The letters stand for digital orthophoto quadrangle. The USGS scans photos, almost exclusively from the National Aerial Photography Program (NAPP), an interagency effort coordinated by the USGS for map revision and a variety of other purposes. Each of these photos covers an area a little more than 5 miles on a side, representing about one-fourth of a standard, 7.5-minute USGS topographic quadrangle map. (Thus the occasional reference to quarter quad.)
So, they're scanned aerial images?
Yes, but that's not all, Once in digital form, the images undergo a computer orthorectification routine, making them geometrically like maps. This means a number of things, including the fact that, when you look at an orthorectified image, no matter where you look, you're looking straight down--just like looking at a map. Rather more importantly, it means that it is possible to make accurate measurements of ground distance on the photos, as one can on maps, and that the photos can be used in automated mapping software along with other digital cartographic data.
Why is it called orthorectification?
Cameras see things the way our eyes see them. The perspective of the camera causes things like foreshortening where, for example, parallel lines seem to converge away from the camera. These phenomena--the way our eyes actually see things--are regarded by cartographers as distortions, and the process of removing them is orthorectification.
Not everybody, of course. These images are useful for professional purposes in automated mapping programs, geographic information systems, and some civil engineering applications. Because of the USGS-Microsoft joint research project, it is possible to look at a low-resolution version of each of the DOQ's in the TerraServer database and even to download and print it.
If, however, you want the higher quality, full-resolution DOQ, the clearest available image, you'll be disappointed unless you have some sophisticated viewing software to look at it; DOQ's don't come packaged with viewers. They're big files, too-- 55 megabytes on average for a black-and-white version, and nearly three times that size for color-infrared.
What if I can't use 55 megabytes of image data?
If what you want to see is simply an aerial photo of a particular place in the United States, you can always get the NAPP photographs, not orthorectified and not digitized. No browse or preview images are available for these. However, they are available for all places in the lower 48 States, and they are inexpensive, starting at $10 for a 9-inch-square black-and-white print or film negative or positive. You can locate and order a NAPP photo online through EarthExplorer, either by place name, geographic coordinates, or map location.
Can I get a DOQ of any place I like?
No. Aerial photos (NAPP photos and others) are being collected for the entire United States, but not for other countries. As for DOQ's, the program is fairly new, and the whole country is not yet covered. As the files went into the Microsoft server, DOQ's were available for about 30 percent of the land area of the lower 48 States. A DOQ status graphic is available showing roughly what areas are covered.
With what other kinds of data can I use DOQ's?
A DOQ can be coregistered with or correlated to other cartographic and geographic data in many forms. From the USGS you can order, and often download, several forms of data that, in given applications, can be used in conjunction with the DOQ's. These data include digital elevation models, digital line graphs, digital raster graphics (digitized topographic maps), and satellite image data. A catalog of data types is available online through EarthExplorer.
What are the file formats in which DOQ's are available?
The monochromatic (black-and-white) photos, representing the majority of the archive, are available in single-file format, usually band-interleaved by pixel (BIP). Color-infrared images are also in BIP format.
What is the smallest object I can see on a DOQ?
That depends on a number of factors, including the viewer and monitor you are using, as well as the quality of the original photograph from which the DOQ was made. So perhaps it is best to discuss the photographs themselves. They are 1-meter resolution images, meaning the smallest item that can be distinguished is about 3 feet across. That does not mean, however, that everything 3 feet long or more can be identified in the photo. For example, it is usually easy enough to identify a driveway in one of these pictures. If a car is parked in the driveway, you can easily tell what it is. But if a car is parked in the middle of a field, where you wouldn't expect to find it, you might not be able to identify it as an automobile. The ability to identify features increases with experience in examining the images.
Why do tall buildings and trees sometimes look tipped over?
That happens because of camera perspective. If you look at a telephone pole from directly above, it's a little circle. If you look at it from the side, it's a vertical line. If you look at it from an oblique angle (from an airplane at a distance, say) it appears to be tipped over. Orthorectification will put the base of the tipped-over pole into its precise geographic location, but won't turn it into a little circle.
That's the way water is. Sometimes, with our own eyes, water looks black. Sometimes it looks silver, sometimes blue, green, or brown. It depends mostly on solar reflection. (In color-infrared photos, water is usually a deep black.)