The production procedures, instrumentation, hardware and software
used in the collection of standard USGS DOQ's vary depending on systems
used at the contract, cooperator or USGS production sites. The majority of
DOQ datasets are acquired through government contract. The process step
describes, in general, the process used in the production of standard
USGS DOQ data sets.
The rectification process requires, as input, a user parameter file to control
the rectification process, a digital elevation model (DEM1) gridded to user
specified bounds, projection, zone, datum and X-Y units, a scanned digital
image file (PHOTO1) covering the same area as the DEM, ground X-Y-Z
point values (CONTROL_INPUT) and their conjugate photo coordinates in
the camera coordinate system, and measurements of the fiducial marks
(CAMERA_INPUT) in the digitized image.
The camera calibration report (CAMERA_INPUT) provides the focal
length of the camera and the distances in millimeters from the camera's
optical center to the camera's 8 fiducial marks. These marks define the frame
of reference for spatial measurements made from the photograph.
Ground control points (CONTROL_INPUT) acquired from ground surveys
or developed in aerotriangulation, are third order class 1 or better, and meet
National Map Accuracy Standard (NMAS) for 1:12,000-scale.
Ground control points are in the Universal Transverse Mercator
or the State Plane Coordinate System on NAD83. Horizontal and vertical
residuals of aerotriangulated tie-points are equal to or less than 2.5 meters.
Standard aerotriangulation passpoint configuration consists of 9
ground control points, one near each corner, one at the center near each
side and 1 near the center of the photograph, are used. The conjugate
positions of the ground control points on the photograph are measured
and recorded in camera coordinates.
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 to 32 microns is used, with an aperture
no greater than 32 microns permitted. 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. Scan files with
ground resolution less than 1 meter or greater than 1 meter but less
than 1.28 meters are resampled to 1 meter.
The principal elevation data source (DEM1) are standard DEM datasets
from the National Digital Cartographic Data Base (NDCDB). DEM's that
meet USGS standards are also produced by contractors to fulfill DOQ
production requirements and are subsequently archived in the NDCDB.
All DEM data is equivalent to or better than USGS DEM standard level 1.
The DEM used in the production of DOQ's generally has a 30-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
nearest neighbor, bilinear, or cubic convolution resampling of the
scanned image. The pixel processing algorithm is indicated in the
header file . An inverse transformation relates the image coordinates
referenced to the fiducial coordinate space back to scanner
coordinate space. For those areas for which a 7.5-minute DEM is
unavailable and relief differences are less than 150 feet, a planar-DEM
(slope-plane substitute grid) may be used.
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. DOQ contractors must meet DOQ standards
for attribute accuracy, logical consistency, data completeness and
horizontal positional accuracy. During the initial production phase,
all rectification inputs and DOQ data sets are inspected for conformance
to standards. After a production source demonstrates
high quality, inspections will be made to 10% of delivery lots
(40 DOQs per lot). All DOQ's are visually inspected for
gross positional errors and tested for physical format standards.