A Digital Elevation Model (DEM) is generally a file containing elevation values representing the terrain height over a specific area of the earth. A DEM can be a raster file: in this case each record is a 3-dimensional element usually associated with geographic coordinates at a fixed grid interval. The denser the grid, the more information the digital elevation model will possess and the more variation in the terrain will be recorded. A DEM with an irregular grid is called a TIN DEM (Triangular Irregular Network) and can be the result of direct 3D point measurements.
Digital Elevation Models are derived from various sources such as photogrammetry, LiDAR, IFSAR and other less commonly used sources. Some large scale DEM have been made available free of charge to the public such as the NASA Shuttle Radar Topography Mission DEM at 3 second grid and ASTER DEM 1.5 second grid which cover most of the planet. Geoscience Australia has released a new free data products for Australia: Digital Elevation Models (DEM) at 1 second (30m resolution) derived from the SRTM Mission (previously restricted only for research purposes).
In general, the higher is the accuracy, the higher is the cost however DEM generated through a photogrammetric process can be a winning solution. Massive amounts of information can be acquired in a short time frame and becomes comparatively inexpensive compared to ground survey methods (See article “Great cost savings and time efficiencies for design engineering projects using photogrammetry”).
Measure of quality:
The quality of a DEM depends on its horizontal and vertical accuracy. The absolute vertical accuracy of a DEM is measured as the average discrepancy between sample points of the grid and surveyed positions (considered as the reference). Following the theory of normal distribution of errors, the accuracy can be stated at the 1-sigma (68% confidence interval) or 2-sigma (95% confidence) interval. For example, a vertical accuracy specification of 0.5m at 2-sigma (2s) means that 95% of the DEM points will be within 0.5m of their true (surveyed) height. A specification of 0.5m at 1-sigma (1s) means that only 68% of DEM points will be within 0.5m of their true height. It is very important that this is clearly specified as the accuracy requirement is fundamental to all other survey parameters, including pixel size, flying height, number of flight runs and cost. (see figure below).
In more simple term, the absolute accuracy is a measure of how well a DEM fits a terrain in terms of shape, position and orientation. The relative vertical accuracy is a measure of the consistency of measurement and the shape of the DEM within itself without external references.
Digital Terrain or Surface Model:
A DEM can be edited to generate a Digital Terrain Model (representation of a bare-earth model that contains elevations of natural terrain features where vegetation, buildings and other non-ground objects have been digitally removed) as opposed to a Digital Surface Model (which contains elevations of natural terrain in addition to top of buildings, trees and any other objects).
In Photogrammetry, DEMs are a by-product resulting from the interpolation of extremely dense 3D point clouds which are extracted from the imagery. Best accuracy is achieved by applying advanced algorithms for image matching and data filtering. The capture of break lines where appropriate is essential to obtain a perfect fit of a DTM with the real-world.
Digital elevation models are usually a fundamental base for any engineering project. Don’t neglect this essential step and make sure you ask an expert before making a decision. The highest vertical accuracy is not always suitable for your application but the wrong data can cost you millions.