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An orthophoto, orthophotograph, orthoimage or orthoimagery is an aerial photograph or satellite imagery geometrically corrected ("orthorectified") 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 Topography, lens distortion, and camera tilt.
Orthophotographs are commonly used in geographic information systems (GIS) as a "map accurate" background image. An orthorectified image differs from Rubbersheeting 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 (DEM) or topographic map is required to create an orthophoto, as distortions in the image due to the varying distance between the camera/sensor and different points on the ground need to be corrected. An orthoimage and a "rubber sheeted" image can both be said to have been Georeferencing; 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 (such as hospitals, schools, and fire stations). Some software can process the orthophoto and produce the linework automatically.
Production of orthophotos was historically achieved using opto-mechanical devices. BBC Tomorrow's World: How maps are corrected and updated using aerial photography and optical machinery. 18 December 1970.
The orthorectification is not always perfect and has side effect especially for the geometry of high-rise constructions. When using the top-most digital surface model (DSM), instead of the bottom DTM, the resulting product is called a true orthophoto.
The document (digital or paper) 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 (like a traditional map) on top of the orthophotomosaic. A similar document, mostly used for disaster relief, is called a spatiomap.
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