Georeferencing from Orthorectified and Non-orthorectified High-resolution Satellite Imagery

The extraction of metric information from image data is highly relevant for the update of geospatial data bases. Photogrammetric methods offer tools to extract such information in a reliable and accurate manner from mostly multiple image setups using appropriate sensor models. These methods require a certain amount of expertise and often expensive software systems to process the image data. An alternative to extract metric information from a multi-image configuration is feature extraction from single, already georeferenced images. The accuracy potential from georeferencing may be sufficient for various applications. This paper describes the method of georeferencing from orthorectified and non-orthorectified high-resolution satellite imagery. During the orthorectification process, georeferencing information enabling the transformation from pixel coordinates to two-dimensional object space coordinates is generated. Mathematically this georeferencing information – often supplied in the commonly used Tiff World File (TFW) format – describes an affine transformation modelling a two-dimensional shift, two scale factors and two rotations. This additional information is often part of an orthorectified image product. Alternatively, the georeferencing information can also be generated with the use of ground control points (GCPs) and can be applied to both orthorectified and non-orthorectified images. For the generation of a set of the six affine transformation parameters, a minimum of three GCPs is necessary. If only two GCPs are available, a two-dimensional conformal transformation using four parameters can be applied for the image to object space transformation. The paper analyses the two methods of coarse georeferencing applied to orthorectified and non-orthorectified highresolution satellite imagery on the basis of three different data sets and different projection systems. The data processing was performed with the software package Barista. The paper quantifies the accuracy yielded with georeferencing, discusses its limitations and concludes with comments about its suitability for potential applications.