Radiometric and geometric characteristics of Pleiades images
Pleiades images are distributed with 50 cm ground sampling distance (GSD) even if the physical resolution for nadir images is just 70 cm. By theory this should influence the effective GSD determined by means of point spread function at image edges. Nevertheless by edge enhancement the effective GSD can be improved, but this should cause enlarged image noise. Again image noise can be reduced by image restoration. Finally even optimized image restoration cannot improve the image information from 70 cm to 50 cm without loss of details, requiring a comparison of Pleiades image details with other very high resolution space images. The image noise has been determined by analysis of the whole images for any sub-area with 5 pixels times 5 pixels. Based on the standard deviation of grey values in the small sub-areas the image noise has been determined by frequency analysis. This leads to realistic results, checked by test targets. On the other hand the visual determination of image noise based on apparently homogenous sub-areas results in too high values because the human eye is not able to identify small grey value differences – it is limited to just approximately 40 grey value steps over the available gray value range, so small difference in grey values cannot be seen, enlarging results of a manual noise determination. A tri-stereo combination of Pleiades 1A in a mountainous, but partially urban, area has been analyzed and compared with images of the same area from WorldView-1, QuickBird and IKONOS. The image restoration of the Pleiades images is very good, so the effective image resolution resulted in a factor 1.0, meaning that the effective resolution corresponds to the nominal resolution of 50 cm. This does not correspond to the physical resolution of 70 cm, but by edge enhancement the steepness of the grey value profile across the edge can be enlarged, reducing the width of the point spread function. Without additional filtering edge enhancement enlarges the image noise, but the average image noise of approximately 1.0 grey values related to 8 bit images is very small, not indicating the edge enhancement and the down sampling of the GSD from 70 cm to 50 cm. So the direct comparison with the other images has to give the answer if the image quality of Pleiades images is on similar level as corresponding to the nominal resolution. As expected with the image geometry there is no problem. This is the case for all used space images in the test area, where the point identification limits the accuracy of the scene orientation.
ANALYSIS AND CORRECTION OF SYSTEMATIC HEIGHT MODEL ERRORS
Abstract. The geometry of digital height models (DHM) determined with optical satellite stereo combinations depends upon the image orientation, influenced by the satellite camera, the system calibration and attitude registration. As standard these days the image orientation is available in form of rational polynomial coefficients (RPC). Usually a bias correction of the RPC based on ground control points is required. In most cases the bias correction requires affine transformation, sometimes only shifts, in image or object space. For some satellites and some cases, as caused by small base length, such an image orientation does not lead to the possible accuracy of height models. As reported e.g. by Yong-hua et al. 2015 and Zhang et al. 2015, especially the Chinese stereo satellite ZiYuan-3 (ZY-3) has a limited calibration accuracy and just an attitude recording of 4 Hz which may not be satisfying. Zhang et al. 2015 tried to improve the attitude based on the color sensor bands of ZY-3, but the color images are not always available as also detailed satellite orientation information. There is a tendency of systematic deformation at a Pleiades tri-stereo combination with small base length. The small base length enlarges small systematic errors to object space. But also in some other satellite stereo combinations systematic height model errors have been detected. The largest influence is the not satisfying leveling of height models, but also low frequency height deformations can be seen. A tilt of the DHM by theory can be eliminated by ground control points (GCP), but often the GCP accuracy and distribution is not optimal, not allowing a correct leveling of the height model. In addition a model deformation at GCP locations may lead to not optimal DHM leveling. Supported by reference height models better accuracy has been reached. As reference height model the Shuttle Radar Topography Mission (SRTM) digital surface model (DSM) or the new AW3D30 DSM, based on ALOS PRISM images, are satisfying. They allow the leveling and correction of low frequency height errors and lead to satisfying correction of the DSM based on optical satellite images. The potential of DHM generation, influence of systematic model deformation and possibilities of improvement has been investigated.
MAPPING FROM SPACE FOR DEVELOPING COUNTRIES
Mapping today is a data acquisition for Geo Information Systems (GIS). Maps are just one form of the output of the geoinformation. Geoinformation are important as basic information for any planning purpose, nevertheless the term mapping is used for such data acquisition. Without such information no modern administration of a country is possible. In several developing countries no actual geoinformation and no regular update exists. Classical, the data acquisition is based on aerial images, but not in all countries photo flight equipment is available and foreign photo flight companies can enter some countries only with extreme bureaucratic effort, causing a delay of photo flights and raising the price. The problem of control point determination for mapping purposes today is solved by GPS-positioning, which can be improved by world wide satellite reference systems like Omnistar to absolute accuracy up to 30cm. With the high and very high resolution optical satellites a competition to aerial images exist. The highest resolution system operational now is WorldView-1 with 50cm distributed ground sampling distance (GSD) corresponding to a photo scale of approximately 1 : 20 000. For topographic maps not in every case the highest resolution is required. Based on long time experience, satellite images with 5m GSD can be used for mapping in the scale 1:50 000 or approximately 0.1mm GSD is required in the map scale. For three-dimensional point determination stereo pairs are required. From the very high resolution satellites only few stereo pairs, taken from the same orbit, are available, but this is partially solved by the stereo systems Cartosat-1 and ALOSPRISM. The image quality of ALOS-PRISM is still not optimal, but the images can be used for the generation of digital elevation models (DEM). The geoinformation also can be generated by onscreen digitizing of orthoimages if a DEM exists. Beside the possibility of the generation of height models by automatic image matching, DEMs can be bought also based on the stereo system SPOT-5 HRS, but for several purposes the nearly world wide available SRTM height model can be used free of charge. An overview about available imaging systems, the possibility of DEM generation as well as detailed description about the possibilities and limitations of the SRTM height models will be given as well as an overview about the experimental relation between GSD and possible map scale. The geometric and information potential of the serious of high and very high optical satellite images is demonstrated.