Elevation Models in GIS Environment

The three most frequently used geospatial features generated by mapping systems are digital elevation models (DEM), digital surface models (DSM), and digital terrain models (DTM). Because each model employs a different methodology, each data product provides a different elevation value.

Digital Elevation Model (DEM)

A digital elevation model is a bare-earth raster grid referenced to a vertical datum. When you filter out non-ground points such as bridges and roads, you get a smooth digital elevation model. The built (power lines, buildings, and towers) and natural (trees and other types of vegetation) aren’t included in a DEM.

DEMs are essential for land-use planning, infrastructure project management, geotechnical engineering, hydrology, and flow-direction analyses, etc.

Inputs for DEM Creation

• Ground Control Points (GCPs).
• Contours.
• Triangulate Irregular Network (TIN).

The DEM can even be obtained from stereo digital aerial imagery at various resolutions, depending on the quality and scale of the imagery.

Satellite Interferometry: Synthetic aperture radar such as Shuttle Radar Topography Mission uses two radar images from antennas captured at the same time to create a DEM.

Photogrammetry: In aerial photography, photogrammetry uses photographs from at least two different vantage points. Similar to how your vision works, it’s able to obtain depth and perspective because of the separate vantage points.

LiDAR: Using light, LiDAR measures reflected light that bounces off the ground and back to the sensor to obtain the elevation of the Earth’s surface. A DEM is created by producing a mesh from the LiDAR ground points and then using one of several interpolation algorithms to develop a jointed fabric that precisely represents the real-world ground model. Creating a DEM from a LiDAR dataset alone can reveal hidden archaeological or geological features that were previously obscured from an aerial perspective or photogrammetric analysis by naturally occurring terrain features.

Characteristics and Accuracy of DEM

A DEM is a measurement of how precise the elevation at each pixel is (absolute accuracy) and how precisely the morphology is presented (relative accuracy). So many factors have an impact on the quality of DEM-derived products:

• Terrain roughness
• Extracting terrain parameters
• Sampling density (elevation data collection method);
• Grid resolution or pixel size;
• Interpolation algorithm;
• Vertical resolution;
• Terrain analysis algorithm
• Rendering of 3D visualizations.
• Intelligent transportation systems (ITS)
• Line-of-sight analysis
• Archaeology
• Auto safety / Advanced driver-assistance systems (ADAS)

Used cases

• Hydrologic Modeling: Hydrologists use DEMs to delineate watersheds, calculate flow accumulation and flow direction.
• Terrain Stability: Areas prone to avalanches are high slope areas with sparse vegetation. This 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).
• Relief maps
• 3D visualization rendering
• Aerial photography or satellite imagery that has been corrected.
• Gravity measurements are reduced (terrain correction).
• Terrain analysis.

Digital Surface Model (DSM)

Light pulses travel to the land surface in a LiDAR system. When a light pulse bounces off its target and goes back to the sensor, it provides the range (variable distance) to the Earth.   Finally, LiDAR produces a massive point cloud with elevation values. Height, on the other hand, can come from the tops of buildings, tree canopy, powerlines, and other features.

A DSM is a representation that depicts the natural and man-made features of the Earth's surface. A DSM can be used to create 3D models for telecommunications, urban planning, and aviation. DSMs are widely used in fields such as urban planning because they represent the bare Earth and all of its above-ground features. For instance, investigating how a new project would affect the views of residents and businesses, inspecting power line corridors, and planning aviation

Used Cases

Ø  Runway Approach Zone Encroachment: DSMs in aviation can detect runway obstructions in the approach zone.

Ø  Vegetation Monitoring: DSMs can identify where and how much vegetation is intruding on a transmission line.

Ø View Obstruction: DSM is being used by urban planners to evaluate how a proposed building will affect the viewsheds of businesses and residents.

Digital Terrain Model (DTM)

A DTM is a DEM that has been supplemented with elements like surface breaklines and inferences, rather than just the original data. This is accomplished by interpolating contour lines onto a DEM. Most commonly, these come in the form of a regular grid, with each pixel assigned a unique elevation value.

A DTM can be interpolated to produce a DEM.  Digital Terrain Model (sometimes called a triangulated irregular network, or TIN) is a three-dimensional, computer-generated model of a certain section of the earth’s surface. The edges of the TIN triangles are more precisely defined by using the brake lines. DTMs are perfectly applied to Geographic Information Systems (GIS) and cartographic interpretations. They have been used in flood or drainage modeling, earthwork construction, land-use analyses, geospatial applications such as watershed, water level, flow accumulation, slope, slope direction, and others.

DTM is a vector set made up of regularly spaced dot representations. At almost the same time, natural features such as a hill or a river are depicted in the vector data as contour lines connecting these dots. It is used to assess the earth's natural processes, floods and water infiltration, vegetation distribution, and large insurance firms use it to determine the scope of damage cover.

Canopy Height Model (CHM)

In particular, the difference in elevation between DSM and DTM can be used to create a Canopy Height Model (CHM). The Spatial Cover Canopy Height Model (CHM) is a raster dataset with a high resolution (1 meter) that maps tree height as a continuous surface. The height of the tree above the underlying ground topography is represented by each pixel in the CHM. LIDAR data is the primary source of information for creating a CHM. Based on light pulses reflected from the earth, LIDAR (Light Detection and Ranging) technology allows for accurate elevation measurement for various ground surface types. 

Summary

A digital elevation model (DEM), as we all know, is a bare-earth raster grid referenced to a vertical datum. When non-ground points like bridges and roads are removed, a gentle digital elevation model is obtained. A DEM does not include both built (power lines, buildings, and towers) and natural (trees and other types of vegetation). DTM and DEM are remarkably similar at this point. DTMs are created using the Stereo Photogrammetry technique. Because of the existence of 3D points at regular intervals and 3D refraction lines, it is capable of displaying distinct terrain features. We believe you have gained a basic understanding of the three varying models used in GIS survey work. It is critical to select a model that meets your GIS requirements.