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Precision LIDAR Data and True-Ortho Images Uwe Lohr TopoSys GmbH, Wilhelm-Hauff-Str. 41, 88214 Ravensburg, Germany Tel: +49 751 3660512; email: u.lohr@toposys.com; website: www.toposys.com
1. Introduction In the recent years an increasing demand on precision digital elevation models (DEM) has to be noted - high-precision in this sense means that the accuracy of the DEM is at least ± 0.5 m in x/y and better than 0.2 m in z. Typical applications of such DEM are monitoring of coastal erosions, simulation of floods in river basins, and simulation of antennae sites or line-of-sight calculations for mobile communication networks. Many of those applications not only request for high precision, but also for short delivery times. The TopoSys laserscanner system FALCON is a LIDAR system which is based on glass fiber tech-nology and especially designed to produce precision elevation models. Recently, FALCON has been complemented by an four channel image scanner. This configuration offers various advan-tages: First of all, elevation and image data is taken at exactly the same time. So, these data sets do not suffer from e.g. such problems that there are new buildings in one data set which in the older one do not exist. Another pro is that the LIDAR digital surface model (DSM) may be taken as data set to rectify image data to a true-ortho projection. 2. Toposys Lidar and line Scanner Camera Since 1996 TopoSys produces precision LIDAR DEM, which mainly are used in large scale appli-cations. Both the LIDAR systems operated by TopoSys GmbH for services in Europe are supple-mented by line scanner cameras for about three years. 2.1. Precision LIDAR The TopoSys LIDAR is a fiber optical line scanner system, which generates 83,000 measurements per second. Due to the glass fiber technology, the distance of measurement spots on ground is regu-lar in across- and along-track direction. In flight direction the scan lines slightly swing, which in-creases the overall distribution of measurement spots (see also Figure 1). TopoSys LIDAR DEM are raster DEM typically available at raster widths between 0.5m and 2 m. At an aircraft speed of 70 m/sec and a survey altitude of e.g. 900 m above ground the LIDAR pro-vides an average density of 4 to 5 measurements per m². The 1 m standard product is calculated from these 4 to 5 measurements per m². The processing of the data will be adapted to the final use of the elevation data. So, the application determines, which of the measurements will be the best representatives within a raster cell, i.e. whether highest or lowest measurements are the best or whether measurements will be averaged.  Figure 1: Advanced scan pattern of the FALCON LIDAR Table 1: Some parameters of TopoSys LIDAR.
2.2. Line scanner camera Table 2: Parameters of TopoSys Line Scanner Camera.
The TopoSys line scanner camera gathers image data in four spectral channels. The design require-ment was that image data must have double the resolution of the raster elevation models. So, from e.g. a survey altitude of about 900 m above ground, image pixel size is 0.5 m, while DEM raster width is 1.0 m. Gathered strips of image are mosaiced and compensated for radiometric differences. Images are available in true-ortho projection, either as four separate channels or as true-color (R,G,B) and color-infrared composites (NIR, R,G) in Geo-TIF format.
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