PRECISE GEOREFERENCING OF CARTOSAT IMAGERY VIA DIFFERENT ORIENTATION MODELS

摘要

The CARTOSAT 1 satellite, launched by the Indian Space Research Organisation (ISRO) in 2005, can provide panchromatic along-track stereo imagery with a ground resolution of 2.5 m. Along with the imagery, encrypted files with rational polynomial coefficients (RPCs) and meta-data are distributed by ISRO. The RPCs allow direct georeferencing within certain limits depending on the on-board systems for registering the orbit path and attitudes of the satellite. At the Cooperative Research Centre for Spatial Information at the University of Melbourne (Australia), the software package Barista for the processing of high-resolution satellite images is being developed. Barista offers three techniques for precise georeferencing of such image data, namely the 3D affine model, bias correction for RPCs, and a generic pushbroom sensor model. The 3D affine model can only be applied when ground control points (GCPs) are available. The RPC model can be improved beyond the limits of direct georeferencing by correcting for the biases contained in the original RPCs. This process requires at least one well-defined GCP per image. Whereas the meta-data for CARTOSAT 1 imagery do not contain all the information required for using the generic pushbroom sensor model for direct georeferencing, they provide initial values for such a sensor model to be determined if enough GCPs are available. In this paper, the authors compare the geopositioning accuracy achievable with CARTOSAT 1 imagery via the 3D affine, bias-corrected RPC and generic pushbroom sensor models. A stereo pair of images covering Hobart, Australia, was processed using Barista. In addition to the imagery, an object point array of altogether 69 3D GPS-surveyed points was utilised. They were distributed all over Hobart and covered about one quarter of the scene. In order to assess the georeferencing accuracy that can be achieved using CARTOSAT 1 images, bundle adjustment was carried out using all three sensor models and nine well-distributed GCPs. The absolute accuracy was then assessed via the remaining 60 points, which served as independent checkpoints. The georeferencing results obtained for CARTOSAT 1 in the Hobart test field are very encouraging. Whereas direct georeferencing using the RPCs provided by ISRO yielded sub-optimal results, the provision of a small number of GCPs is enough to boost the positioning accuracy to subpixel level in planimetry and to make it slightly better than 1 pixel in height, independent from the sensor model used.