Project design guidance provided by USGS. Scanned NAPP imagery provided by NRCS. Ground control provided by USFS, NPS, Bayfield County and Point North Mapping. Image compression services provided by USFS. Bayfield County funded the development of DEMs and the production of the digital orthophotography, both of which were performed by Benchmark GIS, Chapel Hill, NC.
correlation purposes.
The raster image file (PHOTO_1) is created by scanning an aerial photograph film diapositive with a precision image scanner. An aperture of approximately 25 microns is used; using 1:40,000-scale photographs, a 25-micron scan aperture equates to a ground resolution of 1-meter. The scanner converts the photographic image densities to gray scale values ranging from 0 to 255 for black and white photographs. The principal elevation data source (DEM1) are DEM datasets meeting USGS standards which are, as of this writing, in the process of being archived in the National Digital Cartographic Data Base (NDCDB). All DEM data are equivalent to USGS DEM standard level 2. The DEM used in the production of DOQ's has a 10-meter grid post spacing and possesses a vertical RMSE of 7-meters or less. A DEM covering the extent of the photograph is used for the rectification. The DEM is traversed from user-selected minimum to maximum X-Y values and the DEM X-Y-Z values are used to find pixel coordinates in the digitized photograph using transformations mentioned above. For each raster image cell subdivision, a brightness or gray-scale value is obtained using bilinearresampling of the scanned image. An inverse transformation relates the image coordinates referenced to the fiducial coordinate space back to scanner coordinate space. Rectification Process: The photo control points and focal length are iteratively fitted to their conjugate ground control points using a single photo space resection equation. From this mathematical fit is obtained a rotation matrix of constants about the three axes of the camera. This rotation matrix can then be used to find the photograph or camera coordinates of any other ground X-Y-Z point. Next, a two-dimensional fit is made between the measured fiducial marks on the digitized photograph and their conjugate camera coordinates. Transformation constants are developed from the fit and the camera or photo coordinates are used in reverse to find their conjugate pixel coordiates on the digitized photograph.
Quality Control: All data is inspected according to a quality control plan which incorporates attribute accuracy, logical consistency, data completeness and horizontal positional accuracy. During the initial production phase, all rectification inputs were inspected for conformance to standards.
Standards for digital orthophotos: Reston, VA.
A hypertext version is available at:
<URL:ftp://www-nmd.usgs.gov/pub/doq_html/standards_doq.html>