It might be informative to have a look on the data flow in such a system.
The information content of the data in the digital image memories can be
effectively used by two closed loops (Fig.2). Either the image data are
displayed stereoscopically to an operator who moves the measuring device
and controls this movements through the first closed loop. Or the data are
directly used for measurements by digital correlation in a second loop
which is completely integrated in the digital system. |
STEREO- VIEWING V1 SUR Pass ones ue ‘
SYSTEM
À
LS) DIGITAL IMAGE
| MEMORIES
i [T d HUMAN
: OPERATOR
DIGITAL ANALYTICAL
CORRELATOR PROJECTION SYSTEM
Lj
! :
figs foaming. i MEASURING A
DEVICE
Y
COMPUTER-CONTROLLED
DRAWING TABLE
Fig.2 Data flow in a stereophotogrammetric system with digital
image correlation
Another feature of the digital concept which is not available in conven-
tional photogrammetric systems is the possibility of using the graphical
overlay for storage of the data already collected. Points, contour lines
etc. measured by the operator may be superimposed on the image as one
chooses. This allows to check what is already done and if it is done cor-
rectly. As already mentioned by CASE (2), superposition is a prior con-
dition for revision of previously collected data or maintenance of digi-
tized maps, which can be transformed into the display coordinate system
and superimposed to the stereo-model. In a subsequent step, correction
and editing functions for revision purposes will be integrated in the pho-
togrammetric system.
4. Questions of Data Processing
Using turnkey image processing systems for stereophotogrammetric purposes
is coupled with one main problem: processing of the entire stereo model
is only possible by dividing the images into small subregions. The size of
these subregions depends on the capacity of the frame buffer of the system.
Displaying the actual part of the image must be organized by the user's