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Precision LIDAR Data and True-Ortho Images
2.3. Amount of data Operating line scanner camera and LIDAR system simultaneously nearly doubles the amount of data to be stored during a survey flight. Table 3 gives shows the amount of data collected during the survey flight assuming a scan area of 60 km² and a survey height of 900 m above ground. Table 3: Amount of LIDAR and image data after survey flight and during processing
Really significant is the storage capacity needed during data processing, when merging image strips to a complete image and applying radiometric correction. 3. DTM, DSM and TRUE ORTHO Images One input for rectification of aerial images is the elevation model. The traditional way to correct image geometry is by using a bare ground model (digital terrain model DTM). The result of this kind of rectification is acceptable for small scale application; however, large scale images suffer from the fact that 3D objects have a lean. 3 D objects are not represented geometrically correct, because their height is not taken into account. In case of buildings, displacement of footprints and roofs is extremely unpleasant, when e.g. merging such image data with other, geographically cor-rect information. In the past, 3D information of objects was frequently only extracted for “relevant” objects like building, bridges etc. Objects like trees, cars and other irrelevant objects were not considered, be-cause it was too much work to determine their heights. Airborne Laserscanning is a means to generate a complete 3D model of the surface. A LIDAR DSM forms the ideal basis for a true-ortho rectification supposed that both scales (resolutions) of image data and LIDAR elevation data are harmonized. As result of such a work flow, Figure 2 shows a true-color and a color-infrared image of the TopoSys line scanner camera.  Figure 2: True-ortho images rectified with help of the LIDAR DSM. 4. Large Scale Applications Precise LIDAR elevation models are needed in a large number of applications. In this sense “pre-cise” means that the elevation model is available at an accuracy of at least ± 0.5 m in x/y and better than 0.2 m in z (in the local coordinate system). The following shows elevation models of such quality used for
Traditionally, simulation of floods needs very precise elevation models. Aim of such simulations is to determine areas to be prevented, to determine area in which water shall run without causing big damages (retention areas), and to define respective engineering works. Already for some time precise LIDAR DEM serve as input data for simulation. Due to the LIDAR’s ability to penetrate a forest canopy, bare ground models (DTM) can be produced even in vegetated areas. Figure 3 shows the LIDAR DSM as well as the DTM. The depicted area is about 2 * 2 km². Simultaneously taken images are frequently also requested, in order to derive further parameters (like land use), which are needed for an accurate simulation.  Figure 3: 1 m raster LIDAR DSM (left) and DTM (right) of a river basin © Landesamt f. Wasserwirtschaft, Munich, Germany 4.2. Monitoring of costal erosion Figure 4 shows a part of the island Sylt in Germany. The erosion at the western part of the island amounts to about 1 million m³ per year. The total cost for coastal prevention of the western part of the island is more than 10 million Euros per year.  Figure 4: 1 m raster LIDAR DSM (left) of the island Sylt © Amt f. Ländliche Räume, Husum, Germany Precise LIDAR elevation models of the beach area are gathered regularly after the winter storms. LIDAR DTM in combination with bathymetric measurements, taken at the same time of the LIDAR survey, allow to determine the erosion volume as well as the locations, which have to be filled up. As the water surface is clearly visible at FALCON’s laser wave length of 1.5 µm, it is easy to de-termine the land/water boundary. In Figure 4 the different textures of water surface (waves) and beach can be recognized. |