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John Adolphus Flemer.

An elementary treatise on phototopographic methods and instruments, including a concise review of executed phototopographic surveys and of publicatins on this subject online

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Online LibraryJohn Adolphus FlemerAn elementary treatise on phototopographic methods and instruments, including a concise review of executed phototopographic surveys and of publicatins on this subject → online text (page 15 of 33)
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II. Special Surveying-cameras with Constant Focal Lengths.

Among the numerous patterns of this class of instruments
that devised by Dr. Meydenbaur is probably the earliest form.

A. Dr. Meydenbaur 1 s new small Magazine Camera.

This instrument is represented in Figs. 97 and 98, Plate
XL VIII. The camera weighs 750 grams and the plates are
9X12 centimeters in size. The camera- box is mounted, by
means of ball-and-socket joint, upon a vertical rod which is joined
at the bottom to three short legs in such a way that the four
pieces may be folded together to form a stout cane 0.85 m. long.
The lower ends of the three legs of this tripod and the upper
end of the supporting staff are connected by twisted violin-strings.
Tension may be given them by turning the ratchet-wheels indi-
cated in Fig. 98, Plate XL VIII, thus producing a very light
and yet rigid tripod. The plates contained in the magazine M



1 86 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

are exposed successively by being pressed against a metal frame
having marks indicating the principal and horizon lines. This
frame is securely fastened at a constant distance from the lens,
establishing the constant focal length of the camera. Each
plate, after exposure, may be dropped into the leather pouch
b, Fig. 98, Plate XL VIII, secured underneath the camera, by
manipulating a button a'. The leather pouch, together with
one dozen plates, weighs about 500 grams. Dr. Meydenbaur
has used a pantoscopic lens, made by E. Busch in Rathenow,
Prussia, which is said to produce geometrically correct per-
spectives of an angular field of 105. A circular level L serves
to adjust the plates into vertical plane before exposure, using
the ball-and-socket joint for this purpose.



B. Capt. E. Devilled Surveying-camera (new Model).

The following description of Capt. E. Deville's new survey-
ing-camera is taken from Deville's " Photographic Surveying,"
second edition (1895). The camera is shown in Fig. 99, Plate
XLIX, and Fig. 100, Plate L; Figs. 101 and 102, Plate LI,
represent sections of the instrument.

The camera proper is a rectangular metal (aluminum) box
AB, Figs. 101 and 102, Plate LI, open at one end. It carries
the lens L and two sets of cross-levels CC, which may be read
through openings in the outer mahogany casing. The metal
box is supported in the wooden casing by wooden blocks and
by a wooden frame FF\ held in position by two bolts DD.

The plate-holder is made for single plates; it is inserted
into the carrier EE, which may be moved forward and back-
ward by means of the thumb-screw G.

A folding shade, of wood, HH, Figs. 101 and 102, Plate LI,
hooked to the front of the camera, and diaphragms KK inside
of the metal box, intercept all light- rays which do not contribute
to the formation of the image on the photographic plate.



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 187

The camera rests on a triangular base, Fig. 103, Plate LI,
with foot-screws, in shape exactly like the base of the transit
theodolite which is used in conjunction with Deville's survey-
ing-camera, so that either instrument may be placed on the same
tripod at any station. The camera may be set up on the tri-
pod with either the long or the short side vertical.

Both transit and tripod are carried by the surveyor, and one
camera including one dozen plates (in the single holders) with-
out a tripod are taken by one of the men who always accom-
pany the surveyor as packers. The assistant has a second
camera with plates and a separate tripod.

The legs of these tripods when folded together are twenty
inches long. They are placed in separate cases and one is carried
under the box of the transit, to be carried on the back of the sur-
veyor, and the other is attached to the sole-leather case of the
camera iri charge of the assistant surveyor.

The lens of this camera is a Zeiss anastigmat, No. 3 of series V,
141 mm. focal length, with a deep-orange color-screen in front.

Having mounted the camera on the tripod, the plate-holder
carrier E is moved back as far as it will go by turning the screw G,
Figs. 101 and 102, Plate LI; the plate-holder is inserted through
the opening M, the slide is withdrawn, and the carrier moved
forward by revolving the screw G until the plate is brought into
contact with the back of the metal box AB. In order to secure
a perfect contact, the carrier E has a certain amount of free
motion. The camera should now be turned in the proper direc-
tion; the field embraced by the plate is indicated by lines drawn
on the outside of the mahogany casing. The camera is then
carefully leveled, the exposure given, and Ihe plate-holder is
withdrawn by repeating the same operations in the inverse order
as described for its insertion.

For the sake of rigidity and to reduce the number of adjust-
ments to be made in the field to a minimum, the levels CC have
been fixed rigidly to the metal camera-box (without any device
for subseqeunt adjustment). They are, however, very closely



1 88 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

adjusted by the maker of the instrument. For this purpose he
takes the metal camera-box out of the mahogany casing and
deposits the same on a piece of plate glass which previously
had been leveled like an artificial horizon. By filing down
one end or the other of the levels' outer case, each bubble is
brought as near to the middle of its tube as possible. The
graduation on the latter is numbered continuously from end to
end, as illustrated in Fig. 104, Plate LI.

Each camera is accompanied by a piece of plate glass } inch
thick and n inches long, which can be inserted into the carrier
in place of the plate-holder. That end of the glass plate which
projects beyond the camera limits when it is thus inserted is
coated on the back with a varnish composed of gum guaiacum
dissolved in alcohol to which some lampblack had been added.
This coating has very nearly the same refractive index as glass
and is well adapted for precluding all reflections from the back
of the glass plate.

After the camera has been received from the maker the exact
readings of the levels, when the back of the metal box against
which the photographic plate is pressed is vertical, should
be ascertained. To do this, the bolts P, Fig. 102, Plate LI,
next to the opening M, are loosened and. removed. Q may
then slide backwards and be taken out. The piece of plate
glass is now inserted in the carrier E, Figs. 101 and 102, Plate LI,
and pressed into contact with the metal box. The camera is
placed on its tripod and leveled. Immediately in front and
at the same height a transit (or a leveling instrument) T, Fig. 105,
Plate LI, is set up, and after carefully adjusting it, a distant
point P is selected on the same level with the transit and camera.
The intersection of the cross-threads in the telescope is brought
to coincide with P, and the telescope is clamped to the vertical
circle. Turning it around in azimuth, the image of P, reflected
by the plate glass, should appear in the intersection of the tele-
scope's cross-threads. If it does, the face of the plate glass is
vertical and the position of the bubble in the tube of the level,



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 189

directed at right angles to the plate glass, is the correct one for
adjusting the instrument in the future. If it does not, the camera
must be tilted forward or backward by means of the foot-screws
until coincidence is established. The middle of the bubble
of the level may or may not now be in the middle of the tube,
but its position, whatever it is, will be the correct one for adjust-
ing the camera in its subsequent use. The divisions of the
graduation between which the bubble rests should therefore
be ascertained and the middle reading be recorded, and when-
ever the camera is to be leveled, it must be remembered that
the middle of the bubble is to coincide with the recorded middle
reading.

This determination of the level-reading is to be made for
the two positions of the camera in which it is used (horizontal
and vertical position).

i. DETERMINATION OF THE FOCAL LENGTH, THE HORIZON LINE, AND THE

PRINCIPAL LINE.

The next step is to fix the position of the principal point
on the photographic plate and to ascertain the length of the
distance line. Select a station so that a series of distinct and
well-defined distant points may be found on the horizon line
as it is laid down by the maker of the camera. The selected
view may comprise the distant shore line of a lake, a large build-
ing or a row of buildings. Set up the tripod and adjust the transit.
Find two points E and F, Fig. 106, Plate LII, on the horizon
line (with a zenith distance of 90) that both come within the
field of the camera, when set horizontal, and fall near the two
vertical edges of the plate. Measure the horizontal angle cu
between them. Find two other points G and H, also on the
horizon line and such distance apart that both come within the
field of the camera when the same is set up vertical. Now replace
the transit by the camera in the horizontal position and turn
it so that E and F will appear within the limits of the plate, level



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

carefully, and expose the plate. Set the camera in the vertica.
position and turn it in azimuth to take in G and H, level care-
fully, and expose another plate.

The first plate, after development, shows the two points
E and F on a line very nearly parallel to the edges AB and CD,
Fig. 1 06, Plate LII, of the metal box. The principal point, of
course, will be on this line, which is cut into the film, using a
fine needle point and straight-edge for this purpose.

The second plate, exposed in the vertical position of the
camera, will give another horizon line GH, which may be trans-
ferred to the first plate by means of the distances AK and CL
to the corners of the metal box. This (principal) line is like-
wise cut through the film of the photographic plate with a fine
needle point and straight-edge. The principal point P will be
at the intersection of these two horizon lines EF and GH.

The length of the distance line (5P = /), or the focal length
of the camera, may be computed from the horizontal angle w y
included between SE and SF, together with the distances EP = a
and PF = b.

Let 5, Fig. 107, Plate LII, be the second nodal point of the
camera-lens, a and ft the angles ESP and PSF, when



The lengths of a and b are measured directly on the plate.
If we designate the focal length PS by / we have:



a

-r,



b
tan/?=y,



ab

tan a X tan p = -r.



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 19 r

Hence



tan ^a+ / = tan CD
a b



ab



1 tana/
Resolving this adfected quadratic equation we find

a+b \(a + b} 2

j = " + \J : 9 +<*>

2 tan co ^4 tan 2 to

Having now found the focal length and the principal point,
reference marks should be made on the edges of the metal box
to indicate the horizon and principal lines as well as the focal
length on the prints from the negatives.

Measure the distance m, Fig. 106, Plate LIT, from P to AC*
From the corresponding corners A and C, Fig. 108, Plate LII,
of the metal box lay out m on AR and on CT. With a very
fine and sharp file, held in the direction toward the lens, cut
into the edge forming the rear frame of the metal box a clean
and sharp notch at T and another at R.

Repeat the same operation from the corners A and B with the
distance n from P to AB.

The lines OQ and RT will be the horizon and principal lines
of the -photographs, when the camera has been leveled to bring
the bubble into its proper position as mentioned in the foregoing.

From R and T measure the distances Rr, Rr*, Tt, Ttf, equal

to one half of the focal length ( = ) From O and Q measure

Oo, Oo f , Qq, Qq', equal to one quarter of the focal length, and
at each one of these points make another notch with the file



IQ2 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

held in the direction of the lens. Every photograph will now
show twelve triangular projections reaching into the dark border
of the photograph. Four of these projections serve to fix the
horizon and principal lines; the remaining eight give the focal-
length value.

2. ADJUSTMENT OF CAMERA SPIRIT-LEVELS.

It now remains necessary to find the correct readings of the
transverse levels (placed parallel with the sensitive plate) when
the horizon and principal lines pass exactly through their cor-
responding notches of the metal box.

Set up the camera again, facing the same distant view as
before, but in adjusting it bring the bubble of the transverse
level near one end of the tube; note the reading of the level-
tube graduation and expose a plate. When developed, it will
give a horizon line EF, Fig. 109, Plate LII, cutting the border
of the negative in A and 5, at some distance from the pictured
notches O and Q. Now change the adjustment of the camera
by bringing the bubble of the transverse level to the other end
of the tube, note the reading of the level and expose another
plate. This when developed will give another horizon line
N'F f , cutting the border of the negative in C and D.

Great care should be exercised in both cases to keep the
other level (the one at right angles to the sensitive plate) at its
proper reading, in order to expose both plates while in vertical
plan.

After measuring CO and OA or BQ and QD, a simple pro-
portion gives. the proper reading of the transverse level which
will bring the horizon line of the vertically exposed plate through
the two notches O and Q of the metal box.

The correct reading of the transverse level of the second
set of levels is found by the same method, with the camera in
the vertical position.

All these operations must be executed with great care and
precision (and with the help of a microscope of moderate power),



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 193

as the subsequent iconometric plotting of pictured points is
based upon the determination of the ordinates and abscissae of
such points, on the pictures, with reference to the principal and
horizon lines which serve as a system of rectangular coordinates.
It had been assumed that the levels were placed very nearly
in correct adjustment by the maker as mentioned before. If
found too much out, they must, of course, first be approximately
adjusted by setting the metal box on a well-leveled plate. For
this purpose the plate glass supplied with each camera may be
set on the camera base and leveled like an artificial horizon.

3. USE OF THE INSTRUMENTS COMPRISED IN THE CANADIAN
PHOTOTOPOGRAPHIC OUTFIT.

The instruments and tripod of the Canadian instrumental
outfit being very light, steadiness may be secured by means of
a net suspended between the tripod legs, into which a heavy
weight (rock) is placed. With this device, photographs of good
definition and better observations may be obtained than with-
out it, and there is no risk of the instruments (secured to the
tripod) being blown over during one of the sudden and strong
gusts of wind so frequently encountered on elevated and exposed
mountain peaks.

After the phototopographer has arrived at a triangulation
station he adjusts the transit and observes the azimuths and zenith
distances of all signals (marking the triangulation points and
camera stations already occupied) in the vicinity that may be
visible from his position. If accompanied by his assistant,,
each reads one vernier and records the readings independently
of the other into separate record books. After the observa-
tions at that station have been completed the two surveyors
compare notes and any discrepancy that may be discovered in
the recorded data is corrected on the spot by a careful repetition
of the doubtful observations.

The Canadian camera is carried in a sole-leather case which
also contains twelve filled double plate-holders; when more



194 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

holders are needed for a day's work they must be carried in a
separate receptacle. Taking the camera from its case the level-
ing base, Fig. 103, Plate LI, is secured to it by means of the
central clamp-screw, and the camera is then placed upon the
tripod (from which the transit had been removed) without dis-
turbing the position of the latter.

The shade or hood H, Fig. 99, Plate XLIX, is now unfolded
and attached to the hooks at the front of the camera. A plate-
holder is inserted into the carrier and the number of the plate,
in position to be exposed, is noted upon a rough outline sketch
of the panorama views (commanded by the field of the camera
image as indicated by the converging lines cut into the exterior
sides of the camera-box), entering also such notes as may be
of value for the subsequent development of the plate, for the
iconometric plotting of the topography photographed upon the
latter, or for the lettering of the finished map.

Having made sure that the cap is secure on the lens, the
slide is withdrawn from the plate-holder and the sensitized sur-
face of the plate is brought into direct contact with the frame of
the metal box by turning the screw G, Figs. 101 and 102, Plate LI,
devised for this purpose. The surveyor next turns the camera
in azimuth until the lines on the upper face of the wooden casing
show that it is properly directed, or oriented, to include the
panorama section to be photographed between the lines. The
field of view should, of course, in each case coincide with the
outline sketch bearing the number of the plate in position to
be exposed. Sighting along the lines (up and down) shown
on the side face of the wooden camera casing, the observer can
assure himself whether the view on the image plate reaches high
or low enough to control the landscape ; if it does not, he either
puts the longer dimension of the camera upright, or, if already
in that position, he may have to occupy a secondary camera
station, either above or below the one occupied, as the . case
may be.

The observer next levels the camera carefully in the manner



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 1 95

previously described and exposes the plate. Whenever the
sunlight appears inside of the front hood, the latter should be
shaded off during the exposure of the plate by holding some
object (plate-holder slide or hat) above the hood. Under no
circumstances must the sun be allowed to shine upon the lens.

Every evening after the return to camp the surveyor replaces
the exposed plates (in the dark-tent) by new ones, using a ruby-
colored light for this purpose. He also marks the exposed plates
in one corner close to the margin, before removal from the holder,
with his initials, with the number of the dozen and number of
plate, using a soft-lead pencil for this purpose. N. N. IV 5,
for instance, means plate No. 5 of the fourth dozen and exposed
by N. N. This would be the forty-first plate exposed in that
season.

The exposed plates are placed into a double tin or copper
box, Fig. no, Plate LII, which can be closed hermetically and
which will float when filled with two dozen plates (should it
be accidentally thrown into water by the capsizing of a canoe).
These boxes, as soon as filled, are shipped to the head office
in Ottawa, where the plates are developed by a specialist.

The data obtained by aid of the transit theodolite for tri-
angulation purposes are recorded in the field-books in the man-
ner customary for such work.

The horizontal angles observed with the transit (or altazi-
muth) to the terrene points (so-called " reference points ")
marked on the outline sketch, which should accompany each
negative, serve not only for the orientation of the horizontal
projection of the plate on the working-plan to " orient " the
so-called " picture trace " but they also aid materially to ascer-
tain the amount of (and to counteract in a measure) the dis-
tortion of the paper prints or photographic enlargements. The
vertical angles, together with the plotted distances to such refer-
ence points, serve to check and verify the position of the horizon
line on the different photographs as given by the camera alone.

Important camera stations are occupied by the surveyor,



196 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

secondary stations by the assistant with a separate camera. No
trigonometrical observations are made by the assistant when
occupying secondary and tertiary camera stations, as he is not
supplied with a transit. The surveyor locates such stations by
observing upon the signals, erected by the assistant before leav-
ing the station, from his own stations, and he subsequently com-
putes their positions .. as "concluded points." All views are
taken with the same stop,



C. The U. S. Coast and Geodetic Survey Phototopo graphic Cameras,

In the preceding we have already referred to the desirability
that phototopographic surveying instruments designed for use
in rough mountain districts^ where transportation facilities are
restricted and generally confined to portage over rough trails
(or to transportation up steep mountain sides on the backs of
packers), should be made as simple as possible to indefinitely
remain in perfect adjustment.

The phototopographic party generally reaches the mountain
station, after an exerting climb of several hours, in a more or
less fatigued condition, and to obtain the best results the observer
should not be required to spend much time in assembling and
adjusting the instrument before the actual survey work may be
begun. Then, too, the atmospheric conditions are rarely stable
for any length of time, making it most desirable to utilize favor-
able conditions at once. The fields of labor in S.E. Alaska, where
the Coast and Geodetic Survey camera work has been done,
are peculiarly well adapted for the application of the phototopo-
graphic methods, on account of the prevailing cloudy condition
of the atmosphere in the higher altitudes. Distant peaks and
mountain groups, without apparent warning, become suddenly
shrouded in drifting mists, which soon gather into a thick cloud
stratum, shutting the mountains out from view for days and
weeks at a time. During the summer months the prevailing
southerly, vapor-laden winds drifting inland from the Pacific
Ocean find their moisture condensed on approaching the snow-



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 197

and ice-fields or hanging glaciers in the higher altitudes. Clear
days are generally accompanied by calms or they occur when a
northerly drift in the air-currents prevails.

Phototheodolites or instruments in which the elements of a
transit and a camera are assembled into a single apparatus, all
parts being merged into a composite instrument, to remain
united during the various operations of observing and plate
exposures, mostly have the objectionable feature of unstable
adjustments, requiring frequent tests and "readjustments of their
component parts. They are, furthermore, more or less cumber-
some and heavy, making them more liable in transportation
than instruments that may be divided into two or three parts
(each section being complete in itself) and carried by two or
three helpers.

The original type of the U. S. Coast and Geodetic Survey
camera, used in connection with the topographic reconnaissance
made in S.E. Alaska under the U. S. Alaskan Boundary Com-
missioner, was similar in form to Capt. Deville's original survey-
ing camera, except that it was provided with a separate tripod
with ball-and-socket adjustment and that the teeth or index
marks which serve to fix the principal and horizon lines on the
negatives could be pressed into direct contact with the sen-
sitized film of the photographic plate simply by turning a button
after withdrawal of the slide. This camera was provided with
a ground glass, enabling the observer to inspect the entire field
controlled by each plate before exposure was made.

The camera proper was a plain rectangular box, made of
well-seasoned mahogany, 6jX6fX9J inches in size. It was
always used in the same position, with the short faces vertical.
A circular level attached to the upper camera side served for
leveling the instrument, bringing the photographic plate into
vertical plane.

The bamboo tripod legs were made in three sections, each
sixteen inches long and screwed together at the joints. When
dismembered, the tripod was carried in a sole-leather case or



198 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

knapsack, together with the camera, six double plate-holders,
note- book, barometer, etc. . A yellow color screen could be



Online LibraryJohn Adolphus FlemerAn elementary treatise on phototopographic methods and instruments, including a concise review of executed phototopographic surveys and of publicatins on this subject → online text (page 15 of 33)