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Definition
The term "digital orthophoto" is used throughout this document to refer to both the "digital orthophoto quadrangle" (DOQ) and "digital orthophoto quarter-quadrangle" (DOQQ) products. A digital orthophoto is a digital image which has the properties of an orthographic projection. It is derived from a digitized perspective aerial photograph by differential rectification so that image displacements caused by camera tilt and relief of terrain are removed. Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. They serve a variety of purposes, from interim maps to field references for earth science investigations and analysis. The digital orthophoto is useful as a layer of a geographic information system and as a tool for revision of digital line graphs and topographic maps. A digital orthophoto, as described in these standards, can be produced from any scale of photography. However, the program is oriented primarily toward the production of 1-meter digital orthophoto quarter-quadrangles from 1:40,000-scale National Aerial Photography Program (NAPP) or NAPP-like photography. Production of 2-meter digital orthophoto quadrangles will be primarily from National High Altitude Photography (NHAP) or NHAP-like photography (1:80,000 scale), however may be supported by the mosiacking of digital orthophoto quarter-quadrangles. At the present time, hard-copy outputs from the digital orthophotos include 1:12,000-scale orthophoto quarter-quadrangles and 1:24,000-scale orthophoto quadrangles. Image processing algorithms can be applied to the image data to support image classification, three dimensional modeling, and many other spatial applications. The digital orthophoto is created by scanning an aerial photograph diapositive transparency with a precision image scanner. The scanned data file is then digitally rectified to an orthographic projection by processing each image pixel through photogrammetric space resection equations. This process requires, as input, ground control points acquired from ground surveys or developed in aerotriangulation, camera orientation parameters, and a digital elevation model (DEM). Source: United States Geological Survey (USGS) |
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