Comparison of Photogrammetric Applications Based on Narrow Angle Line Scanners with Traditional Photogrammetric Methods

By the Pegasus project digital line scanner images taken from an elevation of 20km, with a ground sampling distance (GSD) of 20cm, will be available. The photogrammetric use of such images is compared with traditional aerial photos, but also digital aerial cameras as well as high resolution space sensors. Line scanner images usually should be combined with direct sensor orientation. But for not turbulent flight conditions it is also possible to determine the scene orientation by bundle orientation based on orientation fixes. For the 6.8° field of view of the planned line scanner camera, this is causing problems of extreme correlations of the orientation unknowns. At least the attitudes or the projection centre line should be available. Under operational conditions analogue images are scanned with 20μm pixel size. Compared with digital airborne images, the information contents of analogue photos agree with this pixel size; scans taken with higher resolution are influenced by film grain. Based on this, a GSD of 20cm corresponds to a scale of analogue photos 1 : 10 000 – this is always large scale. Following the rule of thumb for the information contents that a GSD of 0.1mm in the map scale is required, 20cm GSD allows the generation of topographic maps 1:2000. For topographic mapping and also orthoimages a horizontal accuracy of 0.25mm in the map scale is necessary; that means for the scale 1:2000 accuracy of 50cm should be guaranteed. A standard deviation of 50cm means 2.5 times GSD. With sufficient control such accuracy can be reached without problems, the bottle neck is not the accuracy, it is the information contents. A direct sensor orientation having a precision of 20cm would require the same attitude accuracy like available by IKONOS – it leads to approximately 7m ground accuracy from 680km flying height or 20cm from 20km height. Gyros do not have a good long time precision; they are updated in the case of satellites by star sensors. A similar support of the attitudes is required also for Pegasus. In addition the GPS positions should be guaranteed with +/-10cm. This is a very high level for the requirements, but nevertheless at least the relative accuracy of 20cm should be reached without problems and so with control points pixel accuracy should be possible. Orthoimages should have at least 8 pixels/mm in the presentation scale. With a lower number the human eye can see the individual pixels. 8 pixels/mm or 0.125mm/pixel corresponds to a possible orthoimage scale 1 : 1600. A required accuracy of 0.25mm for the orthoimage leads to 0.4m in the object space or 2 GSD. With a sufficient digital surface model (DSM) this can be reached without problems. But usually digital elevation models (DEMs) with the height values of the bare ground are available. The generation of a DSM with the height values of the visible surface is time consuming and expensive. Here the small field of view of the Pegasus multispectral digital camera has an advantage. If the camera is oriented to the nadir, the largest nadir angle is just 1200m / 20km = 1:16.7. That means an offset of 40cm is caused if the DSM has a height error of 0.4m * 16.7 = 6.7m. The generation of a DSM with such accuracy is usually not a problem. It even can be generated with image strips taken with the Pegasus camera having a sidelap of 50%. Of course this causes only a height to base relation of 16.7, but this is sufficient. Images of a stereo combination with such a small angle of convergence are very similar, so an automatic image matching is quite better like with a large convergence angle. With an IKONOS stereo pair having a height to base relation of 9.0 the matching was possible with a standard deviation of the xparallax of +/-0.25 pixels. Such an x-parallax with a height to base relation of 16.7 and a GSD of 20cm results in a vertical standard deviation of 1.3m, this is quite better than required. A GSD of 20cm will be reached with the large format digital camera DMC and the line scanner camera ADS40 from a flying height of 2000m and with the UltraCamD from a flying height of 2300m. The swath width is in a similar range. Of course from a lower altitude a shorter distance through the atmosphere has to be passed, but the largest problems with the atmosphere are still in the lower range. The mayor difficulty in generating orthoimages with these cameras is the inclined view to the scene corner or side. The DMC has a nadir angle up to 40° and the ADS40 and the UltraCamD and the UltraCamX 30.5°. That means they require for an accurate true orthoimage a DSM with accuracy in the range of 50cm to 70cm.