Geometric accuracy evaluation of theos imagery

This paper presents the georeferencing performance of the imagery from the Thailand Earth Observation System satellite (THEOS) using a generic sensor model developed at the Cooperative Research Centre for Spatial Information (CRCSI), Australia. THEOS provides 2m resolution imagery worldwide together with camera parameters, orbit and attitude data. An assessment of the planimetric accuracy of georeferencing from THEOS imagery, both for single scenes and image strips, has been carried out within a 25 x 110km test field area in Australia, in which a dense array of GPS-surveyed points were established. The test imagery comprised a strip of five images collected within the same orbit. The generic sensor model and integration of the THEOS orientation parameters into the model are first described, along with a brief account of the estimation of camera interior orientation parameters and the concept of strip adjustment. Further investigation reveals presence of errors in the satellite line-of-sight data, possibly caused by mis-alignment of detectors in CCD array, yielding imprecise interior orientation of the sensor. Such errors can be effectively accounted for through modelling via a cubic polynomial, leading to sub-pixel georeferencing accuracy. The test data and experimental procedure are then discussed and the results of the georeferencing of both single images and the 5-image strip as a single entity, via a strip adjustment approach, are presented. The results demonstrate that sub-pixel 2D geopositioning accuracy can be achieved with single THEOS images and within strips of up to three images with as few as six ground control points (GCPs) to effect an orbit adjustment, whereas accuracy decreases to near the 2-pixel level over a strip length of five images.