The term Digital Elevation Model (DEM), Digital Terrain Model (DTM) and Digital Surface Model (DSM) has several meanings and are not always understood properly, correctly or misinterpreted.
In this article I am trying to provide a clear understanding about DEM, DTM and DSM. Down the line you fill find out what is DEM, DTM and DSM and difference among DEM, DTM and DSM.
Let’s start to clear all the mist.
Digital Elevation Model (DEM)
Digital Elevation Models (DEMs) are a type of raster GIS layer. In a DEM, each cell of raster GIS layer has a value corresponding to its elevation (z-values at regularly spaced intervals). DEM data files contain the elevation of the terrain over a specified area, usually at a fixed grid interval over the “Bare Earth”. The intervals between each of the grid points will always be referenced to some geographical coordinate system (latitude and longitude or UTM (Universal Transverse Mercator) coordinate systems (Easting and Northing). For more detailed the information in DEM data file, it is necessary that grid points are closer together. The details of the peaks and valleys in the terrain will be better modeled with small grid spacing than when the grid intervals are very large.
In short: DEM is used to refer specifically to a raster or regular grid of spot heights.
There is a variety of DEM source data available for developed areas and the suitability of this available data is depending on the project specifications. In remote regions around the World, were little or no source Data is available, the DEM can be produced by automatic DEM extraction from stereo satellite scenes, from Satellite sensors such as IKONOS (2-5m resolution), SPOT-5 (5-10m res.) and ASTER (15-25m res.).
Input to generate DEM
Following are the input source to generate DEM.
- Ground Control Points (GCPs).
- Triangulate Irregular Network (TIN).
- The DEM can also be provided from Stereo digital aerial photography at various resolutions, depending on the quality and scale of the aerial photography.
Types of DEM
Digital Terrain Model
A digital terrain model (DTM) can be described as a three – dimensional representation of a terrain surface consisting of X, Y, Z coordinates stored in digital form. It includes not only heights and elevations but other geographical elements and natural features such as rivers, ridge lines, etc. A DTM is effectively a DEM that has been augmented by elements such as breaklines and observationsother than the original data to correct for artifacts produced by using only the original data. With the increasing use of computers in engineering and the development of fast three-dimensional computer graphics the DTM is becoming a powerful tool for a great number of applications in the earth and the engineering sciences.
For practical purpose this “Bare Earth” DEM is generally synonymous with a Digital Terrain Model (DTM).
DEM is far cheaper to produce an a DTM.
Quality and Accuracy of DEM/DTM
The quality of a DEM/DTM 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.
Common uses of DEMs
- Extracting terrain parameters.
- Modeling water flow or mass movement (for example, landslides).
- Creation of relief maps.
- Rendering of 3D visualizations
- 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 analyses in geomorphology and physical geography.
Now let’s a have a look what is Digital Surface Model (DSM).
Digital Surface Model (DSM)
Digital Surface Model (DSM) represents the MSL elevations of the reflective surfaces of trees, buildings, and other features elevated above the “Bare Earth”.
In short: digital surface model represents the earth’s surface and includes all objects on it.
How DEM/DTM is Different from DSM
Now I am leaving on you to check out the difference between DTM/DEM and DSM. Given below are two figures to help you out.
Hope you might have understand the differences.