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Digital Orthophotos - DOQQ / DOQ
A digital orthophoto quadrangle (DOQ) is a computer-generated
image of an aerial photograph in which image displacement caused by
terrain relief and camera tilts has been removed. It combines the
image characteristics of a photograph with the geometric qualities
of a map.
Characteristics of a USGS Digital Orthophoto Quadrangle
DOQs produced by the U.S. Geological Survey (USGS) are either
gray-scale, natural color, or color-infrared (CIR) images with
1-meter ground resolution. They cover an area measuring 3.75-minutes
longitude by 3.75-minutes latitude or 7.5-minutes longitude by
7.5-minutes latitude. Each DOQ has between 50 and 300 meters of
overedge image beyond the latitude and longitude corner crosses
embedded in the image. This overedge facilitates tonal matching and
mosaicking adjacent images.
The standard 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.
All DOQs are referenced to the North American Datum of 1983 (NAD
83) and cast on the Universal Transverse Mercator (UTM) projection.
Primary (NAD 83) and secondary (NAD 27) datum coordinates for the
upper left pixel are included in the header so that users can
spatially reference other digital data to the DOQ.
The file size of a 3.75-minute gray-scale DOQ is 40-45 megabytes,
and a 3.75-minute color or 7.5-minute gray-scale DOQ can be three
times this size.
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Producing a Digital Orthophoto Quadrangle
To produce a DOQ requires (1) a minimum of three ground positions
that can be identified on the photograph to be rectified, (2) camera
calibration parameters, such as the calibrated focal length and the
coordinates of the camera fiducials, (3) a digital elevation model
(DEM), and (4) a digital image produced by scanning an aerial
photograph with a precise, high-resolution scanner.
The digital image is rectified to generate an orthophoto by
processing requirements 1 through 3 above for each image picture
element (pixel), using rigorous photogrammetric equations on a
computer. The finished product is a spatially accurate image with
planimetric features represented in their true geographic positions.
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Accuracy Requirements
The accuracy and quality of USGS DOQs must 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.
Accuracy and quality depend on the following:
- photographs that meet National Aerial Photography Program
standards, which are quarter-quadrangle centered. The photographs
are exposed at a flying height of 20,000 feet above mean terrain;
- a DEM with the same area coverage as the DOQ and that is equal
to or better than a level 1 USGS DEM having a root-mean-square
error in elevation no greater than 7 meters;
- a highly accurate image-scanning process that uses a scanning
aperture between 7.5 and 32 micrometers (µm). A 1:40,000-scale
image scanned with a 25µm aperture produces a ground sample
distance of approximately 1 meter; and
- identifiable ground control positions with coordinates
acquired from ground surveys or aerotriangulation.
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Uses for Digital Orthophoto Quadrangles
A DOQ can be incorporated into any geographic information system
(GIS) or graphics software that can manipulate raster images. It can function as a
cartographic base for displaying, generating, and modifying
associated digital planimetric data. Other applications include
vegetation and timber management, routing and habitat analysis,
environmental impact assessments, emergency evacuation planning,
flood analysis, soil erosion assessment, facility management, and
ground-water and watershed analysis. The accuracy and extraordinary
detail provided by the DOQ allow users to evaluate their data for
accuracy and completeness, make real-time modifications to their
data, and even generate new files.
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Distribution Media
DOQs are distributed in either native (standard) or GeoTIFF
format on a variety of media, including 8-mm tape, CD-R, DVD, and
FTP as uncompressed files. DOQs covering selected counties are also
available packaged as individual, JPEG-compressed files on CD-ROM.
Note that the county DOQ products contain DOQs built to the pre-1997
(old) standard and do not contain the keyword header.
Obtaining Digital Orthophoto Quadrangles
DOQ files can be ordered by contacting us at 800-248-6277.
DOQ coverage is not available for all areas in the United States.
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