----------------------------Original message---------------------------- REF: Date: Sat, 20 Aug 1994 17:48:25 EDT From: "Brill, Margaret" <[log in to unmask]> Subject: DOQs ----------------------------Original message---------------------------- I attended a seminar on Digital Orthophotography a couple of weeks ago, sponsored by the NC Center for Geographic Information and Analysis. I learned that although NC was flown last year by NAPP, the $2m contribution which the state must make in matching funds for the USGS cooperative program has not yet been approved. There is some opposition to the program on the grounds that 1:12,000 is not detailed enough coverage. . . . [text deleted] Margaret Brill Duke University Durham, NC [log in to unmask] ------------------------------ Margaret -- Reference the detail in the USGS DOQ's. The 1:12,000 scale that you refer to in you msg is just "loosely" connected to the detail contained in the DOQ files. Specifically, 1:12,000 scale is the printed scale for map or image products based on the "quarter-quad" (3.75' or one-quarter of a standard 7.5') map area. The DOQ's are generated to cover a quarter-quad plus a bit of "overedge." The standard DOQ digital image files have pixels repre- senting 1 meter on the ground. When those files are displayed on a high-resolution (i.e., standard VGA or work-station resolution of 75 to 90 pixels per inch) image display at full resolution, the displayed scale is in the range of 1:3,000. In other words, there is much more detail in the DOQ than would be expected in a 1:12,000-scale printed product. Try measuring the length of your display line on your CRT; it probably measures about 9 inches if you have a 14-inch (measured diagonally) CRT. At a resolution of 640x480 pixels on the display, the DOQ would display 640 meters in 9 inches, which approximates a scale of 1:2,789. The upshot of all this of course is that the DOQ's can be thought of as "1:3,000-scale" products from the standpoint of the details they contain and the methods in which they generally will be displayed. Hope this information is useful to interested parties. -- Clark [log in to unmask] opinion is personal