Assessment of the rectification accuracy of IKONOS imagery based on two-dimensional models

High-resolution satellite images have become a reality after several high-resolution satellites, such as IKONOS and QUIKBIRD, were launched with 1 m and 0.61 m resolution, respectively, in panchromatic mode. These satellites may change the ordinary concepts of producing varied-scale maps, especially of urban areas, which are now normally based on aerial photographs. This paper focuses on two main issues: First, the attainable geometric accuracy of the digital maps generated from IKONOS satellite imagery by using different two-dimensional transformation models; secondly, the effect of variations in the elevation of the terrain on the resulting overall accuracy. Two typical cases were selected for this research and the attainable accuracy of the checkpoints versus the number and distribution of the ground control points (GCPs) was studied further. The first case is a relatively flat area in the city of Zagazig in Egypt, where the total variation in elevation is about 7 m in the area covered by the image. The second case is a hilly area of Hong Kong, with about 450 m variation in terrain elevation. The two-dimensional models used for the evaluation process include different orders of polynomials and projective models. Moreover, the determination of the effect of land topography, the selection of the best two-dimensional model and the optimum number of GCPs are all investigated in detail. Based on the results obtained, it can be concluded that variations in terrain elevation significantly affect the accuracy of rectification. To achieve better rectification accuracy, ground points should be projected onto a compensation plane for hilly terrain, but they can be used directly without projection on flat terrain. Accuracy up to less than 1 m can be achieved by using most of the 2D-transformation models after projecting ground points onto a compensation plane or onto flat terrain.