Digital elevation model: description, types, types, construction

Digital elevation model (DEM) is a specialized database that demonstrates the surface shape between points of a given level, compiled by interpolating elevation data obtained from ground-based survey sources and photogrammetric collection based on a rectangular modeling grid. GIS software uses digital technology for three-dimensional visualization, creating contours and performing surface analysis.

History of development and modernity

History of development and modernity




The term DTM was introduced in the 1970s to distinguish the simplest form of terrain modeling from more complex types of electronic surface representations. Initially, it was used exclusively for raster representations: the heights specified at the intersection nodes of the regular grid. It took up to several months to build a digital elevation model.

Today, modern drones are able to collect the necessary data, analyze them to the smallest detail and build a visual layout in a more realistic and efficient time frame. Even the most inaccessible vast territories of the Earth can now be viewed and converted into a model using unmanned aerial vehicles (UAVs) equipped with the most modern equipment.





Various types of radars, video cameras and other instruments can be mounted on drones in order to collect the required information for a specific digital terrain model. This advanced technology, combined with the fastest software, delivers the best results in no time.

Japan Remote Sensing Technology Center




On April 26, 2016, NTT DATA and Japan's RESTEC (Japan Remote Sensing Technology Center) global IT company announced that their global 3D digital map service, called AW3D, is the first 5-meter 3D terrain model service to cover the entire globe. including Antarctica. The service operates on the basis of three million images obtained using DAICHI satellites and modernized Earth Observation Satellites (ALOS) from the Japan Aerospace Exploration Agency (JAXA).

In February 2014, NTT DATA and RESTEC launched a limited coverage 3D digital card service. This service represents a significant improvement over existing services that offer permissions of only 30 and 90 meters. NTT DATA, AW3D data is already in use in more than 60 countries.

Terms, definitions and abbreviations

A digital elevation model is a three-dimensional image of a terrain surface, created on the basis of altitude data and presented in the form of a raster - scale squares or a triangular irregular grid.









DEM USGS - raster geographic grid




DEM USGS - raster grid geolocation, which are built in a series of profiles, south-north. Like other USGS parameters, matrices were initially created in the form of sheets corresponding to topographic quadrangles:

  • large-scale -7.5 / 15 minutes;
  • intermediate - 30 minutes;
  • small-scale - 1 degree.

Tiles for building a digital elevation model are available for free download at many state and regional information exchange centers.

DEM is a digital elevation map, that is, a representation of the Earth's surface.

DTM - a set of methods used to obtain or represent the DEM.

DEM filtering is a set of methods used to improve the geomorphological similarity of matrices.

Terrain analysis or parameterization is the process of quantifying terrain detail.

Digital elevation model analysis (DTA) is used as a general term for determining application parameters.

Terrain - maps or images obtained from the database using DTA.

DEM Data Sources

DEM Data Sources




Topography or topography - the form or configuration of the terrain, represented on the map by contour lines, hypsometric shades and shading. Currently, there are five main sources of data for creating a digital elevation model:

  • ground research;
  • airborne photogrammetric data collection;
  • existing cartographic surveys, such as topographic maps;
  • airborne laser scanning;
  • stereoscopic or radar satellite imagery.

These matrix collection methods are compared, considering four aspects:

  • price;
  • accuracy;
  • sampling density;
  • pre-processing requirements.

Traditionally, similar information was collected by surveyors from ground surveys, followed by semi-automatic digitization by stereo plotters. This is the most accurate, but also the most expensive data collection method. Recent developments concern the automatic comparison of stereo images, the use of images with laser scanning, remote sensing either with stereoscopic overlap (SPOT, ASTER), or using interferometric photos.

The second highly effective modern method is the airborne and space interferometric radar system, which is used to accurately obtain data on both the earth's cover and the terrain.

Types of digital elevation models

Types of digital elevation models




Comparison of several elevation surfaces can be used to compare three heights or estimate the volume of objects. Laser scanning is used for the construction of buildings, power lines, open pits, terrain textures and even the geometry of waves on the sea.

There are different methods for modeling elevation: digital elevation models (DEM), digital surface models (DTM), digital terrain models (DTM), and triangular irregular networks (HPS).

The CMP captures natural and built-in functions on the Earth’s surface and is useful in 3D modeling for telecommunications, urban planning, and aviation, since the objects of study are displayed with a height above the Earth.

DTM is a clean raster grid tied to a vertical coordinate system. When a developer filters out points, such as bridges and roads, he gets a smooth digital terrain model. Constructed power lines, buildings and types of vegetation are not included in the DEM. The clean land contour model is particularly useful in planning hydrology, soil, and land use.

DTM has two definitions depending on the country of use. In some countries, it is actually a synonym for DEM and means the elevation surface, representing clean land, tied to a common vertical element.

In the United States, there is another definition of digital elevation models - this is a vector data set consisting of regularly located points and natural elements such as ridges and break lines. It complements the DEM, including linear characteristics of the earth's surface.

In Russia, GOST R 52440-2005 is used for the TsMM, according to which it is intended to create a cartographic database of the spatial reference of geodata obtained in the course of engineering and survey studies, land cadastral works, land surveying, statistical studies, other special works and surveys.

This model is usually created using stereophotogrammetry. Points are located on a regular basis and characterize the shape of the bare terrain. From these regular and contour lines, you can interpolate the DTM in the DTM. It represents the distinctive features of the earth's surface much better due to three-dimensional lines of discontinuity and regularly located three-dimensional points of mass.

Triangulated Irregular Network

Triangulated Irregular Network




To simulate a continuous area based on the measured data, the terrain points lying between the measurements should be connected by computational methods. To do this, the individual points are first connected to a triangular surface, which is available in vector format (TIN: triangulated irregular network) by interpolation.

If necessary, vector data is converted to a raster format, for example, a grid with a fixed cell size. For this, various mathematical methods are used. It is important to test the simulation to decide which of the most realistic to choose for the study area. While some GIS programs, such as Arc GIS, can handle TINs, others only work with raster geomodels. Different layouts are displayed depending on the location of the measurement base points.

Altitude Tools

After choosing a method for a real terrain, a measurement tool is selected. Currently widely used:

  1. Unmanned aerial vehicle.
  2. LiDAR - measures reflected light, which is reflected from the ground and returned to the sensor to obtain the height of the earth's surface.
  3. Stereophotogrammetry from aerial photography.
  4. Stereo multi view for aerial photography.
  5. Setting up a block of optical satellite images.
  6. Interferometry by radar data.
  7. Kinematic GPS in real time.
  8. Topographic maps.
  9. Theodolite or total station.
  10. Doppler radar.

Some methods of remote sensing to obtain a DEM:

  1. Satellite interferometry, a synthetic aperture radar such as the Shuttle Radar Topographic Mission, uses two radar images from antennas taken simultaneously to create a digital terrain model.
  2. Photogrammetry - in aerial photography, photogrammetry uses photographs from at least two different viewpoints. Like the way human vision works, it is able to gain depth and perspective through individual points of view.

Interpolation of digital outline maps

Old DEM generation methods often included the interpolation of digital contour maps that could be obtained by directly examining the surface of the earth. This method is still used in mountainous areas where interferometry is not always satisfactory.

Contour line data or any other sample database sets using GPS or ground survey are not a digital terrain model (DEM), but can be considered as digital terrain models. DTM implies that altitude is constantly available at every location in the study area.

Matrix quality is a measure of how accurate the height of each pixel is (absolute accuracy) and how accurately detail is represented (relative accuracy). Several factors play an important role in the quality of matrix-derived products:

  • uneven terrain;
  • sampling density;
  • height data collection method;
  • grid resolution or pixel size;
  • interpolation algorithm;
  • vertical resolution;
  • terrain analysis algorithm.

Reference 3D products include quality masks that provide information about the coastline, lake, snow cover, clouds and correlation.

Exploring with GIS Global Mapper

Terrain studies with GIS Global Mapper




The first step to use the search tool in Global Mapper to create a point feature at the desired address is to set up a projection for this area. Then, using the online data tool, you can connect to high-resolution images. There are a number of useful layers on the GIS site that you can add. Vector data is loaded as shapefiles using a web browser into Global Mapper by simply dragging and dropping files.

Technology for building digital elevation models:

  1. Download the DEM.zip data archive. The size of the ZIP archive is 2.5 MB.
  2. Unpack the archive into a directory on your hard drive.
  3. Open the DEM.zip archive.
  4. Create a subdirectory named "DEM" in the directory in which the data is saved.
  5. Extract all files from the ZIP archive to a new subdirectory.
  6. The end result will be two subdirectories, one of which contains a 30-meter DTM, and the other a 10-meter DTM.
  7. These datasets have an earlier distribution format for DEM USGS - heights in horizontal (pixel) units and representative for the area covered by the topographic map 1:24 000.
  8. Launch Global Mapper.
  9. Open DEM by choosing File> Open File, then go to the DEM_30m or DEM_10m directory, opening the bushkill_pa.dem file.
  10. Use the Zoom and Pan tools to zoom and scroll through the DEM.
  11. The Full View button (home icon) updates the original full view of the dataset.
  12. To see the DEM with hill shading, find the on / off button for shading the hill, in the lower left corner, where there are sun rays.
  13. Turn on the shading of the hills.
  14. You can change the appearance of the layout by choosing Tools> Customize, changing the settings in the Vertical Options and Shader Options, and select a color from each of the Low Color or High Color buttons in the gradient shader area.
  15. Click the “Apply” button.
  16. Go to the “Vertical parameters” tab and experiment with the “Vertical exaggeration” slider, click “Apply”.
  17. Go to the national map loading tool.
  18. Make sure the current extent is selected in the menu above the map. This indicates the area on the map for which to find data.
  19. Expand the "Elevation Products (3DEP)" section in the left menu and check the box next to any data set that you want to download.
  20. Click the “Find Products” button and use the links provided in the search results to display the area of ​​each data set on the map and load the preferred DEM.
  21. This will create a ZIP archive that can be saved to your hard drive.
  22. Launch Global Mapper and go to the folder where the ZIP archive is saved.
  23. Double click on the file name. Data should be displayed - the program can read them even in a compressed form.
  24. An image of the DEM data should appear in the Global Mapper window.
  25. If DEM Bushkill is still visible, open the Control Center and uncheck DEM Bushkill. Click the "Full View" button.
  26. To view DEM data from the shading hill, find the Enable / Disable Hill Shading toolbar button in the lower left.
  27. Turn on the shading of the hills.
  28. You can change the appearance by choosing Tools> Customize and changing the settings in Vertical Options and Shader Options.
  29. You can view the metadata associated with the layout data through Tools> Control Center. PIXEL dimensions are in degrees, not meters.

Software

Software




Various computer programs are available for processing and interpolating measurement points, including software specially adapted for measuring instruments from manufacturers of surveying equipment (Zeiss, Leica, Wild, Sokkia, Trimble). In archaeological practice, AutoCAD is typically used to process and overwrite real three-dimensional measurement data. You can purchase additional modules or advanced versions to create contour lines and 3D models. For designing 2.5D surfaces, any GIS program can be used. Among other things, geophysical survey data can be easily read and projected using terrain measurement data.

Contour plans can be created in DXF format. Files are exported to AutoCAD. Shaded or color terrain layouts are exported to various graphic formats (TIFF, JPEG, BMP) and integrated into AutoCAD. The resulting models are usually presented in a raster format in which the height value is assigned to the cell defined by the XY angular coordinate with the given page length. In principle, raster variants are similar to image data, except that instead of the color value, the height value is saved.

Converting srtm raster digital elevation models from one format to another in a GIS program is usually not a problem, therefore, a particular format is not necessary here, especially since they are often already fixed in preliminary technical conditions. Depending on the selected output medium, different ways of displaying terrain are selected.

AutoCAD files (* .dwg) are often difficult to export to other vector programs, such as CorelDraw or Adobe Illustrator for further editing. However, for inclusion in publications, AutoCAD plans and drawings can be displayed as PDF files, converted to JPEG files, expanded, or modified using image editing programs.

Method scope

Method scope




Accurate information about the Earth’s surface is fundamental in many sciences. Topography controls the range of processes in the earth's crust (evaporation, water flow, mass movement, forest fires), which are important for the exchange of energy between the physical climate system in the atmosphere and biogeochemical cycles.

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Another area of ​​application for DEM is the global classification of land cover. Accurate mapping and classification of the earth's surface on a global scale is the most important prerequisite for large-scale modeling of geological processes. Numerous studies have demonstrated that radar images are suitable for documenting and classifying natural vegetation and agricultural areas.

For remote sensing, DEMs are used to correct images or obtain thematic information regarding the geometry of the sensor and local terrain.

Thus, for the synergistic use of various GIS sensor systems, the use of digital terrain models is a prerequisite for encoding satellite images and correcting terrain effects.




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