Digital elevation models
Accurate DEMs are arguably one of the most important sources of spatial information used for managing river basins. Surface topography is a key factor in estimating catchment areas and surface water runoff. DEMs are therefore used extensively for watershed modelling and monitoring. DEMs also play an important role in risk assessment for natural disasters such as flooding and landslides. Planning activities are often centred on parameters derived from DEMs. For example, slope and aspect data can help locate optimal agricultural areas, while river valley characteristics (e.g. reservoir capacity) is a critical factor for exploring hydrological power potential. Generally, most civil engineering projects also require a thorough understanding of landscape topography.
A Digital Elevation Model (DEM) is a 3D digital representation of surface topography and can be created for any surface of the earth at varying levels of spatial details and accuracy, depending on available resources and user requirements. A distinction is often made between a Digital Surface Model (DSM), which represents the earth’s surface including all objects upon it, and a Digital Terrain Model (DTM), which represents the surface as it would be without any objects upon it. A wide variety of satellite sensors can be used for creating DEMs including optical sensors with stereo imaging capabilities (e.g. QuickBird, WorldView, Aster) and radar sensors (e.g. TanDEM-X) using Interferometry. High resolution DEMs are available at 1 – 30 meter pixel resolution.
Known Limitations
Digital elevation models can be obtained in a wide range of resolutions and qualities, mainly depending on the availability of input data and budget. The coarser resolution DEMs (>10 meter resolution) are off-the-shelf products, while higher resolution DEMs require processing and in some cases new data acquisitions before they can be delivered. Up-to-date and very high resolution DEMs can be expensive compared to off-the-shelf products offered freely at up to 30 meter resolutions. If relying on stereo optical imagery for DEM creation, cloud cover can be an issue. Although high resolution DEMs are generally very cost effective and relatively accurate, there is inherent uncertainty in the products (e.g. vertical accuracy usually below 1 m), which may be deemed insufficient for certain hydrological applications. Estimations of height in steep terrain valleys and “urban canyons” in densely built up cities can be uncertain due to the terrain “shadow” effects when using stereo optical and SAR imagery caused by the look angle of the sensors.
Future Enhancements
Sensor advancements offer increased spatial resolution and accuracies. For example, Pleides-1A and 1B can be programmed to acquire Tri-Stereo imagery for the production of high quality 1m resolution DEM’s. This capability overcomes problems related to terrain shadow and reveal otherwise hidden elevation data in steep terrain and dense urban areas. A rather new, but yet not widely used, approach for DEM creation is to apply the principle of stereovision to any or all available satellite imagery from a given region. Because the imagery does not need to be captured as traditional stereo pairs, image selection is very easy, and with an absolute accuracy of 3m in all dimensions (SE90) without using Ground Control Points. As an alternative, and as needed for certain hydrological applications, unmanned Aircraft Systems and airborne LIDAR systems can currently offer DEM’s in cm pixel resolution with vertical accuracies of ± 15 cm.
