Evening star., 1938-01-25
AIR PHOTOS CUT
SURVEYING COSTS
Photogrammetry Session
Told Hazardous Years
of Labor Reduced.
BY THOMAS r. HENRY.


Applications of air photography
which are revolutionizing the old art
OF surveying and map making were
described at the annual meeting Of
the American Society Of Photogram.
metry at the National Academy oi
Sciences Building yesterday.
The civil engineer now can run
transit lines through the wastes OF
Northern Alaska or the jungies oi
Costa Rica without ever leaving hi.
desk in Washington, whereas a few
years ago he would have been obliged
to tramp over almost every foot OF
the ground with surveyors transit
over his shoulder and a steel tape
in his pocket.
First the country to be surveyed
is photographed from an airplae fly.
ing over it at 100 miles an hour
The pictures. are sent back to Wash
ington with the pilots notations OF
the height and speed of the plane
and the focal length Of the camera
With this data survey lines can be run
on the photographs and the various
dimensions found can be translated
back into the actual dimensions Of the
surface of the land by applying the
proper formulae.
Special Instrument Devised.
One of the chief instruments fol
doing this was described by Its in.
ventor. R. NJ Wilson, chief Of the
computing division of the Geological
Survey. This is the photo-alidade
an adaptation for armchair survey.
ing Of the "plane-table" instrument
used by engineers in the past.
Large-scale applications Of the vari-
GUS photo-surveying devices by the
photographic Office Of the Navy. the
Geological Survey. the Coast and Geo.
detic Survey. the Tennessee Valley
Authority and the Farm Security Ad.
ministration were displayed. These
have made it possible to complete in 6
few weeks at very little expense sur
veys which formerly would have re.
quired scores of engineers and Their
helpers working for years.
An example shown by the Hydro
graphic once was a survey Of the
coast line of Southern Costa Rica
A good deal Of this region was
densely forested, so that no transit
lines could have been run through 1t
Even without the jungles a surveying
party could not have covered more
than 4 miles day taking observa-
tions every 1000 feet. The air photo.
reveal every little indentation of the
coast, which can be measured al
leisure in Washington and translated
back into the proper dimensions.
Alaska Being Mapped.
Another example is Alaska. oi
whose 566,400 square miles only 314,-
400 ever have been surveyed after
almost half a century of laboriou,
effort with pack horses and dog teams
In some of this Territory 1t Is likely
no human being ever has set foot
Planes are now at work on air sur
veys of large regions Of Southern
Alaska, and it is only . matter OF
time until the project is extended te
the unknown northern mountains.
TO insure accuracy. it was ex-
planned, it is generally necessary to
follow up the Office surveys OF the air
photos by field parties with transit
instruments. These. however. only
locate certain points 10 miles or more
apart. by which to check the photos,
and even without them fair accuracy
Is attained.
One Of the interesting tricks ex-
planned was that Of computing the
height Of objects in an air photo
graph. A picture OF Washington taken
from directly above the Washington
Monument would show buildings in
the outskirts Of the city apparently
leaning slightly. as If they had Just
started to topple over. Such a pic
ture taken on the outskirts would
show the Monument and the Gov
ernment buildings leaning in the
same way.
BY means Of this apparent change
from the vertical, or "parallax," an
engineer could compute almost the
exact height Of the building and, If
1t were necessary. probably Of every
building in the city. This Is q sort
OF up-side-down astronomy, being
very similar to the parallax method
used in determining the distances of
ef8nA nf tha raaror eforr
Vast unexplored areas Of Northern
Canada now are being explored by
means Of aerial Photography, the So
ciety was told by A. c. T. Sheppard
Of the Canadian Institute Of Survey
ing. Requirements for precision
cameras were described by Dr.. Irvine
c. Gardner Of the Bureau of Stand
ards.