An orthophoto is a computer generated manipulation
of an aerial photograph
in which displacement
s 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).