Comparison of orthorectification methods suitable for rapid mapping using direct georeferencing and RPC for optical satellite data

One of the first essential steps in response to a crisis event is to provide current, precise, and rapid information about the extent of the event, the affected population and infrastructure (e.g., information for relief teams). Experiences in crisis mapping show, that remote sensing data are an important information source to get an overview of the situation. For an efficient use of geo-information before or during a crisis of any kind, all information should be orthorectified. Satellite data are delivered normally geocoded with varying accuracy. However, today orthorectification without ground control points (GCP) using only ephemeris and attitude data measured on board the satellite provide an absolute accuracy of about 20 m to 1 km (depending on the satellite). For the intended applications like change detection this accuracy is not sufficient. For this purpose, accuracy in the sub-pixel range is necessary requiring a precise orthorectification. Different methods deployed in different software packages are analyzed and compared in this paper. They are chosen depending on the available metadata. To get an objective comparison of the different orthorectification methods, their accuracy is determined for several test datasets in this study. Additionally to the comparison of the accuracy, further criteria are analyzed like the potential of the algorithm for optimization and automation and the computing time needed, since the timely supply of information is also one of the most important requirements in crisis respond mapping. Results using the commercially available software packages Erdas Imagine, PCI Geomatics and ENVI are compared to products of the in-house developed software XDibias of the German Aerospace Center (DLR). All programs result in high accuracy. Differences are experienced in the computing times and userfriendlyness. While the commercial programs are very user friendly, knowledge about the details of the in-house developed software are very helpful and the automation potential of it has already been proven.