Texas 1 Meter Color DOQQ
Statewide coverage
MrSid Compressed Files


The term "digital orthophoto" is used throughout this document to refer to both the "digital orthophotoquadrangle" (DOQ) and "digital orthophoto quarter-quadrangle" (DOQQ) products. A digital orthophoto is a digital image which has the properties of an orthographic projection. It is derived from a digitized perspective aerial photograph by differential rectification so that image displacementscaused by camera tilt and relief of terrain are removed. Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map.

This program is oriented toward the production of 1-meter digital orthophoto quarter-quadrangles from 1:40,000-scale color infrared (CIR) National Aerial Photography Program (NAPP) photography. NAPP photographs are quarter-quadrangle centered (3.75 minutes of longitude and latitude in geographic extent), which makes them perfectly suited for a quarter-quadrangle-based digital product. From the primary 1-meter data, a variety of standard resampled products will be produced, including 2.5-meter, 10-meter and 30-meter digital images.

The digital orthophoto is created by scanning an aerial photograph diapositive transparency with a precision image scanner. The scanned data file is then digitally rectified to an orthographic projection by processing each image pixel through photogrammetric space resection equations. This process requires, as input, ground control points acquired from ground surveys or developed in aerotriangulation, camera orientation parameters, and a digital elevation model DEM). The rectified digital image is then archived with the USGS and distributed to project contributors via tape and CD.

The photography is scanned at 25 micrometers (microns), or 1000 dpi. Each 240- by 240-millimeter (9- by 9-inch) CIR aerial photograph scanned with an aperture of 25 microns yields approximately 276 megabytes of raw data. Using 1:40,000-scale photographs, a 25-micron scan equates to a ground resolution of 1 meter. A CIR quarter-quadrangle digital orthophoto generated and cropped from a 240- by 240-millimeter photograph, scanned at 25 microns, with the requisite 300-meteroveredge and header records produces a rectified file of approximately 158 megabytes.

The archive and distribution format for this project is MrSid. The digital orthophoto is cast on the Universal Transverse Mercator (UTM) projection on the North American Datum of 1983 (NAD 83). Digital orthophotos are archived and distributed so that when displayed on a computer graphics terminal, projection grid north is at the top.

Accuracy

The Texas digital orthophoto quarter-quadrangles meet horizontal National Map Accuracy Standards (NMAS) accuracy requirements at 1:12,000 scale. The NMAS specify that 90 percent of the well-defined points tested must fall within 33.3 feet (1/30 inch) at 1:12,000 scale. The vertical accuracy of the source DEM must be equivalent to or better than a level 1 DEM, with a root-mean-square-error (RMSE) of no greater than 7.0 meters. A planar-DEM (sloped-plane substitute grid) may be used for areas where the relief difference (high to low elevation) does not exceed 150 feet. All remaining inputs and processes (i.e. aerotriangulation control and methodology, scanner calibration, and sensor calibration) used in digital orthophoto production must be sufficiently accurate to ensure that the final product meets NMAS.