The analysis of historical aerial photos can be accomplished using GIS
Software and real-time, on-screen stereo observation systems. However,
for the interpretation task itself, this is overkill and makes the job
unnecessarily more expensive. In addition, those systems are zero help
in identifying what’s on the ground and deducing its significance. It is
justified when the accurate mapping or dimensioning of features on the
ground is a major goal of the work. However, the great majority of this
type of work is non-numerical.
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The more traditional approach involves aerial film positives,
on a light table, viewed in stereo through a Zoom Stereoscope. These
were mostly made by Bausch & Lomb since the early 1960’s for the
Photographic Intelligence industry, spurred on by the Cold War.
The now-surplus equipment comes in
various models and sizes. However, because most of the primarily
civilian aerial films are not from high resolution imaging systems, the
oldest, original 1960’s B&L “Zoom-70” Stereoscopes can be used with
no loss of detail or acuity. This instrument had an imaging resolution
of 70 Line Pairs per millimeter; therefore, the name. The newer
stereoscopes in this line have much higher resolution (if the aerial
films justify it) and they have longer rhomboid arms so that the film does not have to be cut down to
small film “chips” to fit under the small rhomboids of the Zoom-70.
However, the light table to support all this became a significant, heavy
piece of furniture: not needed for the job.
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B&L Zoom Stereoscope setup for hard copy
film stereo triplet analysis.
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This equipment basically consists of three components: the Zoom-70
Power Pod itself, a very small, tabletop light table underneath to
provide for transillumination of the diapositives, and a simple,
mechanical motion system to raise and lower the stereoscope over the
film. See illustration at right.
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| Because photographic film is gradually going away, another method had
to be devised to view the digital scan files now being more commonly
delivered by the various agencies and companies. In general, a normal
computer screen is not going to work very well for this work because the
use of a standard lens stereoscope (2x power) provides too much
magnification and the observer feels that he or she is looking at the
dots on the screen, instead of the objects on the ground. That is, the
crude screen pixels dominate the view. |
Diagram of the setup for comparing two years of aerials from digital scan files.
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The corresponding view of the workplace for the setup indicated in the
figure above. The shift from the 1971 view to the 1977 view of the site
is almost instantaneous. This makes change detection much easier.
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However,
a few years ago, high-resolution, or 4K monitors began to come into use
as computer monitors. These were known as “Retina Displays” and they
did
provide much more screen detail. The screen resolution was enough to
allow the use of a standard 2X Pocket Stereoscope directly over the
monitor for stereo of the aerial images. The Photo Interpreter would
then use off-the-shelf image processing software (like Adobe PhotoShop)
to do the manipulation of the images and the zooming. |
