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Aerial Photography and Photogrammetry
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Orthorectification
In order to create a map from the photographs, it is necessary to not only georeference them (assign real world coordiates), but also to remove all the distortions.
The lens optics create small distortions, for which calibration is required.
The varying height of the surface photographed results in more substantial distortions. This is obvious in the case of tall buildings, which appear to lean outwards from the principal point of the photograph. (The principal point is the point where the perpendicular projected through the center of the lens intersects the photo image.)
Even relatively gentle topography also has a significant effect. Orthorectification removes these distortions using the Digital Surface Model, together with Ground Control Points, to calculate the exact location of every point in the photograph.
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