An orthophoto is a computer generated manipulation of an aerial photograph in which displacements caused by camera orientation and terrain have been removed. These products combine the image characteristics of a photograph with the geometric qualities of a map.

Unlike a standard aerial photograph, relief displacement in orthophotos has been removed so that ground features are displayed in their true ground position. This allows for the direct measurement of distance, areas, angles, and positions.

The digital orthophoto is created by scanning an aerial photograph transparency (diapositive) with a precision image scanner. The scanned data file is then digitally rectified to an orthographic projection by processing each image pixel through photogrammetric space resection equations. This process requires, as input, ground control points acquired from ground surveys or developed in aerotriangulation, conjugate photo-coordinates of ground control, camera orientation parameters, and a digital elevation model (DEM).

http://rockyweb.cr.usgs.gov/nmpstds/doqstds.html

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