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144    Cha pte r  F i v e

                   In this chapter the sources of image geometric distortion are com-
               prehensively reviewed under three categories. This review is followed
               by a discussion of ground coordinate systems. Two systems, one global
               and one regional, are introduced in detail, in conjunction with the fun-
               damentals of image transformation. Presented next in this chapter are
               various image transformation models. Three special georeferencing
               methods (polynomial, direct, and orthorectification) are covered
               extensively.  Also explored in this chapter are a number of issues
               involved in image rectification, such as the impact of ground control
               and image spatial resolution on the accuracy of image rectification.
               With the emergence of hyperspatial resolution satellite imagery, it is
               increasingly important to orthorectify it. How to perform image ortho-
               rectification forms the content of Sec. 5.7, followed by a discussion on
               image direct georeferencing in Sec 5.8. The last section of this chapter
               is about image subsetting and mosaicking of georeferenced images.



          5.1  Sources of Geometric Distortion
               Many factors contribute to geometric distortions of satellite imagery.
               They are related broadly to the target, the sensor, and the platform.
               Target-related factors include rotation of the Earth during scanning
               and Earth curvature. Sensor-related factors are scale distortion and
               scanning mirror inconsistency. Platform factors refer to its position
               and orientation in space.

               5.1.1  Errors Associated with the Earth
               Earth Rotation
               The Earth spins at a constant angular velocity of 360° a day, or 463 m/s.
               During scanning in acquiring lines of imagery, the Earth rotates some
               distance from the west to the east. The exact amount of linear rotation
               varies with the scan duration and the manner of scanning. Cross-track
               scanning, as with Landsat imagery, takes longer to obtain a frame of
               imagery than along-track scanning commonly associated with SPOT
               (Le Systeme Pour I'Observation de la Terre). After a scan, the Earth
               shifts eastward by a certain distance, for instance, 9.26 m after a scan
               duration of 20 microseconds. When the scanner returns to its former
               position to begin the next scan, this position has moved eastward by
               9.26 m. Instead, the ground of the same distance to the left has been
               displaced here to replace the former position, resulting in a gradual
               westward shift in the ground swath being scanned. As the number of
               scan lines accumulates, the start position shifts to the left cumulatively
               from the first line to the last. Although the raw image is recorded as a
               square (Fig. 5.1a), the actual ground covered by the image is skewed
               toward the left (Fig. 5.1b) as a consequence of the rotation. Earth rotation
               causes a constant displacement in the start position of scan lines only. It
               does not affect the number of scan lines in an image nor the number of
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