Digital elevation model

Digital elevation model A digital elevation model (DEM) is a digital model or 3D representation of a terrain's surface — commonly for a planet (including Earth), moon, or asteroid — created from terrain elevation data. 3D rendering of a DEM of  Tithonium Chasma on Mars

Terminology 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).

Terminology continue.. All datasets which are captured with satellites, airplanes or other flying platforms are originally DSMs (like SRTM or the ASTER GDEM). In the following the term DEM is used as a generic term for DSMs and DTMs.

Types of DEM 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 to as a primary (measured) DEM, whereas the Raster DEM is referred to as a secondary (computed) DEM.  Heightmap of Earth's surface (including water and ice) in equirectangular projection, normalized as 8-bit grayscale, where lighter values indicate higher elevation

Types of DEM Continue.. The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR, land surveying, etc.. DEMs are commonly built using data collected 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. 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 other applications.

What is a Digital Surface Model (DSM)? In a LiDAR system, pulses of light travel to the ground. They return and are detected by the sensor giving the range (a variable distance) to the Earth. Hence, how they earned their name of Light Detection and Ranging. Airborne Light Detection and Ranging (LiDAR) In the end, LiDAR delivers a massive point cloud filled of varying elevation values. But these elevation values can come from the top of buildings, tree canopy, powerlines and other types of features. A DSM captures the natural and built features on the Earth’s surfaĐe.

Digital Surface Model (DSM) – Extruding features are tree canopy A DSM is especially useful in 3D modelling and is relevant in telecommunications, urban planning, aviation and forestry. This is because objects extrude from the Earth, which is particularly useful in these examples: Runway approach zone encroachment – In aviation, runway obstructions in the approach zone can be examined with a DSM to ensure no collisions. Vegetation management – Along a transmission line, overlaying a DSM to see where and how much vegetation is encroaching. View obstruction – Urban planners use DSM to check how a proposed building would affect the viewshed of other residents and businesses.

What is a Digital Elevation Model (DEM)? A digital elevation model is a regularly-spaced bare-earth raster grid referenced to a common vertical datum. When you filter out non- ground points such as bridges and roads, you are left with a smooth digital elevation model. The built (powerlines, buildings and towers) and natural ;trees and other tLJpes of ǀegetationͿ aren’t edžtruding in a DEM. Digital Elevation Model (DEM)

When you void vegetation and man made features from elevation data, you obtain a DEM. A smooth, bare-earth elevation model is particularly useful in fields of study such as hydrology, soils and land use planning/safety. Here are examples how a DEM can be used in GIS: Hydrologic modelling – A DEM is used to delineate watersheds, calculate flow accumulation and find out flow direction. Terrain stability – Areas prone to avalanches are high slope areas with sparse vegetation, which is useful when planning a highway or residential subdivision. Soil mapping – DEMs assist in mapping soils which is a function of elevation (as well as geology, time and climate).

LiDAR High Vegetation What is a Digital Terrain Model (DTM)? When you refer to this USGS LiDAR Base Specification, a digital terrain model (DTM) actually has two definitions depending on where you live. In some countries, a DTM is actually synonymous with a DEM. This means that a DTM is simply an elevation surface representing the bare earth referenced to a common vertical datum. In the United States and other countries, a DTM has a slight different meaning. A DTM is a vector data set composed of regularly spaced points and natural features such as ridges and breaklines. A DTM augments a DEM by including linear features of the bare- earth terrain.

Digital Terrain Model (DTM) DTMs are typically created through stereo photogrammetry like in the example above. Contour lines have been converted into points and are shown in purple. The DTM points are regularly-spaced that characterize the shape of the bare- earth terrain. In the image aďoǀe, LJou Đan see hoǁ the DTM is not Đontinuous and that it’s not a surface model. From these regularly-space and contour lines, you can interpolate a DTM into a DEM. A DTM represents distinctive terrain features much better because of its 3D breaklines and regularly spaced 3D mass points.

Elevation Data Sources Some of the remote sensing methods for obtaining DEM surfaces are: Satellite interferometry with synthetic aperture radar such as Shuttle Radar Topography Mission uses two radar images from antennas at the same time Aerial survey photogrammetry uses photographs from at least two different locations to generate stereopairs LiDAR measures reflected light back to the sensor to obtain a range to the Earth’s surfaĐe.

Production Continue.. 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. 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).

Production Continue.. 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.

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

Production Continue.. The quality of a DEM is a measure of how accurate elevation is at each pixel (absolute accuracy) and how accurately is the morphology presented (relative accuracy). Several factors play an important role for quality of DEM-derived products: Terrain roughness; Sampling density (elevation data collection method); Grid resolution or pixel size; Interpolation algorithm; Vertical resolution; Terrain analysis algorithm; Reference 3D products include quality masks that give information on the coastline, lake, snow, clouds, correlation etc.

Methods for obtaining elevation data used to create DEMs Lidar Stereo photogrammetry from aerial surveys Structure from motion / Multi-view stereo applied to aerial photography 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 Gatewing X100 unmanned aerial vehicle

Uses Digital Elevation Model - Red Rocks Amphitheater, Colorado obtained using an UAV (DroneMapper) Bezmiechowa airfield 3D Digital Surface Model obtained using Pteryx UAV flying 200 m above hilltop

Uses Continue.. Digital Surface Model of motorwayinterchange construction site. Note that tunnels are closed. Example DEM flown with the Gatewing X100 in Assenede Digital Terrain Model Generator + Textures(Maps) + Vectors

Uses continue.. Common uses of DEMs include: Extracting terrain parameters for geomorphology Modeling water flow for hydrology or mass movement (for example avalanches and landslides) Creation of relief maps Rendering of 3D visualizations. 3D flight planning and TERCOM

Uses Continue.. Creation of physical models (including raised relief maps) Rectification of aerial photography or satellite imagery Reduction (terrain correction) of gravity measurements (gravimetry, physical geodesy) Terrain analysis in geomorphology and physical geography Geographic Information Systems (GIS) Engineering and infrastructure design Satellite navigation (for example GPS and GLONASS)

Uses Continue.. Line-of-sight analysis Base mapping Flight simulation Precision farming and forestry Surface analysis Intelligent transportation systems (ITS) Auto safety / Advanced Driver Assistance Systems (ADAS) Archaeology

Sources A free DEM of the whole world called GTOPO30 (30 arcsecond resolution, approx. 1 km) is available, but its quality is variable and in some areas it is very poor. A much higher quality DEM from the 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.

Sources Continue.. 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). 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.

Sources Continue.. 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.

Sources Continue.. The SRTM30Plus dataset (used in NASA World Wind) attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model.  Another global model is Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) with 7.5 arc second resolution.

Sources Continue.. 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.

Sources Continue.. 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). OpenTopography is a community access resource of high-resolution, Earth science-oriented, topography data, and related tools and resources that warehouses a great deal of DEM data. 

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