A hierarchical approach for registration of high-resolution polarimetric SAR images

The presented work aims to automatically register high-resolution polarimetric SAR images with each other and other types of images. A digital topographic map is used as an aid for the registration. SAR images are very different from visual or infrared images. The idea is to identify, for each type of image, objects present on the map and easily detectable in the image. Detecting these objects in the image and matching them between map and image provides a first registration. Several object detectors were developed for the subsequent stages of the registration. Each of these detectors is briefly described. The actual registration uses a hierarchical method. First the SAR image is converted into ground range. Then a rough registration between image and map is obtained based on the position of forests and/or built-up areas. A voting method is used to find the parameters of a simple transformation model and to match the objects between map and image. The third step finds the parameters of an affine transformation based on the objects matched by the voting method. To improve the registration, objects with low 3D structure, e.g. roads and rivers, are used. The method for detecting these in SAR images yields an incomplete results leading to ambiguities for the optimal local displacement. Optimisation methods are used to overcome this problem and yield the parameters of a global transformation model. The accuracy of the registration is now within the accuracy of the map. Once the different images are registered with the map, the results of edge detectors are used to refine the registration between them.