{{Short description|Geometrically corrected aerial photograph}} {{more footnotes|date=February 2009}} thumb|right|Orthographic views project at a right angle to the datum plane. Perspective views project from the surface onto the datum plane from a fixed location. [[File:Ortofoto Città Alta, Rocca.jpg|thumb|Aerophotogrammetry, orthophoto from drone, Città Alta, Bergamo, Italy]] 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). 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''', '''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 topographic relief,<ref>{{Cite web |title=Digital Orthophotography and GIS |url=https://proceedings.esri.com/library/userconf/proc95/to150/p124.html |access-date=2025-08-12 |website=proceedings.esri.com}}</ref> 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 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 (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 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 opto-mechanical devices.<ref>[https://web.archive.org/web/20100430042915/http://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. When using the top-most digital surface model (DSM), instead of the bottom DTM, the resulting product is called a '''true orthophoto'''.<ref name="k568">{{cite book | last1=Neteler | first1=Markus | last2=Mitasova | first2=Helena | title=Open Source GIS: A GRASS GIS Approach | publisher=Springer Science & Business Media | date=2013-04-18 | isbn=978-1-4757-3578-9 | url=https://books.google.com/books?id=G20ECAAAQBAJ&q=true%2520orthophoto&pg=PA267 | access-date=2025-08-05 | page=}}</ref>
==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 mosaic made by merging or stitching orthophotos. The aerial or satellite photographs have been transformed to correct for 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.
thumb|Aerial orthorectified image of a mine for volumetric measurements in stock pile reporting 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.
== See also == * Digital orthophoto quadrangle (DOQ) and digital orthophoto quarter quadrangle (DOQQ) * GRASS GIS (i.ortho.photo module) * IMAGINE Photogrammetry, formerly Leica Photogrammetry Suite * Photogrammetry * Photomapping * Rational polynomial coefficient * SOCET SET * TopoFlight * United States Geological Survey
== References == {{Reflist}}
==Further reading== * 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|first1=E.|last2=Garfinkel|first2=R. |first3=R. |last3=Arbiol|title=Mosaicking of Aerial Photographic Maps Via Seams Defined by Bottleneck Shortest Paths|date=May–June 1998|journal=Operations Research|volume=46|number=3|pages=293–304|doi=10.1287/opre.46.3.293}}. * Petrie, G., (1977), Transactions of the Institute of British Geographers: Orthophotomaps New Series, vol. 2, no.1, Contemporary Cartography., pg. 49–70 * 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 https://erg.usgs.gov/isb/pubs/factsheets/fs05701.html {{Webarchive|url=https://web.archive.org/web/20080602094558/http://erg.usgs.gov/isb/pubs/factsheets/fs05701.html |date=2008-06-02}}
== External links == * [https://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)] {{Webarchive|url=https://web.archive.org/web/20171011154506/https://cms.geoplatform.gov/a16imagery/NDOP |date=2017-10-11}} * [https://www.fgdc.gov/organization/working-groups-subcommittees/ndop/index_html National Digital Orthoimagery Program Subcommittee]
Category:Cartography Category:Remote sensing Category:Photogrammetry