The Providence news. (Providence [R.I.]) 1891-1906, 1898-02-25
PHOTOGRAPHIC SURVEYING.
One OF the Most Important Uses to
Which the Camera Has Been Put.


which the Camera Has Been Put.
Something like 90. OcO square miles of
the mountaincIs country of western
Nanada has been surveyed with the
camera. Nothing comparable to this in
TIE way Of Photographic surveying has
been accomplished on any such scale
anywhere else in the world.
The cans Of failure on the part of
many VINO hnve tried tina methcd and
discarded 1t In disgust are chiefly two.
One OF tn. SO is u defective knowledgs in
descriptive geometry and perspective,
which are essentials of the plotting photo
graphic surveying. The other difficulty
lies ~ the manipulation OF the photo
graphic apparatus itself. and the max.
ing OF proper negative. Ihis IL by no
means the easy thing IL seems. since
photographs that would be highly ac-
ceptable tn the average photographer,
professional or amateur might he quite
useless for the purpose Of the surveyor
For example. the surveyors picture, in
order To be of use, must tie taken with
the camera in precisely the horizontal
positioD, tilting never hairs breadth
in one direction or another the lens lie
uses must be an anastigmat, and abso-
lutely rectilinear, that the image may
not be in the least distorted. and the
plate must be of special kind. and ad-
justed in the camera with dszras of
nicely of which tbe!ordinsry photographer
has no conception.
Mr. Deville, who had in charge tie
Canadian work, points ont In a practical
way how many OF the worst difficulties
may be ea,i y overcome. Thus the
simple device Of having net suspended
between the legs of the tripod, i'm 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 oounseled in determining
b, exact period of proper exposure for
given plate, lens and light. even II car
ried cut with far less scientific precision
than is here suggested, would enable
many An amateur who now wastes
several plates for every good negative to
reverse the proportions OF failures and
successes.
The curious feature of the Photographic
surveying which the amateur would
not care to follow ordinarily, but
knowledge of which might prove Of ser
wee on occasion Is that tic aerial per-
spective, making the distance hazy, is
eliminated by the use Of
orthochromatio plates, which are littlc
affected by the blue rays, com bined with
the use Of an ornnae-col, red screen in
front Of the lens. The result, of course.
Is picture devoid OF atmosphere. and
hence defective from the artlstic stand
point. but probably more valuable for
the purposes Of tho surveyor, who very
likely has taken the picture solely to
show the exact location, altitude and
contour of distant mountain.
IRE use of these methods of restricting
the light necessitatss very long exposure.
and this in turn makes requisite the use
OF plates coated on the back with n light
absorbing material (so-talled nonholstion
plates), else the light reflected from the
back surface of the plats would fog the
film. Mr. Dsville is careful TO point out
that this coating must be In optical con-
tact with the surface Of the glass; ia
other words, actually painted upon 1t,
the mere placing Of a black cloth or parer
against the back Of the plate bSing of no
service Whatever.
when all these technical difficulties Of
photographic Surveying are made clear,
it no longer seems 20 strange that the
method has teen but s.owly adopted by
surveyors even in thoss districts where
its advar tnges wouli te most marked.
The surveyors who tried II usually
failed TO get any available results, and
they did not realize that the fault iav
with themselves ard not with the
method st Louis Globa. Democrat