of each target is a small fraction (about one third) of the
time required to read an equivalent set of images on ten
separate frames).
(f) Because successive images of a multiply exposed target are
physically close together and would be measured in rapid
succession, their film coordinates would largely share any
uncompensated systematic errors. This could be expected to
Lead to triangulated X,Y,Z coordinates in which relative
systematic errors of successive targets are virtually non-
existent. Accordingly, extraordinary accuracies become
possible insofar as detection of change is concerned, as,
for example, when successive observations are made of an
object undergoing thermal deformation.
(g) If a given camera is perfectly stable in exterior orientation
throughout a series of multiple exposures recorded in the
incremental film-shifting mode, and if fiducial marks are
recorded for each exposure, all such exposures can be con-
strained in STARS software to share common exterior projec-
tive parameters. Alternatively, if significant changes in
angular elements (o,w,k) are suspected, the exposure can be
constrained to share only translational elements (X ,Y ,Z ).
In either case the result is an appreciable strengthening of
the photogrammetric adjustment. |
(h) If the multiply exposed object considered under (g) is also
stable throughout the series of exposures, STARS software
can be exercised to effect a succession of least squares
transformations (translations/rotations) to combine all of
the separate sets of images into an equivalent single set.
(2) With retroreflective targets exposures can be made so that
only the targets themselves are recorded on the film. How-
ever, occasions arise where is is desirable to record the
object also, perhaps as a faint 'phantom' image. This can
be accomplished if supplementary illumination, not emanating
from the camera, is available. Then, a sufficiently long
exposure can be made by means of the camera shutter to bring
out the background to whatever extent is desired.
(j) When a large object or structure must be photographed from
a considerable distance in daylight, some areas of interest
may be in full sunlight and others in deep shadow. This can
present an extremely difficult photographie Lighting problem
when conventional targets are employed, for fill-in illumina-
tion must be comparable in intensity to sunlight. With retro-
reflective targets, on the other hand, all necessary illumination
is provided by a single, relatively low powered flash unit
Located close to the camera, and targets in sunlight and shadow
all end up equally well exposed with clear backgrounds.
The above by no means exhausts the possibilities opened up by the use of
retroreflective targeting in conjunction with instrumentation and software
specifically designed to exploit its characteristics. A fuller exposition
is to be found in Brown (1980). Although designed for optimal utilization
of retroreflective targeting, STARS by no means neglects situations in
which conventional targeting is employed.
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