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{{Short description|Geometrically corrected aerial photograph}}
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[[Image:OrthoPerspective.svg|thumb|right|Orthographic views project at a right angle to the
An '''orthophoto,''' '''orthophotograph''' or '''orthoimage''' is an [[Aerial photography|aerial photograph]] geometrically corrected ("orthorectified") such that the scale is uniform: the photo has the same lack of distortion as a map. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the Earth's surface, having been adjusted for [[Topography|topographic relief]],<ref>Smith, Gary S. "DIGITAL ORTHOPHOTOGRAPHY AND GIS." ESRI Conference. http://proceedings.esri.com/library/userconf/proc95/to150/p124.html</ref> [[Barrel distortion|lens distortion]], and [[camera tilt]].▼
[[File:Ortofoto Città Alta, Rocca.jpg|thumb|Aerophotogrammetry, orthophoto from [[unmanned aerial vehicle|drone]], Città Alta, [[Bergamo]], Italy.]]
[[File:Aerial Orthophoto.png|thumb|This photo is properly projected on elevation model, yet on a single building scale, a small tilt is noticeable. This is an orthophoto, but not a true orthophoto (not all vertical features are reprojected).]]
[[File:Aerial True Orthophoto.png|thumb|This photo is assembled from several overlapping photos from UAV, completely removing any residual tilt of the buildings. This is a true orthophoto.]]
▲An '''orthophoto
Orthophotographs are commonly used in [[geographic information system]]s ([[GIS]]) 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|digital elevation model (DEM)]] 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 ground need 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 (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 mechanical devices.<ref>[https://www.bbc.co.uk/archive/aerialjourneys/5320.shtml BBC Tomorrow's World: How maps are corrected and updated using aerial photography and optical machinery. 18 December 1970.]</ref>
The orthorectification is not always perfect and has side effect especially for the geometry of [[high-rise]] constructions.
==Orthophotomosaic and orthophotomap {{anchor|Orthophotomosaic|Orthophotomap}}==
[[File:7.5 minute series orthophotomap (topographic).jpg|thumb|right|Orthophotomap of [[Rockport, Texas]]. The orthophotomap contains additional roads, [[terrain]] [[elevation]] and [[bathymetry]] layers.]]
[[File:Ortofotomapa Asturias 2010-OVIEDO CENTRO.jpg|thumb|right|Orthophotomap of [[Oviedo]] city center, [[Spain]].]]
An '''orthophotomosaic''' is a [[raster image|raster]] [[image mosaic]] made by merging or [[Image stitching|stitching]] orthophotos. The aerial or satellite photographs have been transformed to correct for [[Perspective distortion (photography)|perspective]] so that they appear to have been taken from vertically above at an infinite distance.<ref>{{citation |page=370 |title=Glossary of the Mapping Sciences |author= American Congress on Surveying and Mapping, American Society for Photogrammetry and Remote Sensing |publisher=American Society of Civil Engineers |year=1994 |isbn=9780784475706}}</ref> [[Google Earth]] images are of this type.
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]].
[[File:Stock Pile Reporting.jpg|thumb|[http://www.aeroglobe.com.au/ Aerial] orthorectified image of a mine for volumetric measurements in stock pile reporting.]]
== See also ==
* [[Digital orthophoto quadrangle|Digital Orthophoto Quadrangle (DOQ) and Digital Orthophoto Quarter Quadrangle (DOQQ)]]
* [[Leica Photogrammetry Suite]] Orthorectification Software
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* [[Socet set|Socet set Orthophoto Software]]
* [[U.S. Geological Survey]]
* [[Rational Polynomial Coefficient]]
== References ==
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* Bolstad, P., (2005), GIS Fundamentals: A First Text on Geographic Information Systems, Eider Press, White Bear Lake, MN, 2nd ed.
* Demers, Michael N., (1997). Fundamentals of Geographic Information Systems, John Wiley & Sons.
* {{cite journal|last1=Fernandez
* Petrie, G., (1977), Transactions of the Institute of British Geographers: Orthophotomaps New Series, vol. 2, no.1, Contemporary Cartography.
* Robinson, A.H., Morrison, J.L., Muehrcke, P.C., Kimerling, A.J., Stephen Guptill, (1995) Elements of Cartography: John Wiley & Sons Inc., Canada, 6th ed.
* United States Geological Survey, US Department of Interior, USGS Fact Sheet May 2001 http://erg.usgs.gov/isb/pubs/factsheets/fs05701.html
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== External links ==
* [http://www.usgs.gov/ United States Geological Survey]
* [https://web.archive.org/web/20160613054129/http://www.ndop.gov/ National Digital Orthophoto Programs (NDOP) - Original Site via Wayback Machine]
* [https://cms.geoplatform.gov/a16imagery/NDOP National Digital Orthoimagery Program (NDOP)]
* [https://www.fgdc.gov/organization/working-groups-subcommittees/ndop/index_html National Digital Orthoimagery Program Subcommittee]
[[Category:Cartography]]
[[Category:Remote sensing]]
[[Category:Photogrammetry]]
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