Written by AT
Digital Elevation Model (DEM) is the digital representation of the land surface elevation with respect to any reference datum. DEM is frequently used to refer to any digital representation of a topographic surface. DEM is the simplest form of digital representation of topography. DEMs are used to determine terrain attributes such as elevation at any point, slope and aspect. Terrain features like drainage basins and channel networks can also be identified from the DEMs. DEMs are widely used in hydrologic and geologic analyses, hazard monitoring, natural resources exploration, agricultural management etc. Hydrologic applications of the DEM include groundwater modelling, estimation of the volume of proposed reservoirs, determining landslide probability, flood prone area mapping etc.
Three main type of structures used are the following.
· Regular square grids
· Triangulated irregular networks (TIN)
· Contours.
A digital elevation model (DEM) is a digital model or 3D representation of a terrain's surface, created from terrain elevation data. There are three similar names as digital elevation model (DEM), digital terrain model (DTM) and digital surface model (DSM). DEM is a subset of the DTM, which also represents other morphological elements.
Creation of DEMs:
Several methods are available to create DEM.
1. Conversion of printed contour lines
The first method is conversion of printed contour lines and use it in raster or vector form.
2. Photogrammetry:
This can be done manually or automatically:
a. Manually, an operator looks at a pair of stereophotos through a stereoplotter and must move two dots together until they appear to be one lying just at the surface of the ground
b. Automatically, an instrument calculates the parallax displacement of a large number of points.
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. A 7.5-Minute DEM covers 30- x 30-meter data
spacing.
Production of DEM:
Mappers may prepare digital elevation models in a number of ways, but they frequently use remote sensing rather than direct survey data. One powerful technique for generating Digital elevation models is interferometric synthetic aperture radar where two passes of a radar satellite. 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. The SPOT 1 satellite (1986) provided the first usable elevation data for a Sizeable portion of the planet's landmass. Older methods of generating DEMs often involve
interpolating digital contour maps that may have been produced by direct survey of the land surface.
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;
Common uses of DEMs
As an architect, I feel that the most important application of DEM ffor me is to understand the basic massing of site. The better visualisation we can get, the better is our approach to designing the builtmass on the respective site. There have been many occasions where DEM has given us detailed and accurate insight of the contours of site and helped us deliver sustainable solutions for proposed projects in hilly areas of Himalayan foothills, north east India and A&N islands. Other more important uses are as following but may not be limited to this list.
1. Extracting terrain parameters.
2. Modeling water flow or mass movement (for example, landslides).
3. Creation of relief maps.
4. Rendering of 3D visualizations
5. Creation of physical models (including raised-relief maps).
6. Rectification of aerial photography or satellite imagery.
7. DEM is used to determine the attributes of terrain, such as elevation at any point, slope and aspect.
8. DEM is also used to find features on the terrain, such as drainage basins and watersheds, drainage networks and channels, peaks and pits and other landforms.
9. Modelling of hydrologic functions, energy flux and forest fires, etc can be done using DEM data.
Following figure shows an example of a generated site model with DEM
DEM applications
1. Planning Guide for TPS in contoured areas
2. Design assistance while modelling architectural buildings in contoured areas
3. Determining road and infrastructure network suck as pipelines etc
4. Determining underground access networks such as metros and tunnels etc
5. Estimating elevation
6. Estimating slope and aspect
7. Determining drainage networks
8. Determining watershed
9. Terrain stability – Areas prone to avalanches are high slope areas with sparse vegetation, which is useful when planning a highway or residential subdivision.
10. To create a profile from digitized features of a surface.
Digital Elevation Model (DEM), consists of a sampled array of elevations for a number of ground positions at regularly spaced intervals, as shown in this fig.
*Indian Space Research Organisation provides all DEM data of India through it portal Bhuvan.
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