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Magnum One (Mid-American Digital) (Disc Manufacturing).iso
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README.DOC
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1991-06-13
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Digital Orthophoto Production FLOPPY Disk Readme File
An orthophoto is by definition "a photographic reproduction
prepared from a perspective photograph in which the displacement
of images due to camera tilt and terrain relief have been
removed" (Manual of Photogrammetry 1966). For the sake of
completeness the definition may be expanded to include "so that
the orthophoto has the same metric accuracy as a map and it also
has a uniform scale."
The basic equipment used to make a digital orthophoto is a
scanning microdensitometer and a digital computer. A scanning
microdensitometer is a precision instrument that uniformly scans
a photograph in a rectilinear fashion so that the designated area
is completely covered by adjacent picture elements (pixels) of
the desired size. A single numerical value is assigned to each
pixel proportional to the photon flux at that point (Horton
1978). The entire aerial photograph is digitized resulting in a
volume of data of from 20 million bytes to slightly over 80-
million bytes. The computer required to process these large data
sets must be fast, have a large random access memory (RAM),
and have large data storage devices.
The input data needed for generating a digital orthophoto
include: digitized image, a digital elevation model (DEM), and
both exterior and interior image orientation parameters. At this
time the digital orthophoto is made using many conventional
procedures. Control for the aerial photograph is developed
by aerotriangulation methods and is transferred to the
diapositive to be scanned. The diapositive is placed on the
microdensitometer and digitized, the digital data being written
to tape. The data is then processed using the software developed
by the USGS.
The software solves the exterior orientation of the
photograph by using space resection (collinearity) equations.
Next, raster fiducials are fit to the calibrated camera fiducials
using a linear conformal transformation and the transformation
parameters are saved.
The first two profiles of the digital elevation model (DEM)
are input to form a profile pair of DEM cells. The photo
coordinates of the cell corners in the raster system are computed
using DEM cell corner coordinates. The DEM cell is then
subdivided into the desired resolution. The raster coordinate
(line and sample) of each subdivision is then computed to the
nearest integer using bilinear transformation parameters. For
each subdivision the 8-bit binary grey scale value that
corresponds to the raster coordinate is assigned to a subdivision
image coordinate. The processing of each cell in a profile pair
and of each profile pair is accomplished sequentially.
The result of this program is a digital orthophoto that can
be used as an image map base in a graphic display system or
written to film to form a hardcopy orthophoto.
The digital orthophoto on this disk is the southwest quarter
of the Black Earth SW, WI quarter quadrangle. The area covered
is approximately 1.875-minutes with a ground resolution for each
pixel of 3.75-meters. The orthophoto is cast on North American
Datum of 1983 (NAD 83) in Universal Transverse Mercator (UTM)
zone 16. The UTM coordinates of the northwest corner are:
X = 275670.0 meters
Y = 4781265.0 meters.
The radiometric resolution is 8-bits and the file contains 1,000
lines of 1,000 samples each. There is no header i.e., header
bytes = 0.
All instructions to display the image are explained in the
ASCII file IMDISP.DOC. Although this program has been used by
the U.S. Geological Survey, no warranty, expressed or implied, is
made by the USGS as to accuracy and functioning of the program
and related program material nor shall the fact of distribution
constitute such warranty, and no responsibility is assumed by the
USGS in connection herewith. Users should use care in judging
the quality of the image for specific applications. Poor image
resolution may be due to low resolution monitors commonly found
on personal computers.