how well models like kriging or inverse distance weighting generalize. For point cloud-based DEMs, leave-one-out cross-validation removes individual points for prediction and comparison, providing a robust estimate of local accuracy without requiring external data. Additional statistical metrics address non-normal error distributions common in DEMs. The linear error at 90% (LE90) quantifies the value below which 90% of elevation errors fall, offering a percentile-based accuracy measure that is less sensitive to outliers than RMSE; TanDEM-X DEMs, for example, target an LE90 of 10 meters for absolute vertical accuracy. The normalized median absolute deviation (NMAD), defined as NMAD = 1.4826 × median(|z_pred - z_true| / median(z_true)), is suited for robust assessment of non-Gaussian errors, capturing typical deviations while mitigating the influence of extreme values in heterogeneous terrains. For instance, the Copernicus DEM GLO-30 (2021) achieves relative vertical accuracy of LE90 ≤4 m on slopes >20%, validated against ICESat-2, while NASADEM (2020) reports global RMSE improvements over SRTM.[65][66] Qualitative assessments complement quantitative metrics by identifying systematic issues not captured by statistics alone. Visual inspection involves rendering the DEM with hillshading or contour overlays to detect artifacts such as striping or sinks, which may arise from sensor limitations. Slope consistency checks compare derived slope maps against expected geomorphic patterns, flagging inconsistencies like unnatural flat areas that indicate processing errors. Standardized guidelines ensure consistent reporting of DEM quality. The American Society for Photogrammetry and Remote Sensing (ASPRS) provides positional accuracy standards for LiDAR-derived DEMs, specifying that high-accuracy (e.g., Class 1 or equivalent) data should achieve a vertical RMSE_z of less than 15 cm for bare-earth terrain, as per legacy guidelines.[67] Internationally, ISO 19157 establishes principles for geographic data quality, including components like positional accuracy and completeness, with guidelines for evaluation procedures applicable to elevation datasets. Specialized tools facilitate large-scale validation. ICESat and ICESat-2 laser altimetry data serve as a global reference for DEM assessment, enabling automated interpolation of footprints to DEM grid points for error computation over vast areas without ground surveys. Software tools for pairwise DEM comparisons, such as those implementing difference raster analysis, allow quantification of discrepancies between models like SRTM and TanDEM-X by generating error maps and statistics. Recent advancements incorporate machine learning for predictive error modeling. Post-2015 developments use neural networks trained on metadata (e.g., terrain slope, vegetation cover) to forecast DEM errors at unsampled locations, improving uncertainty estimates; for example, recent machine learning approaches, such as stacking ensembles, have achieved substantial RMSE reductions (e.g., over 60% in some hybrid cases) for SRTM by incorporating auxiliary data like land cover.[68] These approaches enhance traditional metrics by providing probabilistic quality layers integrated into DEM products.Applications
Terrain Analysis and Geomorphology
Digital elevation models (DEMs) are fundamental in terrain analysis and geomorphology, providing a quantitative basis for deriving topographic attributes that reveal landform characteristics and surface processes. These attributes, computed through algorithms applied to DEM grids, enable the study of erosion patterns, landscape evolution, and tectonic influences without direct field measurements. Key derivations include slope, aspect, curvature, and hypsometric properties, which help classify landforms and infer geomorphic histories.[69][70] Slope and aspect are primary terrain attributes derived from DEMs using finite difference methods, which approximate gradients across neighboring grid cells. Slope gradient, representing the steepness in degrees or percent, is calculated as the maximum rate of change in elevation from a central cell to its eight neighbors, essential for modeling erosion rates in geomorphic processes. Aspect, the downslope direction in compass bearings, is determined from the direction of this maximum gradient, aiding in the analysis of exposure and weathering variations. These computations typically employ a 3x3 kernel for local derivatives, with slope influencing sediment transport and aspect affecting solar insolation on slopes.[71][72] Curvature analysis from DEMs quantifies the second-order shape of the terrain, distinguishing convex and concave features critical for landform classification. Profile curvature, measured parallel to the slope direction, indicates acceleration or deceleration of downslope processes; concave profiles in valleys promote flow convergence and sediment deposition. Plan curvature, perpendicular to the slope, reflects lateral flow divergence; concave plan forms identify valleys where water converges, while convex forms denote ridges. These curvatures are derived via finite differences on slope grids and combined in object-based classifications to delineate elements like peaks, shoulders, and footslopes, enhancing automated mapping of geomorphic units.[73][70] Hypsometry uses DEM-derived elevation distributions to assess landscape evolution, plotting cumulative area against normalized elevation to form hypsometric curves. The hypsometric integral (HI), a scalar summary of this curve, is computed as
where is the mean elevation, and and are the minimum and maximum elevations within a basin. Values near 1 indicate youthful, high-relief landscapes with minimal erosion, while lower values suggest mature or old stages dominated by dissection; this aids in inferring tectonic uplift or denudation histories from elevation histograms.[74][75]
Feature extraction in geomorphology leverages DEMs to automate the identification of linear and basin features through flow accumulation algorithms. Flow accumulation sums the number of upstream cells contributing to each grid cell based on derived flow directions, typically using an eight-direction pour-point model after filling sinks. High accumulation values delineate valleys and basins as convergent zones, while low values (near zero) highlight ridges as divergent or non-contributing areas; thresholding these maps enables extraction of networks for analyzing drainage patterns and topographic skeletons. This method is effective across resolutions from 3 m to 30 m, improving efficiency in hilly or mixed terrains.[76][77]
Geomorphometric indices derived from DEMs quantify relief and roughness, informing tectonic and erosional studies. The relief ratio, defined as total basin relief divided by maximum basin length, measures average slope steepness and correlates with dissection intensity in uplifting regions. The terrain ruggedness index (TRI), which captures topographic heterogeneity, is computed as the square root of the sum of the squared differences in elevation from a central cell to its eight neighboring cells, divided by the number of neighbors; high values indicate rugged terrains prone to rapid erosion, as seen in tectonic active zones like the Himalayas. These indices facilitate basin-scale comparisons and integration with structural geology models.[78][79]
In case studies, DEM differencing has quantified glacial retreat by subtracting pre- and post-event surfaces to measure volume loss. For instance, global analyses from 2000 to 2023 revealed glaciers lost 273 ± 16 gigatonnes annually, with acceleration post-2010, using stereo-optical DEMs co-registered for elevation change mapping in regions like the Himalayas. Similarly, volcanic edifice mapping employs DEM curvature and slope thresholds to delineate boundaries; a study of Sardinian scoria cones integrated slope-total curvature with modified algorithms to trace 13 edifices accurately, accounting for erosion and aiding hazard assessment.[80][81]
Software like SAGA GIS supports comprehensive terrain metrics computation from DEMs, including slope, curvature, topographic position index, and ruggedness within user-defined neighborhoods. Its modules, such as Basic Terrain Analysis, generate multiple derivatives simultaneously for integration with tectonic models, enabling scalable geomorphic interpretations.[82]
Environmental and Hydrological Modeling
Digital elevation models (DEMs) play a pivotal role in environmental and hydrological modeling by providing topographic data essential for simulating dynamic processes such as water flow, sediment movement, and ecosystem responses. These models enable the integration of terrain attributes into process-based simulations, allowing researchers to predict environmental changes under various scenarios, from local watershed dynamics to global climate impacts. By representing surface elevations, DEMs facilitate the derivation of hydrological parameters like flow paths and slopes, which are critical inputs for algorithms that model fluid dynamics and ecological interactions.[83] In watershed delineation, DEMs are used to compute flow direction and accumulation, defining drainage basins and sub-basins through algorithms such as the D8 method, which assigns flow to one of eight neighboring cells based on steepest descent, or multiple flow direction approaches that distribute flow proportionally across multiple cells to better represent divergent terrains. Sink filling is a preprocessing step that artificially raises depressions in the DEM to create a depressionless surface, preventing artificial storage and ensuring continuous flow paths for accurate basin boundary identification. This process is fundamental in hydrological software like ArcGIS and SWAT, where it supports runoff routing and pollutant transport simulations.[84] For flood inundation modeling, DEMs serve as the primary input for topographic representation, acting as bathymetry in hydraulic simulations to predict water depth and extent during flood events. Integration with Manning's equation, which calculates flow velocity as $ v = \frac{1}{n} R^{2/3} S^{1/2} $ where $ n $ is the roughness coefficient, $ R $ is hydraulic radius, and $ S $ is slope derived from the DEM, allows models like HEC-RAS to simulate overland and channel flow dynamics. High-resolution DEMs, such as those from LiDAR, enhance the accuracy of inundation maps by capturing micro-topography that influences flood propagation.[85] Erosion and sediment transport predictions rely on DEM-derived topographic factors within models like the Revised Universal Soil Loss Equation (RUSLE), where the LS-factor quantifies slope length and steepness to estimate soil loss potential as $ A = R \cdot K \cdot LS \cdot C \cdot P $, with LS computed from flow accumulation and gradient grids. This factor captures how terrain influences rill and interrill erosion, enabling simulations of sediment yield in agricultural and forested landscapes. Applications in tools like GeoWEPP demonstrate how DEM resolution affects LS accuracy, with finer grids reducing underestimation of erosion hotspots.[86] In climate applications, DEM differencing—subtracting elevation changes between sequential models—quantifies glacier mass balance through rates like $ \frac{dh}{dt} $, revealing thinning or thickening trends that inform ice sheet dynamics and sea-level contributions. For instance, differencing ASTER and ICESat DEMs has shown annual mass losses exceeding 100 Gt/year for the Greenland Ice Sheet.[87] Coastal DEMs are also used to model sea-level rise impacts, simulating inundation and shoreline retreat by overlaying projected water levels on terrain to assess habitat loss and infrastructure vulnerability.[88] Biodiversity modeling employs DEMs to derive topographic roughness indices, such as the vector ruggedness measure, which quantifies terrain heterogeneity to assess habitat suitability for species sensitive to elevation gradients. These indices help predict species distributions in rugged landscapes, integrating with ecological models like MaxEnt to map potential refugia under climate change. Studies in montane ecosystems have shown that roughness correlates with beta-diversity, aiding conservation planning.[89] Examples of broader applications include the Intergovernmental Panel on Climate Change (IPCC) assessments, which incorporate DEMs in distributed hydrological models for runoff prediction under future climate scenarios, enhancing projections of water availability in river basins. In the 2020s, AI-enhanced forecasts, such as those using machine learning to refine DEM-based inputs in LSTM networks, have improved real-time hydrological predictions by up to 20% in accuracy for flood-prone regions.[90] Challenges in these applications arise from scale effects in nested models, where coarser resolutions like 90m SRTM DEMs can lead to discrepancies in simulated hydrological responses compared to 30m datasets, often affecting flow routing and peak flow estimates depending on terrain characteristics. Addressing this requires multi-resolution fusion techniques to balance computational efficiency and fidelity.[91]Data Sources and Accessibility
Global DEM Datasets
The Shuttle Radar Topography Mission (SRTM), conducted in 2000 by NASA and the National Geospatial-Intelligence Agency, produced one of the first near-global digital elevation models using synthetic aperture radar interferometry with C-band and X-band systems.[92] It covers latitudes from 56°S to 60°N, encompassing approximately 80% of Earth's land surfaces, and is available at resolutions of 1 arc-second (about 30 m) and 3 arc-seconds (about 90 m).[58] The vertical accuracy is approximately 16 m at 90% confidence level (LE90), though this varies with terrain and vegetation.[93] The ASTER Global Digital Elevation Model (GDEM), a collaborative effort between NASA and Japan's Ministry of Economy, Trade, and Industry (METI), derives from optical stereo photogrammetry using data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument.[94] It provides near-global coverage from 83°N to 83°S, spanning 99% of Earth's landmass, at a 30 m resolution.[95] Version 3, released in 2019, incorporates additional stereo pairs to reduce voids and artifacts compared to earlier iterations, enhancing overall data completeness.[96] A 2025 update, IC2-GDEM, corrects ASTER GDEM elevations using ICESat-2 altimeter data to improve accuracy, achieving root mean square error reductions of 16% to 82% globally.[97] The Copernicus Digital Elevation Model (DEM), based on interferometric synthetic aperture radar (InSAR) data from the TanDEM-X mission operated by the German Aerospace Center (DLR) and distributed by the European Space Agency (ESA), offers full global coverage from 90°N to 90°S.[6] The GLO-30 instance provides a 30 m resolution digital surface model, with the original TanDEM-X data at 12 m, and was made freely available to the public in 2021. It achieves relative vertical accuracy better than 4 m root mean square error (RMSE) globally, with specifications targeting 2 m in low-relief areas and 4 m in high-relief terrain.[98] Other notable global DEM datasets include the Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010), an archived product from the U.S. Geological Survey offering resolutions from 7.5 to 30 arc-seconds (approximately 250 m to 1 km), derived from a combination of SRTM and other sources for multi-scale terrain analysis.[99] The EarthEnv-DEM90 fuses ASTER GDEM and SRTM data to produce a void-filled 90 m resolution model, emphasizing smoothed, multi-scale elevation for environmental applications.[100] The Multi-Error-Removed Improved-Terrain (MERIT) DEM, at 90 m resolution, applies hydro-correction to remove systematic errors like vegetation and slope biases from base datasets including SRTM and ASTER, and remains under ongoing refinement as of recent years.[101] Recent machine learning-based advancements include FABDEM (2022), a 30 m global bare-earth DEM derived from Copernicus GLO-30 by removing forest and building height biases, achieving median errors as low as -0.11 m in validations.[102] FathomDEM (2025), an update using a hybrid vision transformer model on radar-derived data, further reduces mean absolute errors to half of FABDEM and a quarter of Copernicus DEM while preserving global coverage at 30 m resolution.[103] Among freely available global digital elevation models at approximately 30 m (1 arc-second) resolution, recent independent validations (including a 2024 study using novel ranking methods) identify the Copernicus DEM GLO-30 (derived from TanDEM-X radar data, ~2011–2015) and its derivative FABDEM as the most accurate options. The Copernicus DEM GLO-30 provides a global digital surface model (DSM) with absolute vertical accuracy better than 4 m (90% linear error) and strong relative accuracy, excelling in consistency and minimal voids. It is freely accessible via the Copernicus Data Space Ecosystem (registration required for full access). FABDEM (Forest And Buildings removed Copernicus DEM, released ~2022) applies machine learning (random forest models) to correct vegetation and building biases in the Copernicus GLO-30, producing a closer approximation to bare-earth digital terrain model (DTM) elevations. Studies show FABDEM often ranks highest overall, particularly in vegetated or built-up areas, reducing large positive errors while maintaining performance in other terrains. Other notable free global 30 m options include:- NASADEM (reprocessed SRTM with ICESat calibration): Improved over original SRTM, with ~1.5 m RMSE in bare ground areas.
- ALOS AW3D30 (JAXA, optical stereo): Stable performance, ~5 m RMSE, strong in rugged terrain.
- Original SRTM and ASTER GDEM: Earlier datasets, generally lower accuracy in comparisons.