An orthophoto, orthophotograph or orthoimage is an aerial photograph or satellite imagery geometrically corrected such that the scale is uniform: the photo or image follows a given map projection. Unlike an uncorrected aerial photograph, an orthophoto can be used to measure true distances, because it is an accurate representation of the Earth's surface, having been adjusted for topographic relief, lens distortion, and camera tilt. Orthophotographs are commonly used in geographic information systems as a "map accurate" background image. An orthorectified image differs from "rubber sheeted" rectifications as the latter may accurately locate a number of points on each image but "stretch" the area between so scale may not be uniform across the image. A digital elevation model is required to create an accurate orthophoto as distortions in the image due to the varying distance between the camera/sensor and different points on the groundneed to be corrected. An orthoimage and a "rubber sheeted" image can both be said to have been "georeferenced"; however, the overall accuracy of the rectification varies. Software can display the orthophoto and allow an operator to digitize or place linework, text annotations or geographic symbols. Some software can process the orthophoto and produce the linework automatically. Production of orthophotos was historically achieved using mechanical devices.
Orthophotomap
An orthophotomosaic is a raster image made by merging orthophotos — aerial or satellite photographs which have been transformed to correct for perspective so that they appear to have been taken from vertically above at an infinite distance. Google Earthimages are of this type. The document representing an orthophotomosaic with additional marginal information like a title, north arrow, scale bar and cartographical information is called an orthophotomap or image map. Often these maps show additional point, line or polygon layers on top of the orthophotomosaic. A similar document, mostly used for disaster relief, is called a spatiomap.
