Sustainability of Digital Formats
 Planning for Library of Congress Collections

A Digital Orthophoto Quadrangle (DOQ) is a digital image that has the properties of an orthographic projection, derived from a digitized perspective aerial photograph or other remotely sensed image data by differential rectification so that image displacements caused by camera tilt and terrain relief are removed. The DOQ combines the image characteristics of the original photograph with the georeferenced qualities of a map. Ground features are displayed in their true position, allowing for the direct measurement of distance, areas, angles, and positions. DOQs are cast to the UTM projection and referenced to the NAD 27 or NAD 83 datum. DOQs may be black and white (B/W), natural color, or color-infrared (CIR) images.

Digital orthophoto and DOQ refer to both the 1-meter ground resolution quarter-quadrangle digital orthophoto (3.75- x 3.75-minutes in extent) and the one or two-meter ground resolution quadrangle digital orthophoto (7.5- x 7.5-minutes in extent). The acronym DOQQ is used for quarter-quadrangle orthophotos when the distinction from the quadrangle DOQ orthophotos is necessary. The native DOQ is formatted with an ASCII keyword header followed by a series of 8-bit binary image lines. Color DOQs are 24-bit band-interleaved-by-pixel (BIP) images. The header contains a wide range of data for identifying, displaying, and georeferencing the image. Digital orthophotos are also available from USGS in the GeoTIFF format, cast to the UTM projection and referenced to the NAD 83 datum. GeoTIFF DOQs consist of a georeferenced TIFF, with all geographic referencing information embedded within the .tif file.

Since an orthophoto shows features that may be omitted on maps, a DOQ can be used as a base image to assemble, review, and revise other data, especially digital line graphs (DLG) and topographic maps. When the DOQ is combined with other digital products, such as digital raster graphics (DRG) or digital elevation models (DEM), the resulting image provides additional visual information for the extraction and revision of base cartographic information.

DOQs are generated and distributed by many agencies in the U.S. Federal Government and by states.

In 1997, the WMC and Microsoft Corporation entered into a Cooperative Research and Development Agreement to make DOQs easily available to the public through the TerraServer USA Web site, which receives nearly 40,000 hits per day.