Digital Orthophoto Quadrangles

Table of Contents

Background

The U.S. Geological Survey (USGS) is the lead Federal agency for the collection and distribution of digital cartographic data. The U.S. Department of Agriculture's Farm Service Agency (FSA), the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS,) the U.S. Forest Service (USFS), and the USGS are partners in the National Digital Orthophoto Program (NDOP). The USGS Earth Science Information Centers (ESICs) distribute digital cartographic and geographic data produced through the USGS National Mapping Program.

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 analyses. The digital orthophoto is useful as a layer of a geographic information system (GIS) and as a tool for revision of digital line graphs and topographic maps.

Unlike a standard aerial photograph, relief displacement in orthophotos has been removed so that ground features are displayed in their true ground position. This allows for the direct measurement of distance, areas, angles, and positions. Also, an orthophoto displays features that may be omitted or generalized on maps.

The National Aerial Photography Program (NAPP) imagery and NAPP-like photography are the primary sources of aerial photography used in the production of 1-meter digital orthophotos for the National Digital Orthophoto Program (NDOP). NAPP photography is quarter-quadrangle centered (3.75-minutes of latitude by 3.75-minutes of longitude in geographic extent) and taken at an aircraft altitude of approximately 20,000 feet above mean terrain using a 152-millimeter focal-length camera. The scale of the NAPP photography is approximately 1:40,000. Orthophoto quadrangles may also be produced through the mosaicking of digital orthophoto quarter-quadrangles. Color infrared (CIR) photography may be used as a source. However, the resulting DOQ may either be a single black-and-white composite of all bands or a color DOQ with all three bands. Although NAPP is the primary image source, this does not prevent the use of additional aerial photographs or digital images in the future.

IMAGE Example of Washington, D.C. DOQ image (282 kb)

Extent of Coverage

The DOQ coverage area includes the conterminous United States, Alaska, Hawaii, and Puerto Rico.

Acquisition

Processing Steps

Digital orthophotos require several types of inputs to produce an orthogonally rectified image from the original perspective image captured by the sensor. These inputs are the following: 1) the unrectified raster image scanned from the diapositive or directly acquired from a digital sensor, 2) a digital elevation model with the same area of coverage as the digital orthophoto, 3) the image and ground coordinates of photo identifiable ground control points, 4) calibration information about the sensor collector device and, 5) a user parameter file. These five inputs are used to register the image file to the scanner and to the sensor platform, to determine the orientation and location of the sensor platform with respect to the ground, and to remove the relief displacement from the image data.

For more information on the processing steps used to create digital orthophotos, see the Standards for Digital Orthophotos 12/96.

Data Characteristics

Spatial Resolution

Resolution is the minimum distance between two adjacent features or the minimum size of a feature, that can be detected by a remote sensing system. The resolution is generally larger than the computed ground sample distance of the DOQ. The ground sample distance (GSD) is the distance on the ground represented by each pixel in the x and y components. The ground sample distance of the digital orthophoto is a result of the scanning aperture of the microdensitometer used to capture the digital image and the resampling algorithm. For example, if a scanning aperture of 25 micrometers is used on a 1:40,000 photo-scale image, the ground (pixel) sample distance is approximately 1 meter. A 7.5 micrometer scan yields a pixel size of 0.3 meters while a 15 micrometer scan equates to a 0.6 meter. For the processed DOQ, the GSD is 1 meter for quarter-quad digital orthophoto and 2 meters for quadrangle digital orthophotos. If digital orthophotos are produced at a finer sampling distance than 1 or 2 meters, they may be processed by resampling to 1 or 2 meter horizontal GSD. Digital orthophotos produced at a coarser sample distances are not resampled to a finer horizontal ground sample distance.

The geographic extent of the digital orthophoto is equivalent to an orthophoto quarter-quadrangle (3.75-minutes of latitude and longitude), plus a minimum of 50 meters to to a maximum of 300 meters of overedge is included, sufficient to offer coverage to encompass the four primary and secondary horizontal datum corner points. The overedge is useful for edgematching and mosaicking of quadrangles by offering areas outside the primary area of interest, which facilitate tonal matching between images. Every orthophoto is a rectangle, but may not necessarily be the same size as its adjoining neighbor. The normal orientation of the data are by lines (rows) and samples (columns).

Spectral Range

In order to assure that the image brightness values of the orthophoto closely portray the source imagery, very little image enhancement, other than a limited amount of analog dodging, is performed when preparing the photograph for scanning. Some deviation of brightness values may also occur during the scanning and rectification processes. Radiometric accuracy and quality are verified through visually inspecting and comparing the digital orthophoto to the original unrectified image.

Data Organization

A gray scale USGS digital orthophoto has the following characteristics:

Data Availability

Procedures for Obtaining Data

To place orders and to obtain additional information regarding technical details and price schedules, contact:

Earth Science Information Centers (ESICs)

Online requests for these data can be placed via the USGS Global Land Information System (GLIS) interactive query system. The GLIS system contains metadata and online samples of Earth science data. With GLIS, you may review metadata, determine product availability, and place online requests for products.

Products and Services

Uncompressed DOQ files are available on 8-mm tape, compact disc-recordable (CD-R), 3480 cartridge tape, and via semi-anonymous file transfer protocol (FTP).

In addition to the standard format, this data is also available in the GeoTIFF format. TIFF is an acronym for Tag(ged) Image File Format. Further information on GeoTIFF software specifications can be found at http://mcmcweb.er.usgs.gov/sdts/geotiff.html

Compressed DOQ files are distributed in a JPEG format on CD-ROM. Each CD-ROM contains DOQ coverage for an individual county or area. In addition to the image files, each CD-ROM includes compression and decompression software for DG/UX and MS-DOS users and C-language makefiles that can be compiled for use on other systems.

Each compressed DOQ on a county CD-ROM consists of a binary image file and an associated metadata file. The metadata file includes descriptive information, such as file identification, data sources and dates, data storage, coordinate systems and datums, and image compression.

To see what is available over your area, a status graphic map may be viewed at the following location: USGS Digital Orthophoto Quad Status Page.

Applications and Related Data Sets

The DOQ data may be combined with other geographically referenced data to conduct automated analyses in support of various decision-making processes. These digital cartographic and geographic data may also be used as one layer in a geographic information system (GIS), as a tool for various kinds of spatial analyses, and as information for plotting base maps.

Raster data such as the DOQs, rather than vector data, may be more effectively used for some applications. Much like the symbology on a topographic map, vector data tend to be more generalized (i.e., most ground features are distinguishable using raster data, while the vector data, only selected, cartographically significant information may be shown). Also, it may be more economical to produce raster images; however, DOQs are not meant to be replacements for vector data.

References

U.S. Geological Survey, 1996, Standards for digital orthophotos, specifications: Reston, Virginia, U.S. Geological Survey [variously paged]. [This document is also available online.]

U.S. Geological Survey, 1989, North American datum of 1983--map data conversion tables: U.S. Geological Survey Bulletin 1875, 3 v.

Appendix

Datums and Coordinates

Digital orthophoto quarter-quadrangles will be cast on the North American Datum of 1983 (NAD 83), Universal Transverse Mercator (UTM) projection, with coordinates in meters. Digital orthophoto quadrangles will be cast on either North American Datum of 1927 (NAD 27) or NAD 83, UTM projection, with coordinates in meters. The principal horizontal primary datum for the quarter-quadrangle digital orthophoto will be NAD 83. The principal secondary horizontal datum for quarter-quadrangle digital orthophotos will be the NAD 27, the Puerto Rico Datum, the Old Hawaiian Datum, or other approved datum.

The four primary datum corners are imprinted into the image as four solid white crosses (brightness values = 255) and the four secondary datum corners as four dashed white crosses (brightness values = 255).

Example of Datum Corner Points

The image header contains the primary and secondary datum corner X, Y coordinates, representing the four theoretical quadrangle corners in each datum.

Accuracy

Digital orthophoto quadrangles and quarter-quadrangles must meet horizontal National Map Accuracy Standards (NMAS) at 1:24,000 scale and 1:12,000 scale, respectively. The NMAS specify that 90 percent of the well-defined points tested must fall within 40 feet (1/50 inch) at 1:24,000 scale and 33.3 feet (1/30 inch) at 1:12,000 scale. The vertical accuracy of the source DEM must be equivalent to or better than a level-1 DEM, with a root-mean-square-error (RMSE) of no greater than 7 meters. The DOQ RMSE is the square root of the average of the squared discrepancies. These discrepancies are the differences in coordinate (x and y) values derived by comparing the data being tested with values determined during aerotriangulation or by an independent survey of higher accuracy. All remaining inputs and processes (e.g., aerotriangulation control and methodology, scanner and sensor calibrations) used in digital orthophoto production must be sufficiently accurate to ensure that the final product meets NMAS specifications.

Data Header

Header metadata are affixed to the beginning of the image data and are composed of a variable number of ASCII-text metadata entries. To simplify in-place header editing, each entry is 80 characters long and ends with an asterisk (*) as character 79 and an invisible newline character as character 80.

The header line is equal in length to the length of an image line. If the sum of the byte count of the header is less than the sample count of one DOQ image line, then the remainder of the header is padded with the requisite number of 80-character blank entries, each terminated with an asterisk and newline character. If the number of 80-character, blank-padded entries is not an even multiple of one image line, then a partial blank-padded entry, terminating asterisk, and newline character is added to the header just before the terminating 80-character keyword entry that begins with END_USGS_HEADER.

Examples of Data Header Entries

Notes on the North American Datum (NAD)

Early surveys were often based on a local datum or reference system usually determined by astronomic observations. In 1900, enough observations were obtained to complete a national geodetic datum known as the U.S. Standard Datum. It contained 2,500 survey points and was based on the Clarke 1866 reference ellipsoid. In 1913 the name was changed to the North American Datum upon adoption by the governments of Canada and Mexico. Geodetic survey station MEADES RANCH in Osborne County, Kansas, was selected as the reference point for this datum.

By the 1920's the survey points had expanded to more than 25,000. The addition of the new survey points were then incorporated into the readjustment known as the NAD 27.

The development of light wave and microwave distance measuring equipment, aerial photography, and eventually satellite positioning systems has enhanced the capabilities to provide precise positional data. By 1972, the NAD 27 had developed into a network of almost 250,000 geodetic survey stations. These additional stations were forced to fit the original NAD 27 network which caused distortions in the positional data. In 1971, the National Academy of Sciences recommended the readjustment of the NAD 27 to meet the demands for increased positional accuracy.

This lead to the development of the NAD 83, which included both a readjustment of the survey observations and a redefinition of the datum. The readjustment required the automation of all observations performed. A technique known as Helmert blocking was employed to provide a simultaneous least-squares adjustment of all observations to minimize mathematical distortions in the resulting coordinate information. To facilitate the use of satellite surveying and navigation systems, the new datum was redefined using the Geodetic Reference System 1980 (GRS 80) as the reference ellipsoid because this model more closely approximates the true size and shape of the Earth. Unlike the Clarke 1866 ellipsoid, GRS 80 is geocentric (the center of the reference ellipsoid is at the mass-center of the Earth) and completely compatible with the satellite systems.

The NAD 83 is the culmination of 12 years of automation and analysis of every major horizontal control survey performed. The readjustments to NAD 83 has resulted coordinate shifts that exceed 400 meters in some parts of the country.

                        COMPARISON OF DATUM ELEMENTS
----------------------------------------------------------------------------
		             NAD 27			NAD 83
----------------------------------------------------------------------------
Reference Ellipsoid	Clarke 1866		GRS80
			a = 6,378,206.4 M *	a = 6,378,137.0 M *
			f = 1/294.9786982 **	f = 1/298.2572221 **

Datum Point		Triangulation Station	NONE
			MEADES RANCH		(Mass-center of Earth)

Longitude Origin	Greenwich Meridian	SAME
			(BIH Zerio Meridian)

Adjustment		25,000 Points		250,000 Points

Best Fitting		North America		Worldwide	
----------------------------------------------------------------------------

 * Length of equatorial semimajor axis (a) of the ellipsoid.
** Flattening (f) of the ellipsoid is derived from other defined parameters.

Example of Band Storage


Examples of band storage for digital orthophotos.

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[ Monochrome (Fig. 2-2) | BIL (Fig.2-3) | BSQ (Fig.2-4) | BIP (Fig.2-5) ]

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Figure 2-2 Example of monochrome DOQ storage. Example of a standard digital
orthophoto stored as a simple monochrome image file consisting of 6094 image
lines plus a single header line. Each line, including the header, is 5790
bytes in length. File size is 33.6 MB.  M is equal to monochrome.



line   byte no.                                line length in bytes
num.   1                                                       5790
    1 header record  = 72, 80-character entries + 30 blank pads
    2 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
    3 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
    4 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
    5 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
    6 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
    7 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
    8 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
                 "                  "                  "
                 "                  "                  "
   24 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
   25 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
   26 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
   27 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
                 "                  "                  "
                 "                  "                  "
                 "                  "                  "
 2996 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 2997 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 2998 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 2999 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 3000 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
                 "                  "                  "
                 "                  "                  "
 6089 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 6090 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 6091 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 6092 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 6093 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 6094 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
 6095 mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

Figure 2-3 Example of an RGB Orthophoto stored Band Interleaved by Line
(BIL). Each band contains 6094 lines. File size is 100.9 MB.

r - red band g - green band b - blue band

line   byte no.                                line length in bytes
num.   1                                                       5790
    1 header record = 72, 80-character entries + 30 blank pads
    2 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
    3 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
    4 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    5 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
    6 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
    7 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
                 "                  "                  "
                 "                  "                  "
 6094 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
 6095 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
 6096 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

                 "                  "                  "
                 "                  "                  "
12187 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
12188 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
12189 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
                 "                  "                  "
                 "                  "                  "
18281 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
18282 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
18283 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

Figure 2-4 Example of RGB Orthophoto stored as Band Sequential (BSQ) with
6094 lines of each band appended to each other. File size is 100.9 MB

r - red band g - green band b - blue band

line   byte no.                                line length in bytes
num.   1                                                     5790
    1 header record = 72, 80-character entries + 30 blank pads
    2 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
    3 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
                 "                  "                  "
                 "                  "                  "
                 "                  "                  "
 6093 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
 6094 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
 6095 rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
 6096 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
 6097 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
 6098 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
                 "                  "                  "
                 "                  "                  "
                 "                  "                  "
12187 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
12188 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
12189 gggggggggggggggggggggggggggggggggggggggggggggggggggggggggg
12190 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
12191 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
12192 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
                 "                  "                  "
                 "                  "                  "
                 "                  "                  "
18280 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
18281 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
18282 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
18283 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

Figure 2-5 Example of RGB Orthophoto stored Band Interleaved by Pixel (BIP).
6094 image lines plus a header line, each 17370 bytes in length. File size
is 100.9 MB.

r - red band g - green band b - blue band

line   byte no.                                line length in bytes
num.   1                                                   17370
    1 header record = 217, 80-byte entries + 10 blank pads
    2 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb
    3 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb
    4 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb
    5 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb
    6 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb
    7 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb
                 "               "              "
                 "               "              "
 6095 rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb  rgb

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