A digital orthophoto quadrangle (DOQ) is a computer-generated image of an aerial photograph. It has been orthorectified--altered so that it has the geometric properties of a map; DOQ's in fact meet National Map Accuracy Standards. Thus the user can measure distances accurately on a DOQ.
The standard DOQ from the U.S. Geological Survey is a black-and-white (gray-scale) or color-infrared image covering 3.75 minutes of latitude by 3.75 minutes of longitude. Thus, four such photos can be combined, or mosaicked, to cover the area represented by a standard USGS 7.5-minute, 1:24,000-scale topographic map. Mosaicking is facilitated by the fact that the images overlap. The DOQ's are referenced to the North American Datum of 1983 and use the Universal Transverse Mercator projection. Their resolution is such that each pixel represents a square meter.
The standard gray-scale DOQ includes an ASCII keyword header followed by a series of 8-bit binary image lines. The average file size for a black-and-white DOQ is 55 megabytes . Header information includes photographic source type, date, production software, and date of the digital elevation model metadata used in orthorectification. Also, primary and secondary datum coordinates for the upper left pixel are included to assist in referencing other digital geospatial data. To view or manipulate one of these images, the user must extract the header information from the image and be equipped with editing software designed to handle large image files.
A minority of the DOQ's are not gray scale, but color-infrared, a false-color scheme used for certain scientific and analytic purposes. These files average about 150 megabytes and are stored in band-interleaved-by-pixel, or BIP, format.
DOQ applications include land and timber management, routing and habitat analysis, environmental impact assessment, evacuation planning, flood analysis, soil erosion assessment, facility management, and groundwater and watershed analysis. The accuracy and extraordinary detail of DOQ's allow users to evaluate their data for accuracy and thoroughness, to modify their data promptly, and even to generate new files.
A DOQ can be used in a wide variety of geographic information systems and is an excellent cartographic base on which to overlay any number of associated thematic layers for displaying, generating, and modifiying planimetric data or associated data files.
DOQ production begins with an aerial photograph and requires four elements: (1) at least three ground positions that can be identified within the photograph; (2) camera calibration specifications, such as focal length; (3) a digital elevation model (DEM) of the area covered by the photograph; (4) and a high-resolution digital image of the photograph, produced by scanning. The photograph is processed pixel by pixel to produce an image with features in true geographic positions.
USGS DOQ's meet National Map Accuracy Standards at 1:12,000 scale for 3.75-minute quarter quadrangles and at 1:24,000 scale for 7.5-minute quadrangles (corresponding to standard, 7.5-minute USGS topographic maps).
DOQ's are distributed mostly on compact disc-recordables but are also available on 8-mm tape and by file transfer protocol download, all in uncompressed form. DOQ's are not available for all areas of the United States; however, a few counties are covered by JPEG-compressed CD-ROM's, each containing decompression software for DG-Unix and MS-DOS users and C-language makefiles that can be compiled for use on other systems.
From the TerraServer TerraServer Web site, users can view and download web-compatible version of USGS DOQ's. Because these images have been converted to JPEG format so they can be viewed with a web browser, the sharpness and quality is not as good as the source DOQ's from which they were created.
USGS DOQ's may be obtained from a selected list of USGS Business Partners.
DOQ's can also be ordered online via EarthExplorer.
Additional information about DOQ's is available at http://wmc.wr.usgs.gov/orthophoto_basic.html.