A DOQQ is a digitized image of an aerial photograph, corrected for displacements caused by camera angle and relief. The result is a spatially accurate image with planimetric features appearing in their true geographic positions. Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. The primary digital orthophoto quarter quad (DOQQ) is a 1-meter ground resolution, quarter-quadrangle (3.75-minutes of latitude by 3.75-minutes of longitude) image cast on the Universal Transverse Mercator Projection (UTM) on the North American Datum of 1983 (NAD83).
Figure 3. USGS Digital Orthophoto Quarter Quadrangle (DOQQ)
The geographic extent of the DOQQ is equivalent to a quarter-quad plus a slight amount over the quarter-quad boundary. The overedge ranges from a minimum of 50 meters to a maximum of 300 meters beyond the extremes of the primary and secondary corner points. The overedge is included to facilitate tonal matching for mosaicking and for the placement of the NAD83 and secondary datum corner ticks. The normal orientation of data is by lines (rows) and samples (columns). Each line contains a series of pixels ordered from west to east with the order of the lines from north to south. The standard, archived digital orthophoto is formatted as four ASCII header records, followed by a series of 8-bit binary image data records. The radiometric image brightness values are stored as 256 gray levels ranging from 0 to 255. The metadata provided in the digital orthophoto contain a wide range of descriptive information including format source information, production instrumentation and dates, and data to assist with displaying and georeferencing the image. The standard primary series USGS DOQQ is a quarter-quadrangle image (3.75- by 3.75-minutes). USGS DOQQ's are derived from photography meeting National Aerial Photography Program (NAPP) specifications, and meet National Map Accuracy Standards for 1:12,000-scale quarter quadrangles. DOQQ's adhere to the USGS NMPTI Standards for Digital Orthophotos.