Accuracy- and Quality-Models for Airborne Scanner Images.

Digital recording systems which scan the surface of the earth line by line are called scanners. Scanners are not only successfully used in spaceborne applications, but also more and more in airborne applications. To meet the requirements of various remote sensing applications, special system designs of scanners have been developed; e. g. whisk-broom scanners for additionally recording thermal data, and three-line cameras are remarkable for their along-track stereo-capability. As a prerequisite for further use, for example in a Geographic Information System, remote sensing data has to be geo-coded; this way the relation to a ground co-ordinate system is established. At present accuracy- and quality- models for airborne scanner imagery (recorded by a one- or more-line scanning device) equivalent to the already existing accuracy-models for aerial images of analogue frame cameras do not exist. For project- and flight- planning such models would be a valuable tool for accuracy considerations and in the following for time scheduling and cost accounting. Aim of this research project is the development of accuracy and quality models for geo-coding of airborne scanner images. At the Institute of Photogrammetry and Remote Sensing of the Vienna University of Technology an universal method was developed which can be applied to scanner images recorded by all current scanning devices (whisk- broom scanners or push-broom scanners with one or more detector-lines). Based on least squares adjustment orientation parameters for each single image line are determined modelling position and attitude parameters along the flight path by spline functions. In addition to points also straight lines as well as general curved lines (described by polynomials or spline functions) can be used as control- and tie-information. Furthermore also direct observations of position and attitude parameters along the flight path through Global Positioning System (GPS) or Inertial Navigation System (INS) can be taken into consideration. Hence, this method is well suited for accuracy simulations, which serve as foundation for the development of accuracy- and quality-models for geo-coding of airborne scanner images.

In the course of this research project accuracy aspects of the geo-coding process of multi-spectral airborne scanner images have been investigated. Multi-spectral airborne scanners are remote sensing devices which scan the surface of the Earth line by line. Scanners gain increasing importance for example in the field of environmental monitoring. As a prerequisite for further use, for example in a Geographic Information System, scanner images have to be geo- coded; this way the relation to a ground co-ordinate system is established. The process of geo-coding is performed in two steps: a) Determination of the exterior orientation parameters (i.e. position and attitude) of the scanning device along the flight path in the course of a least squares adjustment. Different types of observations can be introduced into the adjustment: control and tie information (points and lines) as well as direct observations of the exterior orientation parameters provided by an integrated GPS/INS-system (Global Positioning System / Inertial Navigation System). As mathematical model for the exterior orientation along the flight path spline curves can be used. b) In the second step the resampling of the scanner images is performed. For this step the exterior orientation parameters along the flight path (from step a), the scanner image as well as a digital terrain model are necessary. In the course of this research project two datasets of airborne scanner images of Vienna from the years 1997 and 2000 could be geo-coded. In both flight missions the multi-spectral airborne rotation scanner DEDALUS AADS 1268 (ATM) was used; the Austrian Health Institute (Österreichisches Bundesinstitut für Gesundheitswesen) is the owner of the datasets. The significant difference between these two data sets is, that during the flight mission 1997 no integrated GPS/INS-system was available but during the flight mission 2000 a GPS/INS-system was already in operation. The results of the research project can be summed up shortly: High quality geo-coding of airborne scanner images without GPS/INS-support is costly but feasible under the condition that enough control- and tie information is available. This aspect is especially relevant for older scanner images which can provide a valuable input for a time series in the context of environmental monitoring. Although GPS/INS-systems have reached a high maturity in the meanwhile, during the geo-coding process of the scanner images of the year 2000 some kind of in-homogeneities in the GPS/INS-observations were detected. By extending the mathematical model of the least squares adjustment, the negative effects of these in-homogeneities could be corrected or - at least - reduced. The results of this research project are presented at several international symposia; more details can be found in the proceedings (http://www.ipf.tuwien.ac.at/).