A Primer on Working with DOQs
A digital orthophoto is a digital image of an aerial photograph in which displacements caused by the camera and the terrain have been
removed. It combines the image characteristics of a photograph with the geometric qualities of a map.
The standard digital orhthophoto produced by the USGS is a black-and-white, or color infrared, 1-meter ground resolution quarter
quadrangle image. The accuracy and quality of USGS digital orthophotos must meet National Map Accuracy Standards at 1:12,000 scale for 3.75-minute
quadrangles and at 1:24,000-scale for 7.5-minute quadrangles. The accompanying information
serves to help users to overcome some of the obstacles frequently encountered when working with
DOQs. Material has been gathered from various USGS sources.
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The standard quarter-quadrangle image covers 3.75 minutes of latitude by 3.75 minute of longitude, at a scale of 1:12,000 cast on the
Universal Transverse Mercator projection based on the North American Datum of 1983. Each image also has between 50 and 300 meters of
overedge to facilitate tonal matching for mosaicking of adjacent images.
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.
For more details about DOQs see this
User Guide developed by the USGS NSDI Clearinghouse
Adobe Acrobat PDF Reader
Using New Header DOQs (keywords) in ARC/INFO and ARCVIEW
You can make use of several utilities for creating .hdr files
for viewing DOQs in ARC/INFO or ARCVIEW.
You can use this AML routine for ARC 7.X
You can use this menu driven Windows program(95 or NT).
You will need to ftp 3 other files:
Note: If your browser changes the .dll extension to .exe you
must rename them for the program to work.
Here's how to prepare a DOQ to be read into ArcView. The information you need
for the .hdr file (steps 2 and 3 below) is contained in the header of the DOQ.
You can view this information by opening the DOQ file in a word processor, like
wordpad. You can also create the .hdr file by using one of the utilities I have
included in the links above. The example used here uses the "Napa_swm.doq" DOQ.
The header file contained in it is attached below for you to follow along.
1. Naming of the DOQ image file:
nrows and ncols are "must have" values. They are contained in
"SAMPLES_AND_LINES" in the DOQ header (LINES = nrows and SAMPLES = ncols)
- use .bil extension for single band gray scale images
- use .bip extension for 3 band color images
example: rename "napa_swm.doq" to "napa_swm.bil"
2. Create a text header file, it must be an ASCII text file:
- use the same name as the .bil but with a .hdr extension
- be sure the word processor doesn't add ".txt" at the end
example "napa_swm.hdr.txt" won't work
3. Add lines to the .hdr text file that contain the following information:
nrows #number of rows or lines in image
ncols #number of columns or samples in image
ulxmap #upper left corner x value of pixel 1,1
ulymap #upper left corner y value of pixel 1,1
skipbytes #Number of bytes to skip that make up the header
xdim 1 #dimension of pixel in x direction
ydim 1 #dimension of pixel in y direction
nbands 1 #number of bands in image, nbands 3 for color images
ulxmap and ulymap are used to geo-reference the DOQ in UTM coordinates.
They are contained in "XY_ORIGIN" in the DOQ header
(the first value is X and the second value is Y)
skipbytes is used to avoid display of the header information found at the top of
each DOQ. This value is called "BYTE_COUNT" in the DOQ header.
xdim and ydim refers to the dimension of the pixels, the default is 1 by 1.
If you omit the xdim and ydim items you will see no change in your DOQ display.
nbands is 1 if the image is a gray scale image. For a color image nbands is 3 and
the extension on the name of the DOQ should be .bip
So napa_swm.hdr looks like this:
The following is the header information from "napa_swm.doq" data file:
WEST_LONGITUDE -122 22 30.0
EAST_LONGITUDE -122 18 45.0
NORTH_LATITUDE 38 18 45.0
SOUTH_LATITUDE 38 15 0.0
PRODUCTION_DATE 1997 9 15
BAND_ORGANIZATION "SINGLE FILE"
SAMPLES_AND_LINES 6221 7601
XY_ORIGIN 554240.000 4241020.000
SECONDARY_XY_ORIGIN 554336.454 4240823.773
NW_QUAD_CORNER_XY 554639.911 4240673.227
NE_QUAD_CORNER_XY 560103.961 4240712.028
SE_QUAD_CORNER_XY 560155.516 4233777.073
SW_QUAD_CORNER_XY 554686.779 4233738.292
SECONDARY_NW_QUAD_XY 554641.288 4240466.818
SECONDARY_NE_QUAD_XY 560105.475 4240505.620
SECONDARY_SE_QUAD_XY 560157.029 4233570.810
SECONDARY_SW_QUAD_XY 554688.154 4233532.029
IMAGE_SOURCE "Black and White film"
SOURCE_IMAGE_ID "NAPP 6359-203"
SOURCE_IMAGE_DATE 1993 7 6
SOURCE_DEM_DATE 1980 1 1
AGENCY "WESTERN MAPPING CENTER (WMC)"
PRODUCER "Hammon, Jensen, Wallen & Associates"
STANDARD_VERSION 1996 12
METADATA_DATE 1997 9 15
Information on BARD's compressed DOQs
The compressed DOQs ( also known as COQs for Compressed Orthophoto Quadrangle )
range in size from 2.5 Megs to just over 5 Megabytes using JPEG compression.
You can view the DOQs in Netscape after downloading by renaming the extension to jpg.
Programs available below are for the old DOQ header format
To use the DOQs in uncompressed format, if desired, you can uncompress them using the following
JPEG decompression software
Version for DOS executed from the DOS prompt command line - djpeg.exe
Version for the Sun computer compiled under Solaris 2.7. -
The Files Below are tar files with source code for compiling your own header manipulating routines.
To compile your own version of a program that will read and display DOQ header information.
To compile your own version of a program that will display and create a .hdr file on a windows 95 or NT system
DOQ Standards (PDF)
Standards for Digital Orthophotos - Contains preliminary pages and pages
that briefly describe the various changes that have been made to the standard.
Part 1: General, Standards for Digital Orthophotos -
Provides general information on definitions, objectives, product description, sources, file structure,
and format for digital orthophotos.
Part 2: Specifications -
Standards for Digital Orthophotos
Defines the specifications for geographic extent, collection, processing, datums and coordinates,
accuracy, ground sample distance, image radiometry, image mosaicking, data quality, header data,
archiving, and distribution formats for digital orthophotos.
Fact sheets from the USGS
If you develop products, scripts, extensions, or translators for use with DOQQs and would like to make them available to the public, please send e-mail to firstname.lastname@example.org. If you have a paper, tutorial, or other useful document that we should refernce from here, please tell us about it.