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technology has given rise to automated techniques for 
performing this task. 
The goal of this investigation is to develop and test a 
system which can automatically generate a digital terrain 
model using digitized aerial photography and a general 
purpose computer. This system utilizes a scanning micro 
densitometer to convert aerial photography to a form which 
can be "understood" by the computer. The densitometer 
scans the photography in a very fine grid pattern and 
records numerical values indicating the gray-shade at each 
point of the grid. In order to establish photogrammetric 
relationships this arbitrary scanner coordinate system must 
be transformed to the fiducial axis system. This requires 
locating the fiducial marks in scanner coordinates and 
transforming them to the calibrated values in the photo 
graphic XY system. When the parameters of the transforma 
tion are determined, photo coordinates can be readily 
computed for other points in the scanned image. Basic to 
any photogrammetric system is the ability to locate conju 
gate imagery on a pair of overlapping photos. In the 
automated mapping system this process is performed by a 
technique known as digital image correlation. In this 
method, two arrays of image densities are compared 
numerically to indicate the degree of similarity of the 
images. Pass points and ground control points are located 
by this method allowing a numerical relative and absolute 
orientation, or a simultaneous bundle adjustment to be 
performed. Once the orientation parameters are known a 
regular grid of digital terrain model points can be corre 
lated and absolute XYZ positions determined. This data is 
then input to a contour mapping program to generate the 
topographic map. 
Two sets of photography have been used to test the system. 
The first set, taken of an area near LaCrosse, Wisconsin, 
was used to develop the DTM generation software. Using 
these photos, a digital terrain model was generated auto 
matically and compared with a DTM read manually in a Kern 
PG-2 stereoplotter. The second set, taken over Mazomanie, 
Wisconsin, was used to test ground control panel recogni 
tion and scanner calibration. These photos contained 
specially designed control targets which were automatically 
located and converted to photo coordinates. These coor 
dinates were then compared to values measured with an H. 
Dell Foster monocomparator to indicate the accuracy of the 
automatically located panels. Bundle adjustments were 
performed using the monocomparator and automatically 
derived photo coordinates and results compared. 
SYSTEM HARDWARE 
The instrument used to convert the photography to digital 
form was an Optronics 1700 scanning microdensitometer. 
This scanner employs a rotating drum and lead screw to 
uniformly move the photo past the scanning optics. At 
each point along the scanning grid a narrow beam of light 
is transmitted through the film. The transmitted light 
intensity is compared with the incident intensity to obtain 
a digital value which indicates the relative darkness of