Generation of DEMs and orthoimages based on non-stereoscopical IKONOS images

Due to the capability of the satellite IKONOS to rotate the sensor up to an angle of 26 degrees off-nadir, the satellite can take stereo image pairs. IKONOS stereo imagery consists of two images of the same location taken from two different perspectives on the same orbital pass. The total convergence angle between two images of the stereopair is between 30 degrees and 45 degrees. These angles are suitable for terrain extraction and stereo visualization. One image is above 60 degrees elevation angle and the other one is above 72 degrees elevation. The "high leg" image (with elevation above 72 degrees) is intended for orthorectification against the DEM generated from the stereo pair. (Space Imaging, 2002) However, it is possible to create a stereopair with two IKONOS images that were not meant to be stereoscopic. The test site is the city of Brussels, Belgium. The two IKONOS images were taken on June 8, 2000 and June 5, 2001. The specifications of the image of June 8, 2000 are; scan direction: 0 degrees, nominal collection azimuth: 203.9063 degrees, nominal collection elevation: 68.56165 degrees. The specifications of the image of June 5, 2001 are; scan azimuth: 179.93 degrees with reverse scan direction, nominal collection azimuth: 83.6720 degrees, nominal collection elevation: 72.40775 degrees. Thus the two images form a stereopair taken "across track", where normally the stereo pair taken on demand is "along track". After selecting only the overlap of the two images, they are imported in the digital photogrammetric software VirtuoZo, to generate a DEM and an orthoimage, with success. Besides the problems such as clouds and changes in the image due to the great time interval between the two images, there is also a great advantage. The archive of IKONOS images that were taken is becoming more and more extended. Because of this growing archive of data, it seems likely that in order to obtain a stereopair of a certain area, it will not always be necessary to order a complete new stereo pair. Simply looking at the archive might give a good stereopair. This could mean a lower cost for a project that does not need the latest images