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Geo-Information based on Very High Resolution Optical Satellites
In the same area also a QuickBird scene has been used with similar results. The orientation with 3D-affine transformation and with DLT do require more and well distributed control points but did not reach the same accuracy at independent check points like bias corrected RPC and geometric reconstruction. This may be caused by the slow down factor of 1.7 used by QuickBird. The inner scene accuracy of QuickBird in relation to the GSD of 0.62m requires a 2D-affine transformation after terrain relief correction, so at least 3 control points should be used for bias corrected RPC and geometric reconstruction. Both methods do have similar sub-pixel accuracy.  QuickBird Zonguldak: Results at independent check points for the different orientation methods as a function of the number of control points OrbView-3 was only available as slightly improved original sensor image. The scenes show very clear rotations during acquisition, so the first and the last lines are not parallel. This does not allow the use of 3D-affine transformation and DLT; they are limited to 5m up to 20m accuracy. OrbView-3 is a quite less expensive satellite like IKONOS and QuickBird, it has no TDI, by this reason staggered CCD-lines are used – a combination of 2 CCD-lines shifted half a pixel in the CCD-line direction against each other. The pixels projected to the ground do have a size of 2mx2m, but they are over-sampled by 50%, leading to a GSD of 1m. By this reason the image quality is slightly less than for IKONOS and the control point identification was limited to a standard deviation of 1m. Like for IKONOS and QuickBird the direct sensor orientation without ground control was with 9.3m in the average below 10m. With the same control points like for the scenes described before it was not possible to reach sub-pixel accuracy with bias corrected RPC orientation. For the both used scenes after terrain relief correction based on a shift to the control points in the average root mean square errors of the ground coordinates of 1.9m and based on affine transformation 1.6m has been reached. In relation to the pixel size of 2m this is still sub-pixel accuracy but not in relation to the 1m GSD. The accuracy of the scene orientation is important for the generation of digital elevation models (DEM) and for GIS products. For DEMs the best accuracy is important, for GIS-products this has to be seen in relation to the map scale which is indirectly specified also for GIS products. As a rule of thumb the GIS data acquisition requires an image resolution of approximately 0.1mm in the map scale. So with IKONOS and OrbView-3 scenes a map scale 1 : 10 000 is possible, for QuickBird 1 : 6000. The mapping accuracy is sufficient with 0.25mm in the map, corresponding to the preceding rule to 2.5 pixels. With all 3 VHR sensors this accuracy requirement has been reached. GIS DATA ACQUISITION For the GIS data acquisition the ground resolution, radiometric quality, the sun elevation and the view direction of the used scenes are important. In city areas a larger nadir angle may cause problems to look down to the streets. A lower sun elevation causes shadows, worsening the street identification.  influence of sun elevation to IKONOS image quality sun elevation 63° sun elevation 41° Of course also the atmospheric conditions are important, haze, cloud shadows and the haze around clouds are reducing the image quality. Independent from this, there are still some clear differences between the 3 analyzed VHR sensors. The effective image resolution can be identified with an edge analysis, but this is also depending upon contrast improvement which in most cases has been used. So for all 3 sensor types the GSD has been confirmed as effective resolution even if the details are not so clear in OrbView-3 images like in IKONOS.  IKONOS 1m GSD, sun elevation 46°  QuickBird 0.62m GSD, sun elevation 65°  OrbView-3 1m GSD, sun elevation 63° |