U.S. geological survey standard for digital orthophotos
Author(s)/Corporate Author (s)United Nations. Economic and Social Council;
United Nations. Economic Commission for Africa;
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The U.S. Geological Survey has added the new category of digital orthophotos to the National Digital Cartographic Data Base. This differentially rectified digital image product enables users to take advantage of the properties of current photoimagery as a source of geographic information The product and standard were implemented in July 1992. The digital orthophotos are quadrangle based and cast on the Universal Transverse Mercator projection and extend beyond the primary datum theoretical 3.75-minute or 7.5-minute quadrangle neat line approximately 300 meters to form a rectangle. The oyeredge may be used for mosaicking with adjacent digital orthophotos. To provide maximum information content and utility to the user, metadata (header) records exist at the beginning of the digital orthophoto file. Header information includes the photographic source type, date, instrumentation used to create the digital orthophoto, and information relating to the DEM that was used in the rectification process. The North American Datum of 1983 (NAD 83) is considered the primary datum. Additional header information is included on transformation constants from the 1927 and 1983 North American Datums to the orthophoto internal file coordinates to enable the user to register overlays on either datum. The quadrangle corners in both datums are also imprinted on the image. Flexibility has been built into the digital orthophoto format for future enhancements, such as the provision to include the corresponding digital elevation model elevations used to rectify the orthophoto. The digital orthophoto conforms to National Map Accuracy Standards and provides valuable mapping data that can be used as a tool for timely revision of standard map products, for land use and land cover studies, and as a digital layer in geographic information system.