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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.
About DOQs
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 ClearinghouseUsing 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 doqnewhdr.aml
You can use this menu driven Windows program(95 or NT). Doq2Arc2.exe
You will need to ftp 3 other files:
1.tb510.dll
2.xnmba420.dll
3.xnmte420.dll
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 example "napa_swm.hdr"
- 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:
nrows 7601
ncols 6221
ulxmap 554240.000
ulymap 4241020.000
skipbytes 6221
xdim 1
ydim 1
nbands 1
The following is the header information from "napa_swm.doq" data file:
BEGIN_USGS_DOQ_HEADER QUADRANGLE_NAME "NAPA" QUADRANT SW 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 RASTER_ORDER LEFT_RIGHT/TOP_BOTTOM BAND_ORGANIZATION "SINGLE FILE" BAND_CONTENT BLACK&WHITE BITS_PER_PIXEL 8 SAMPLES_AND_LINES 6221 7601 HORIZONTAL_DATUM NAD83 HORIZONTAL_COORDINATE_SYSTEM UTM COORDINATE_ZONE 10 HORIZONTAL_UNITS METERS HORIZONTAL_RESOLUTION 1.0 SECONDARY_HORIZONTAL_DATUM NAD27 XY_ORIGIN 554240.000 4241020.000 SECONDARY_XY_ORIGIN 554336.454 4240823.773 NATION US STATE CA 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 RMSE_XY 0.987 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" PRODUCTION_SYSTEM "ORTHOVIEW" STANDARD_VERSION 1996 12 METADATA_DATE 1997 9 15 DATA_FILE_SIZE 47292042 BYTE_COUNT 6221
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 softwareThe 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.