Optimized Patch Backprojection in Orthorectification for High Resolution Satellite Images

نویسنده

  • Liang-Chien Chen
چکیده

The objective of this investigation is to build up a fast orthorectification procedure for high resolution satellite images. The proposed scheme comprises two major components: (1) orbit modeling, and (2) image orthorectification. In the orbit modeling, we provide a collocation procedure to determine the precision orbits. In the image orthorectification, the area of interest is sequentially subdivided into four quadrate tiles until a threshold is met. The threshold of maximum terrain variation in a tile will be optimized according to the computation efficiency and accuracy requirements. Once the ground tiles are determined, we perform adaptive patch backprojection to correspond to the image pixels. Selecting the highest elevation in the tile, the four corners of the tile are projected on the image to form a set of anchor points. Another set of anchor points with the lowest elevation are generated in the same manner. Assuming that the relief displacement in a moderate tile is linear, a groundel within the tile is projected into the image space according to the groundel elevation and the two associated anchor point sets. Tests of images include SPOT 5 supermode and QuickBird panchromatic satellites. Experimental results indicate that the computation time is significantly reduced without losing accuracy. * Corresponding author. 1. INTORDUCTION The most rigorous way to register a remotely sensed image with a relevant spatial data layer is performing orthorectifictation to the image. The generation of orthoimages from remote sensing images is an important task for various remote sensing applications, such as cartography, environmental monitoring, city planning, etc. Moreover, GIS (Geographic Information Systems) technology often needs multi-temporal images for detection of lancover changes. Thus, ortho-rectified images have become important due to their short production time. Nowadays, most of the high resolution satellites use linear pushbroom arrays, such as SPOT5, Ikonos, QuickBird and others. A number of investigations have been reported regarding the geometric accuracy for those pushbroom linear array images (Westin, 1990; Chen and Lee, 1993; Orun and Natarajan, 1994; Toutin, 2003). Traditionally, the first step of image orthorectification is to model the orientation parameters by using ground control points. Then, incorporating a DTM, an image, and the orientation parameters, a non-linear equation is formulated to determine the along-track image coordinates in terms of the sampling time for a ground element. The acrosstrack image coordinates can thus be calculated according to the collinearity condition equations. The traditional solution of orthorectification for pushbroom images is time-consuming due to a vast amount of non-linear equations have to be solved. This weakness is so obvious for those high resolution satellite images that an efficient way is required. Konecny et al (1987) emulated SPOT images as centre perspective, then, implemented the idea on an analytical plotter to achieve real time operation while maintaining some accuracy. Inspired by the idea, we propose a “Patch Backprojection” procedure for accelerating the computation in orthorectification for high resolution satellite images with large amount of pixels. Because of the small field-of-view (FOV) of high resolution satellite, the relief displacements in a small area with moderate terrain variations may be assumed linear. We, thus, propose a method to do the orthorectification patch by patch. The patch size may be adapted for different terrain characteristics. We first divide the area of interest into a number of tiles. For corner point with highest elevation, we compute the image coordinate for each corner point of tiles using indirect method. The indirect method also applies to the corner point with lowest elevation. Using an affine transformation as a mapping function of image coordinates and object coordinates. In addition, we will analyze terrain variations for the selection of the adaptive window of the tiles. We also analyze the model error of the proposed method including transformation error and interpolation error. Affine transformation, patch size, tilt angle, and elevation range are the most important factors to be considered. In the validation, we first analyze the model error of the proposed method. It has two parts, transformation error and interpolation error. Affine transformation, patch size, tilt angle, and elevation range are the most important factors to be considered in the analysis of model errors. Then, we check the accuracy of the determined orientation parameters. Finally, the accuracy of the generated orthoimage will be examined. Pushbroom scanner images including SPOT5 and QuickBird are considered in this investigation.

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تاریخ انتشار 2004