TCEQ Digital Orthophoto Quarter
This project has involved a cooperative joint funding agreement between the TCEQ and various state and federal entities to produce digital orthophoto quarter quads (DOQQs) and their associated digital elevation models (DEMs). DOQQs are digital images of high-altitude aerial photographs where the displacements caused by the camera angle and surface variations of the photographed terrain have been removed. These corrections give DOQQs the same geometric properties as paper maps; thus, it is possible to use DOQQs for the direct measurement of distances, areas, angles, and positions.
The standard DOQQ image covers 3.75 minutes of latitude by 3.75 minutes of longitude (one-quarter of a USGS 7.5-minute quadrangle), has a scale of 1:12,000, and a ground resolution (the area of the ground represented in each pixel in x and y components) of one meter. DOQQs must meet National Map Accuracy Standards (NMAS) of 10 meters. On a typical DOQQ, 90 percent of the well-defined points on the DOQQ that are tested for positional accuracy must fall within 10 meters of their actual location.
The aerial photographs used to produce DOQQs are obtained through the National Aerial Photography Program (NAPP). The photographs are taken at an altitude of 20,000 feet above mean terrain elevation using a camera with a focal length of 152.4 mm (approximately 6 inches), resulting in a photographic scale of 1:40,000. A typical NAPP photograph is centered around a 3.75-minute quarter quadrangle.
DOQQs and DEMs for the state of Texas may be ordered through the Texas Orthoimagery Program (TOP) at the Texas Natural Resources Information System (TNRIS), or the USGS EROS Data Center.
One-Meter Compressed DOQQs now available from TNRIS!!
The one-meter color-infrared DOQQs (Digital Orhtophoto Quarter Quads) from 1995 and 1996 are available from the TNRIS website for the entire state of Texas.
To search by county, quad name, or degree block, go here.
If you already know the correct EISYS Number (i.e. - 2597032), go directly to the FTP area.
Note: You must load the MrSID Image Support extension to display the compressed DOQQs in ArcView. This is already bundled with ArcView 3.2, but if you don't have it, download it here and save it to your ESRI Ext32 folder, then go to File: Extensions in ArcView to make it active.
If you are using ArcGIS, no extension is necessary.