Page 178 - Digital Analysis of Remotely Sensed Imagery
P. 178

CHAPTER 5






                                         Image Geometric


                                                 Rectification







                     emotely sensed images obtainable from Earth resources sat-
                     ellites form a vital information source for managers of the
               Renvironment and natural resources. Thanks to their ease of
               acquisition and up-to-datedness, satellite images, together with
               aerial photographs, are playing an increasingly significant role in
               numerous geographic information system (GIS) applications. Unlike
               existing maps, raw satellite images and scanned aerial photographs
               have a local coordinate system. They do not have the right projection.
               Neither do they have any scale nor a proper orientation. During data
               acquisition the platform on which the sensor is mounted is in a state
               of constant motion. Any deviation of the sensor position and orienta-
               tion from the norm will lead to geometric distortions in the resultant
               satellite images.
                   It is important to geometrically rectify these images for three reasons:
                    •  Firstly, the end product of a large number of remote sensing
                      applications about the Earth’s resources and environment
                      occurs in the thematic map form. These maps must conform
                      to certain geometric mapping standards. Image rectification
                      ensures that geometric distortions inherent in the remote
                      sensing imagery are eliminated or reduced to an acceptable
                      level.
                    •  Secondly, in order to be analyzed with data from other sources
                      in a GIS, raw satellite images and aerial photographs have to
                      be projected to a common ground reference system, which
                      enables images obtained at different times to be spatially
                      registered with one another.
                    •  Finally, if remotely sensed data are used to detect changes or
                      update existing maps, they must be reprojected to a coordinate
                      system with a known geometry identical to that of the digital
                      maps to be revised.

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