The process that removes effects of relief displacement, optical sensor distortions and geometric perspective from an initial image, is called orthorectification. In normal photographs, objects closer to the camera, appear larger than objects of equal size, that are further away. This creates an obvious complexity in measuring objects accurately or determining their precise location, in a reference coordinate system. In order to use perspective imagery as a map or in a GIS environment, all geometric distortions must be corrected. The resulting image is called orthophoto.
Orthophoto can be generated from any perspective image (regardless of the source), as long as three main things are known ( aerial triangulation performed succesfully) :
- Interior orientation (describing internal geometry of the camera system)
- Exterior orientation (describing the geometric relationship between the image and the ground)
- DTM (Digital Terrain Model) must be supplied as input to the orthorectification process, in addition to the internal and external sensor geometry described above.
The quality of digital orthophotos is ensured by respecting three quality levels:
- Geometric fidelity: Geometric fidelity is the trueness of features in the orthophoto to the shapes and alignments of the real world objects they represent.
- Absolute accuracy: Absolute accuracy is a measure that indicates how closely the coordinates of a point in the orthophoto agree with the true coordinates of the same point on the ground, in a reference coordinate system. All ground control points used previously in aerotriangulation or from another source are located and measured on the orthophoto for ensuring requested accuracy.
- Radiometric accuracy: Radiometric accuracy is a measure of the colour balance, luminosity and contrast of the image. The radiometric properties of the orthophotos varies depending on the nature of the topography.
If orthophotos can be characterized as images that are geometrically corrected for relief variation, true orthophotos add the dimension of correcting for the distortion of buildings (true orthophotos do not show building lean). For big cities orthophotos will show displacement of skyscrapers and many of the streets will be obscured. This is important for mapping applications such as digitizing street centerlines.