Documentation | Help File | Glossary | Rectification

The computational process of transforming and georeferencing vector or raster (image, grid) layer to a known coordinate system to give the layer the integrity of a map. The lowest level rectification transformation rotates and Scale the layer to match the coordinate system, and saves the coordinate system information for the rectified layer. For layers containing systematic distortion, higher level rectification transformations can also skew and Warp the layer to change the relative position of each pixel or vector vertex to more accurately reflect true ground features, and relative distances between features, of the subject terrain. (Refer to Rectification - Polynomial Transformation Method for information about supported rectification transformations.)

Any raw aerial image is naturally distorted in ways that make the proportions of the image differ from true relative distances between the positions of real objects on the ground. The distortion results from an image orientation that is not true to North, curvature of the earth relative to the camera position, angle and tilt of the camera lens relative to the plane of the ground at the instant the image is taken, irregular scaling throughout the image reflecting the varying distances of points on the ground from the camera position, variations in the elevation of the terrain relative to the position of the camera, and even imperfections within the camera lens itself. A scanned topographical map image is without distortion but lacks coordinate system information to control the rotation and scaling. Vector maps can similarly contain systematic distortions and (as with many CAD drawings) be without coordinate system information. Rectification can make such raster or vector layers usable in GIS map project with other layers.

While rectification is normally performed to transform a layer to a cartographic coordinate system, it can also convert a layer to some other useful coordinate system. For instance, a layer could be conformed to the proportions of another map layer (a reference layer) of the same terrain, even if the coordinate system of the reference layer is unknown. Another example would be the rectification of a series of very low altitude (perhaps from the height of a tower or a man standing) aerial photographs of a an archeological site to fit to an archeological grid that is unrelated to map cartography.