Remote sensing for non-experts: An introduction to elevation models
If you’re a Geospatial analytics enthusiast, this article is for you. Have you ever come across terminologies like Digital Elevation Model, Digital Surface Model, Elevation or Height Maps, when reading about Geospatial data? Today, we’ll learn the basics of these digital models that aid geospatial data analytics. Maybe the naming sounds intuitive enough, implying that all these terms represent the heights of the world locations. While you’re not wrong, there’s more to it. If it’s only about height and elevation, why do we use so many different phrases? In this blog, we would be explaining the difference between these terms, with visual examples.
Understanding popular digital models used in geospatial analytics
Elevation Map: An Elevation/Topographic map shows the elevations (in meters or feet) in a region depicted on a map with respect to the sea level in a digital format. It can have negative values as well. An elevation map can be represented in multiple formats, e.g., Heat map, Contours, 3D models, etc.
There are primarily three types of elevation models based on the surface type they capture:
- Digital Elevation Model (DEM): A DEM is a digital model representing the bare-Earth surface, removing all-natural and built features. It is a 3D computer graphics representation of the bare ground topographic surface of the Earth excluding trees, buildings, and any other surface objects.
- Digital Surface Model (DSM): The DSM captures both the natural and built/artificial features of the environment. E.g., Electric towers, buildings, trees, wires, etc. As DSMs represent the bare-Earth and all of its above-ground features, they are particularly useful in urban planning. 3D surface models can improve the understanding and explanation of complex urban scenarios, especially as built-up areas change with time due to urban expansion.
- Digital Terrain Model (DTM): A DTM augments a DEM, by including vector features of the natural terrain, such as rivers and ridges. The main difference between the two models lies in the fact that the DEM generally takes into account all persistent objects on the ground (vegetation, buildings, and other artifacts) — while the DTM shows the development of the geodesic surface. DTM is essentially a 3D digital representation of a surface, consisting of X, Y, and Z coordinates. A DTM includes heights and elevations as well as natural features such as rivers and ridge lines.
*Although the definition captures a generic view, but it changes as per the geographic locations. In some countries, DTM is synonymous with a 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).
Height Map: A Height map or Heightfield is a raster image used mainly as Discrete Global Grid in secondary elevation modelling. Each pixel stores values, such as surface elevation data, for display in 3D.
Height Map displayed in 2D Height Map displayed in 3D
Triangular Irregular Network (TIN): A TIN is a vector-based representation of an Elevation model. The TIN representation has information about altitude, slope and aspect.
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References:
https://geodetics.com/dem-dsm-dtm-digital-elevation-models
https://planningtank.com/geographic-information-system/digital-elevation-model
https://www.gisresources.com/confused-dem-dtm-dsm/
https://gisgeography.com/dem-dsm-dtm-differences/