The Marietta journal. [volume] (Marietta, Ga.) 1866-1909, 1898-05-05

PHOTOGRAPHIC SURVEYING.
One OF the Important Uses to Which the
Camera Has Been Put.

Camera Has Been Put.
Something like so, 600 square miles
of the mountainous country of west
ern Canada has been surveyed with
the camera. Nothing CON. parable
to this in the way of Photographic
( surveying has been accomplished
on any such scale anywhere else in
| the world
| The causes of failure on the part
of many who have tried the method
and discarded it in disgust are
chiefly two. One of these is defect
i've knowledge in descriptive geom-
etry and perspective, which are es-
sentials of the plotting Photographic
surveying. The other difficulty lies
III the manipulation of the photo
graphic apparatus itself and the
making of proper negative. This
is by no means the easy thing it
seems, since photographs that would
be highly acceptable to the average
| photographer, professional or anna
teur, might be quite useless for the
purpose of the surveyor. For exam
pie, the surveyor picture, in order
to be of use, must be taken with the
camera in precisely the horizontal
position, fitting never hair
| breadth in one direction or another;
the lens he uses must be an anastig-
mat and absolutely rectilinear, that
the image may not be in the least
distorted, and the plate must be of
a special kind and adjusted in the
camera with degree of nicely of
which the ordinary photographer
has no conception.
Mr.. Deville, who had in charge
the Canadian work, points out in a
practical way how many of the
worst difficulties may be easily over
come. Thus the simple device of
having net suspended between the
legs of the tripod, in which stones
are placed to steady the camera,
would save many negative in
windy weather that now is lost
through the vibration of the instru-
| ment. And the care which is coun-
seled in determining the exact peri
cd of proper exposure for a given
plate, lens and light, even if carried
out with far less scientific precision
than is here suggested, would enable
many an amateur who now wastes
several plates for every good mega
tive to reverse the proportions of
failures and successes.
| One curious feature of the photo
graphic surveying, which the anna
teur would not care to follow ordi-
narily, but knowledge of which
might prove of service on occasion,
Is that the aerial perspective, max
ing the distance hazy, is practically
eliminated by the use of orthochro-
matte plates, which are little affect
ed by the blue rays, combined with
the use of an orange colored screen
in front of the lens. The result, of
course, is picture devoid of atmos-
phere, and hence defective from the
artistic standpoint, but probably
more valuable for the purposes of
the surveyor, who very likely has
taken the picture solely to show the
exact location, altitude and contour
| of a distant mountain.
The use of these methods of re-
stricting the light necessitates very
long exposure, and this in turn
makes requisite the use of plates
coated on the back with a light alb
sorbing material $50 called nonhala-
ton plates), else the light reflected
from the back surface of the plate
would fog the film. Mr.. Deville is
careful to point out that this coating
must be in optical contact with the
surface of the giass-in other words.
| actually painted upon it, the mere
placing of a black cloth or paper
against the back of the plate being
of no service whatever.
When all these technical difficult
ties. of Photographic surveying are
made clear, it no longer seems so
strange that the method has been
but slowly adopted by surveyors,
even In those districts where its adj
vantages would be most marked.
The surveyors who tried it usually
failed to get any available results.
and they did not realize that the
fault lay with themselves and not
with the method. st. Louis Globe.
Democrat.