The sun. [volume] (New York [N.Y.]) 1833-1916, 1898-02-20
PHoToGRAPH1o SURVEYING,
Said to he Ono or the Nest Important Useo to
which the Camera Has Been Put.
Stands,

From the 81. Louis Globe. Democrat.
Something like $4,000 square miles of the
mountainous country of western Canada has
been surveyed with the camera. Nothing COM
parable to this in the way of Photographic sur
veying 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 dis.
gust are chiefly two. One of these is q defective
knowledge in descriptive geometry and per-
spective. which are essentials of the plotting
photographic surveying. The other difficulty
apparatus itself and the making OF u proper
negative. This Is by no means tho easy
h hIphs y a seeI,s, h snnt &c Photographs that would DO
professional or amateur. might be quite useless
ior the purpose OF the surveyor. For example.
the surveyors picture, in order to DO of use.
must be taken with the camera in precisely the
horizontal position. fitting never a hairs breadth
in one direction or another: the lens he uses
must be an anastigmat. and absolutely ~rectilin.
ear. that the image may not be in the least dis
torted; ana TIE Plate must be Of q special kline,
nicely OF which tho ordinary photographer has
MIR. Deviilc, who had in charge the Canadian
work. points out in a practical way how many
OF the worst difficulties may be easily overcome.
between the legs OF the tripod. in which stones
are placed to steady the camera, would save
many a negative in windy weather that now is
lost through the vibration of the instrument.
And tho care which Is counseled in determining
the exact period of proper exposure for a given
plate. lens, aud light, even II carried out with
far less scientific precision than is here sug-
gcsted, would enable many an amateur who now
wastes several plates for every good negative
to reverse the proportions OF failures and sue
CesseB.
One curious feature of the Photographic sur
veying. when 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 tho distance hazy. is
practically eliminated by the use OF orthochro-
matte plates. which are little affected by the
blue rays, combined with tho use of an orange
colored screen in front OF the lens. The result,
OF course, Ia q picture devoid OF atmosphere, and
hence defective from tho artistic standpoint,
but p~robpbly moro valuable for tho purposes OF
the surveyor. who very likely has taken the
picture solely to show tho exact location. alti-
tude. and contour of a 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 a lightabsorbing material (so-
called nonhalation platesl. else the light reflect
ed from tho 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 glass; in other words. actually
painted upon IL, the mere placing or a black
cloth or paper against tho back OF the plate
being OF no service whatever.
When all these technical difficulties OF photo
graphic surveying are made clear, IL no longer
seems so strange that the method has been but
slowly adopted by surveyors. even In those dis
tricts whore Its advantages 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 tho method.