St. Johnsbury Caledonian. [volume] (St. Johnsbury, Vt.) 1867-1919, 1899-07-12
Photographic Surveying.

Something like $4,000 square miles OF
the mountainous country of western
Canada has been surveyed with a cam
era. Nothing comparable 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 detective knowledge in
descriptive geometry and perspective,
which are essentials of the plotting
pho .ographic surveying. The other
difficulty lies in the manipulation of the
Photographic apparatus itself and the
making OF proper negative. This is by
no means the easy thing it seems, since
photographs that would be highly ac-
ceptabe to the average photographer,
professional or amateur, might be quite
useless for the purpose of the surveyor.
For example, the surveyors picture, in
order to be of use, must be taken with
the camera in precisely the horizontal
position, fitting never hairs breadth in
one direction or another the lens he uses
must be an anastigmat, and absolutely
rectilinear that the image may not be in
the least distorted and the plate must
be of special kind, and adjusted in the
camera with degree OF nicely Of which
the ordinary photographer has no con-
ception.
Mr.. Devile, who had in charge the
Canadian work, points out in a prac-
teal way how many of the worst diff
culties may be easily overcome. Thus
the simple device of having net sus-
pended 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 instrument And
the care which is counseled in determin-
ing the exact period of proper exposure
for given plate, lens, and light, even if
carried out with far less scientific preci-
sion than is here suggested, would enable
many an amateur who now wastes seve-
raj plates for every good negative TO re.
verse the proportions of failures and sue
cesses.
One curious feature of the photo
graphic surveying, which the amateur
would not care to follow ordinarily, but
a knowledge of which might prove OF
service on occasion. is that the aerial
perspective, making the distance hazy,
is practically eliminated by the use OF
orthochromatic plates, which are little
affected 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 atmosphere, and
hence defective from the artistic stand.
point, 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 distant mountain.
The use of these methods OF restricting
the light necessitates very long exposure
and this in turn makes requisite the use
of plates coated on the back with light.
absorbing material (so-called nonhala
ton pates), 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 optica
contact with the surface of the glass; ii
other words. actually painted upon it
the mere placing of black cloth or paper
against the back of the plate being OF nc
service whatever.
When all these technical difficulties o
Photographic surveying are made clear
it no longer seems so strange that thu
method has been but slowly adopted b!
surveyors, even in those districts wher
it's advantages would be most marked
The surveyors who tried it usually faile'
to get any available results, and the,
did not realize that the fault lay witt
themselves and not with the method
TSt. Louis Globe-D democrat.