Using Digital Orthophoto Quadrangle (DOQ) Images

DOQ Attributes

Each DOQ is approximately 50 MB in size and features a 1-meter pixel resolution. This makes it a very good source of detailed visual information as well as a valuable layer in a geographic information system. The DOQ is georeferenced to the Universal Transverse Mercator (UTM) coordinate system. The image uses NAD83 as the primary datum and NAD27 as the secondary datum. Corner tick marks are located on each image, with solid white crosses used for the NAD83 datum and dashed white crosses used for the NAD27 datum. The UTM coordinates in Northings and Eastings, for each tick mark and the upper left corner pixel, can be found on the header file.

Each DOQ consists of an image file and a header file. File names follow the DOS name convention with an eight-character name, a "dot", and a three-character file extension. The structure of the name is "onnnnnnn.qqx" where "o" is the leading lower-case letter o, and "nnnnnnn" is approximately the latitude and longitude of the quadrangle. The "qq" represents the quarter of the larger 7.5 minute quadrangle, SE, SW, NE, or NW. The "x" identifies the image file with the lower-case letter c and the header file with the lower-case letter h. File o410725.sec is the image file for the southeast quarter of the Ellington 7.5-minute quadrangle. File o4107204.neh is the header file for the northeast quarter of the Broad Brook 7.5-minute quadrangle.

A full DOQ image file requires approximately 50 MB of storage. Each image file on the CD has been compressed and is stored in the JPG format, which reduces the CD image storage to approximately 4.5 MB but causes the loss of some image detail as a result of compression. Images in the TIFF format, with no compression loss, can be purchased individually from the USGS if necessary. Data is stored on the header files in one of two formats. Header files created after 1996 store the information in ASCII records using a KEYWORD=VALUE format that can be easily viewed using a variety of utilities and programs. Header files created prior to 1996 store records in a binary format in variable length records. These can be displayed using the technique described below for fixing the registration.

Before data can be extracted from the CD, it is necessary to identify the proper DOQ file name. The file doqlist.txt contains a list of 7.5-minute quadrangle names and the associated quarter-quadrangle DOQ file names in the "onnnnnnn.qqx" format described above. This file is located in the directory CDROM/document/doqtext. The image and header files are located in the directory CDROM/data. Additionally, each of the DOQ CDs produced by the USGS contains a complete set of documentation. Please refer to one of the 27 text files to learn more about these images.

DOQs and ERMapper

DOQ images can be imported into ERMapper and used with satellite images and GIS data. Remember that these images use the UTM coordinate system with the NAD83 datum so they may need to be re-projected if your other data uses different referencing systems. The CEO has the USGS DOQ CDs for the following Connecticut counties: Fairfield, Litchfield, New London, Tolland, and Windham. We also have DOQs for the city of New Haven. DOQs for other areas of Connecticut can be downloaded in a DOS PKZIP format over the Internet from the MAGIC site at the University of Connecticut.

From ERMapper select the menu [Utilities | Import Image Format | DOQ (USGS)]. This will open up an input/output file window. Select the header file for the DOQ you wish to import. Using the Broad Brook Northeast quarter DOQ example above, the input file would be: /CDROM/data/o4107204.neh. Enter a unique file name for the output file such as: /usr/people/bonneau/dataset/doq/bbrookne.ers then click on the OK button and the import will begin. Once the import has completed, it will be necessary to correct two problems with the import utility; both the cell size and the registration point are wrong.

To fix the cell size: Use a text editor such as jot, nedit, or vi to edit the header file (.ers) of the ermapper dataset that was created by the import process. Find the "CellInfo" block which will look something like this:

                CellInfo Begin
                        Xdimension      = 0.8705851241556
                        Ydimension      = 0.9036013304257
                CellInfo End
Simply change the Xdimension and Ydimension to reflect the real cell size of these files (1 meter)
                CellInfo Begin
                        Xdimension      = 1.0
                        Ydimension      = 1.0
                CellInfo End
To fix the registration: Open an xterm from within ERMapper (Utilities | User Menu | Open a Terminal Window). Type the following "importdoq -t <quad_to_import>", where "<quad_to_import>" is the full path name to the file on CD (e.g. /CDROM/data/o4107204.neh. The data from the header for this quad will scroll up on your screen. Find the values in Section 3 - Item 10 labeled "[10]Pri X,Y of Pixel 1,1". These are the correct NAD83, UTM easting and northing values that you will need to enter in the ERMapper header file, write them down.

Go back to editing your header file and find the "RegistrationCoord" block which looks like this:

                RegistrationCoord Begin
                        Eastings        = 639620.0008953
                        Northings       = 4658093.22494
                RegistrationCoord End
Replace the easting/northing values that are in there with the ones you just wrote down.

Exit the xterm by typing "exit".

Your DOQ should be ready to use! (Reload the dataset if you are already displaying it.)

The newly created ERMapper DOQ file will be approximately 50 MB in size. You may want cut out a smaller subset of the image to retain in your personal storage area and delete the full DOQ file. Another option is to resample the image to reduce both the resolution and size. If the image is converted from one-meter to three-meter resolution the file size will be reduced to 1/9th of the original size, or approximately 5.5 MB.

CEO Home Page
27 July 1998