Radar-derived Dems for Urban Areas

The role for Digital Elevation Models (DEMs) has been expanding rapidly in recent years. Applications are increasing and encompass industries as diverse as telecommunications, forestry, municipal engineering and precision agriculture to name only a few. The accuracy and detail required varies with application, and a number of technologies are able to provide DEMs appropriate to particular mapping scales. To date, DEM availability, accuracy and price issues have tended to be a disincentive for many users. The problem is evident on a global basis. However, even in the well mapped areas of North America and Europe there is a desire from users for more detail over broader areas and at lower cost. In response to this perceived demand, Intermap Technologies has commenced the development of a data bank of medium and high detail/accuracy DEMs under the name of Global TerrainTM. In particular, it is the goal to create a DEM data base of 5 meter postings and 1.5 meter (1 ) vertical accuracy for large portions of North America and Europe over the next two years. Most of this data base will be created using STAR-3i, an interferometric airborne SAR carried in a Lear Jet. STAR-3i has been operated commercially by Intermap since January 1997. Its products include DEMs and Ortho-Rectified Images (ORIs), both acquired at sample spacing of 2.5 meters although normally averaged down to 5 meters or coarser. Previous application areas for radar have typically been rural. Because of the spatial detail now available with STAR-3i, and with the price competitiveness afforded by the Global TerrainTM strategy, there is now considerable interest in the applicability of these DEMs and ORIs for urban applications such as mobile telecommunications. However, urban areas are very complex as seen by the radar, and there are a number of effects that may impact the appearance of the DEM and/or the ORI. This work reports on the ‘performance’ of STAR-3i DEMs in two urban test areas of Denver, Colorado, USA. The test areas were chosen to be representative of industrial, high-rise and low-rise residential regions outside the dense urban core. As a basis of comparison, DEMs and orthorectified images from air-photos were acquired from a commercial vendor for both areas. Additionally, a high precision DEM derived from an airborne laser scanning system was obtained for one of the areas. This paper reports on the results to-date of the comparison. In order to put the results in context, the various factors that contribute to the observed DEM response to buildings and other objects are examined. A major objective of the work, which is on-going, is to determine to what extent the STAR-3i DEM can be used for 3D building extraction and, on the other hand, for creation of a ‘bald-earth’ DTM. In this paper we provide illustrations and statistical data to demonstrate the viability of STAR-3i DEMs for both of these applications in a typical North American city. The Global TerrainTM DEMs therefore represent a technically attractive and cost competitive alternative for many urban applications.