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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 16 of 33)
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attached to the inner side of the camera-box just behind the
lens mount. The four-inch transit used in conjunction with
this camera had a separate tripod.

With a view toward simplicity in structure and a light weight
to be transported in the mountains, the new Coast and Geodetic
Survey phototopographic instrument has been made into three
distinct parts or sections, the transit, the camera, and one tripod,
serving for both.

The superstructure, embracing the Y's, telescope, and ver-
tical circle, may be lifted off the horizontal circle, to which it
ordinarily is secured by two capstan-head screws, uniting the
base-plate of the Y support with the vernier plate of the hori-
zontal circle. Plate XCVIII shows the transit as used for
trigonometric observations.

The camera, complete as such, may be mounted on the ver-
nier plate of the horizontal circle with the same capstan-head
screws that secure the superstructure of the transit. Plate XCIX
shows the camera-theodolite in its usual position (long sides of
the photographic plate horizontal). The truncated aluminum
cone under the camera-box is secured to the latter by means
of a central clamp-screw (within the hollow cone), and the
base-rim of the cone is then fastened to the vernier plate of the
horizontal circle with the two capstan-head screws already men-
tioned.

Both transit and surveying camera are used on the same
tripod. The understructure (with the horizontal circle) is con-
nected with the tripod by means of the triangular tripod plate
shown in upper part of Plate XCIX. This triangular plate is
screwed to the tripod and the three leveling- screws of the under-
structure are placed on the arms of this plate, a clamp device
securing the conical ends of the leveling- screws to the tripod
plate serves to prevent a possible disturbance of the under-
structure when the exchange from transit to camera is made.



SURVEYING- CAMERAS WITH CONSTANT FOCAL LENGTHS. 199

The adjustments of transit and camera are stable and with
ordinary care will suffer no frequent changes. To reduce weight
aluminum has been used when practicable without sacrificing
rigidity and strength.

The camera is packed in a stout packing-case, together with
eight double plate-holders, focusing-cloth, note-book, etc. The
transit is packed by itself.

A small triangular net or hammock, that may readily be
attached to the legs, should be provided when stations are occu-
pied in windy weather. These light instruments lack stability,
but by placing a suitable weight (a rock will do) in the net sus-
pended between the tripod legs no noticeable vibration will occur.

The inner edge of the rear frame of the inner (aluminum)
camera-box is supplied with notches to mark the horizon and
the principal lines. The constant focal length (about 5 31 /32
inches) of the lens is also laid off on the inner edge of this rim,
one half to either side of the principal line and one half to one
side of the horizon line. All these notches will be printed on
the edges of the negatives, giving ready means for checking
distortions in the prints.

As will be noted, this camera-box is similar in form to Capt.
Deville's new model, having also the same lens (Zeiss anastigmat,
6JX8J, series V). The plates used in this camera are 5X8
inches. The outer box or casing is made of well-seasoned J-inch
mahogany reinforced with strips of brass. The sensitized film
of the photographic plate may be brought into direct contact
with the rear frame of the inner camera-box by means of a milled-
head screw attached to the front board of the outer box with a
counteracting spiral spring similar to G, Figs. 101 and 102,
Plate LI.

The lens is about 2 1 /ie inches from one long side and 3 15 /i6
inches from the other long side of the outer wooden case, making
the horizon line correspondingly nearer one long side of the
camera. When the main field of the terrene falls below the
station the camera is mounted with the lens low and vice versa



200 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Three sets of cross-levels are attached to the inner camera-box,
each being covered with a glass window inserted into the outer
wooden case, so that a set of cross-levels will appear on the upper
camera side for each of the three positions in which the camera
may be mounted. Plate C shows the vertical position of this
camera; it is used when the terrene to be pictured subtends
rather large vertical angles for both elevation and depression.

D. L. P. PaganinVs new Phototopo graphic Instrument for Re-
connaissance Surveys on Scales oj 1 150000 and i : 100000
(Model of 1897).

To overcome the difficulties encountered in the topographic
reconnaissance work (1:100000 scale) in Eritrea (East Africa)
due principally to the torrid climate and in Sardinia (i : 50000
scale) on account of the danger of contracting malarial fevers
L. P. Paganini has devised another surveying- camera, smaller,
more simple in form, and easier in manipulation than the type
just described.

This instrument (model of 1897) combines rapidity in the
field operations with a minimum expenditure of money, and it
materially reduces the period during which the operator has
to be exposed to the vicissitudes of climate and weather. It is
compactly built, essentially light in weight, and preserves the
various adjustments, when once made, almost indefinitely, at
least for a long time if the instrument is carefully handled.

This instrument, together with all accessories, compass,
frame, tripod head, shutter, dark-cloth, etc., may conveniently
be packed into a knapsack to be carried by a single packer, the
entire outfit weighing only about 15 kilogrammes.

This photogrammeter is composed of the following parts:

First. A photographic camera;

Second. A horizontal graduated circle attached to the ver-
tical axis of rotation, with superimposed alidade bearing
verniers and spirit-levels;



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 2OI

Third. An azimuth compass;

Fourth. A tripod with folding legs which may easily be
taken apart.

i. THE PHOTOTOPOGRAPHIC CAMERA PROPER.

The camera of Paganini's latest photogrammeter is rigidly
constructed of aluminum. It has been given a prismatic form
with an equal-sided trapeze for base. The rear side of the
camera- box is formed by a metal frame which supports either
the ground glass or the sensitized photographic dry-plate. The
plates are 18X24 cm. in size and they are exposed with the longer
side horizontally.

Attached to the center of the camera front is a metal collar
or tube into which a tube may be screwed having a thread of
i mm. rise and holding the objective in the outer end. This
objective tube is provided with a flat ring soldered near the
objective end to the tube in such manner to leave a cylindrical
space between the ring and objective tube into which the fixed
collar attached to the front side of the camera may enter
when the objective tube is screwed into the camera collar. The
latter has a millimeter graduation on its outer surface extending
in the direction of the optical axis. The rear (beveled) edge
of the flat ring is divided into ten equal parts, and when the
objective is brought nearer to or farther from the image plane
by revolving the lens tube about its axis the beveled edge of
the flat ring will slowly be moved over the millimeter scale. Fur-
thermore (this beveled edge being divided into ten equal parts),
the position of this circular scale on the beveled edge with refer-
ence to the longitudinal millimeter scale will permit the focal
length to be read to tenths of a millimeter for any position of
the lens.

The objective of this camera is a Zeiss wide-angle anastigmat
with a principal focal length of 182 mm. With a .small dia-
phragm stop it produces a picture of 40 cm. diameter, equivalent
to an angular field of 104. With the diaphragm aperture //35



2O2 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

it covers a plate of 20X26 cm. very well, and as the adopted
size of plate is only 18X24 cm - we obtain a very clear and sharp
definition over the entire plate even when using a larger dia-
phragm opening.

The perspectives obtained with this camera command an
angular field of 67 horizontally and 53 vertically. With six
plates we can, therefore, cover an entire panorama view (of 360)
from which vertical angles of elevation or depression may be

tt
deduced up to ( = 26 30').

Two adjoining plates have a common vertical margin of
3 30' in width. Horizontal angles between points falling within
this panoramic zone or belt of 53 angular width may also be
deduced from the six plates iconometrically.

This instrument has been carefully constructed with optical
axis of the camera perfectly normal to the image plane, and
the point of intersection of optical axis with the image plane
the principal point is photographically transferred to every
photographic perspective as the point of intersection of the pic-
tures of two very fine silver wires crossing each other at right
angles. These wires are secured, quite close to the image plane,
in such manner to be easily removed and replaced by others
in case they should accidentally be ruptured. Directly below
the camera-box, attached to its lower side or base, are three
Z- shaped metal arms with rectangular bends. One is placed
immediately below the objective and the other two in the rear,
below the cross-wires. The lower (horizontal) arms of these
metal Z bars are equidistant from the camera base, and they
are perforated by smooth circular openings which readily receive
three stout screw-bolts securely and permanently attached at
right angles to the alidade of the horizontal circle. Each one
of these bolts has two nuts of which the lower one supports its
corresponding Z-bar arm, while the upper nut has been added
as a locking device to secure the position of the lower nut together
with the corresponding Z-bar arm at any desired height, after



SURVEYING -CAMERAS WITH CONSTANT FOCAL LENGTHS. 203

the proper adjustment of the camera's position on the horizontal
circle has been made.



2. THE HORIZONTAL GRADUATED CIRCLE.

The horizontal circle of this photogrammeter has a diameter
of 14 cm.; it is graduated into half -degrees and reads from o
to 360. The vernier reads to single minutes, but 30 seconds
may be estimated. The vertical axis of rotation of the instru-
ment projects above the plane of the alidade and the latter is
secured in a plane at right angles to the axis of rotation by means
of a stout collar covering the latter.

The instrument is supplied with three leveling-screws, clamp-
and tangent-screws, spirit-levels, etc., the general arrangement
being the same as for any other surveying-theodolite.

The metal tripod head supports a circular spirit-level for
an approximate adjustment into position of the vertical axis,
the final leveling being accomplished by means of the three
leveling-screws, which form the support of the instrument on
the tripod head, together with a pair of cross-levels attached
to the alidade surface.

The superstructure is secured to the tripod head, precluding
an accidental upsetting, by means of a central clamp-screw with
spiral spring and a long handle for an easy manipulation between
the tripod legs, the same as shown for the other Italian photo-
grammeters.

3. THE AZIMUTH COMPASS.

On the upper surface of the camera-box is a cylindrical recep-
tacle of pure aluminum,, to receive the magnetic compass which
is modeled after the so-called Dixey or Schmalkalder pattern,
so well adapted for topographic work. It has the usual pris-
matic eyepiece and vertical hair-sight, which are permanently
fixed in the vertical plane which passes through the optical axis
of the camera objective.



204 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

4. THE TRIPOD.

The tripod of this photogrammeter is practically the same
as has been described for the other Italian phototheodolites,
except that the legs are folding and may be carried together
with the instrument by one packer.

5. ADJUSTMENTS AND USE OF THIS INSTRUMENT.

Before commencing the regular photo topographic survey of a
given area with this instrument, certain observations should
be made which, however, will serve equally well for all sub-
sequently occupied panorama stations, and certain adjustments
should be made or verified which, owing to the stable and solid
construction, will also remain in stabiliment for a long time
thereafter.

Of course, the operator should, after all the adjustments
have once been rnade, check them from time to time, particu-
larly when it may be supposed, or when it is known, that the
apparatus had been subjected to unavoidable shocks or care-
less handling by packers while in transit over rough roads or
trails. With an adjusted instrument and careful handling the
final adjustment that will be necessary to be made before occupy-
ing the camera station for observing reduces itself to a small
matter.

The first step when the phototopographic survey of a given
area is to be made consists in a most accurate determination
of the constant length of the focal distance, which is preferably
done on a bright day in the following manner: The objective
is moved back or forth until some far-distant but well-defined
points of the terrene appear well outlined and quite sharp in
definition on the ground-glass plate under the dark-cloth, using
a small magnifying-glass to ascertain the position of the plate
(the focal length) giving the best definition. It is advisable to
focus in this manner successively upon several well-defined
distant points, reading the scale indications for each point focused



SURVEYING -CAMERAS WITH CONSTANT FOCAL LENGTHS. 205

upon and entering the records in the field-book. We will thus
obtain a definite value for the principal focal length, as indi-
cated by the mean of the recorded scale- readings.

We next determine the diaphragm aperture which is to be
used for the entire panorama, selecting the smallest stop that
will yet give a uniformly good definition for the entire field con-
trolled by the plate 60 in azimuth and 45 in altitude having
special reference to a good definition of the summits of distant
mountains that may be shown on the perspective.

The adopted diaphragm aperture the instrument in ques-
tion has a revolvable diaphragm disc with numbered apertures
of graduated sizes at different hours of the day, at different
altitudes, and under different illumination (" light- intensity ")
will only in part control the length of exposure, as the latter also
depends in great measure on the rapidity (" sensitometer num-
ber ") of the plate that may have been selected for use in the
survey, and on the so-called rapidity of the lens, together with
the color screen, if such is used.

A systematic exposure of a few trial plates under different
conditions regarding hour of the day, elevation of station above
sea horizon, and illumination lens, diaphragm aperture, and
plate remaining the same will give valuable data for future
reference and for judging the required length of exposure cor-
rectly under similar conditions.

One indispensable condition to be fulfilled when proceeding
to make the final adjustments of this instrument would be to
make a complete turn of the camera about the vertical axis,
very much the same as when exposing the plates for a complete
panorama. If the instrument is well assembled with all its
parts in rigid adjustment, and if the tripod is securely placed in
position on firm ground and no disturbing causes interfere, the
instrument should, while being thus rotated, maintain its axis
(and its image plane) perfectly vertical.

To realize this condition the cross- levels attached to the
upper surface of the alidade should be well adjusted. This



206 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

may be accomplished by means of the three leveling-screws
which transverse the tripod head and form the support of the
superstructure, in precisely the same manner as such adjust-
ment would be made with an ordinary surveying-theodolite.

After establishment of the verticality of the axis of rotation
there still remains the adjustment into position of the rigid system
of the three orthogonal axes, which have their common origin-
point of intersection in the principal point of the perspective.
This system of coordinates is composed of:

First. The optical axis of the photographic camera, repre-
senting the principal ray of the photographic perspective.

Second. The two axes intersecting each other at right angles
one horizontal and the other vertical which are visible
on the ground glass and which are photographically trans-
ferred to the photographic perspective, where they repre-
sent the horizon and the principal line respectively.

The rectangular intersection of these two lines (last named)
is perfectly obtained by the instrument-maker. Two very fine
silver wires are strung quite close to the image plane, where
they are held in position by thin metal plates, the required ten-
sion being applied by means of pressure-screws. The correct
position of these cross-wires with regard to the rear metal frame
of the camera-box is assured by four fine incisions made with a
dividing-machine into the rear metal surface of the frame against
which the ground-glass plate or the sensitized photographic
plate rests when they are in position.

The degree of accuracy with which this rigid system of axes
is placed into position (and the degree of accuracy in their direc-
tions regarding horizontality and verticality) in a great measure
controls or determines the attainable accuracy in the iconometric
plotting, based upon pictured points referred to that system
of lines as coordinates.

The optical axis of the camera, which by construction inter-
sects the perspective plane the image plane at right angles
in the principal point, should be horizontal, and therefore the



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 2O/

optical axis should intersect the plane containing the two axes
of coordinates (the cross- wires) also at right angles. It is fur-
thermore required that the wire representing the horizon line
of the perspective be horizontal, or, in other words, the optical
axis of the camera and the horizon line of the perspective are
to be in the same horizontal plane that is, in the horizon plane
of the camera station.

This condition may easily be fulfilled by using the screws
which support the three Z bars as three leveling-screws. The
three lower arms of these Z bars are first placed approximately
at equal altitudes above the plane of the alidade circle (which
previously had been leveled) by means of the supporting nuts,
having first loosened the upper counter nuts to permit the Z-bar
arms to move freely over the upright screw-bolts. Now observe
the image on the ground-glass plate and bisect a distant point
with the pictured intersection of the cross-wires. Change the
pointings to the distant bisected point to different positions along
the line defining the horizon by revolving the camera in azimuth
from left to right, or vice versa, at the same time raising or lower-
ing the screws which support the rear Z-bar arms of the camera,
until a point is found bisected by the intersection of the cross-
wires, which, when turning the camera about the axis of rotation,
does not leave the horizon wire, rather continues to be bisected
by that wire while being moved over it from one extremity to-
the other, falling neither above nor below that wire, during the
full azimuthal swing through the entire length of the ground
glass.

If this distant bisected point was not in the horizon plane
of the camera-theodolite, the curve which it describes on the
ground -glass surface during the revolution of the camera through:
an azimuthal field of 60 will be traceable with the eye, the
point while in transit will pass over the vertical wire above
or below the point of intersection of the two cross- wires, according
to its position in nature, whether below or above the horizon
plane of the instrument.



208 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The farther removed from the intersection of the cross-wires
the crossing of the pictured distant point over the vertical wire
is the greater will be the curvature of its hyperbolic trace on
the ground -glass plate and the greater will be the vertical dis-
tance between the bisected point and the horizon plane.

From the preceding remarks it will be evident that the Z bars
supporting the camera in the rear may be adjusted by simply
watching the courses of points of different elevation as they are
traced (by the points while in transit) on the ground glass during
the azimuthal swing of the camera. If the distant point, having
the same elevation as the optical axis of the camera, leaves the
horizon wire and passes over the vertical wire above the inter-
section of both, the supporting Z bars (those under the rear
frame) are to be lowered and vice versa. The distance between
the point of crossing and the point of intersection of the cross-
wires gives a measure for the amount of change to be made in
the elevation of the rear Z-bar supports.

After a few carefully made trials, the position of the horizon
wire will be such that any point bisected at one extremity of
the horizon wire will continue to be bisected by that wire during
the revolution of the camera through a horizontal field of 60.
When this has been accomplished the counter-screws of the
upright screw-bolts are tightened to secure (the Z bars) the
camera in this position with reference to the horizontal circle.

This adjustment once made and secured will be maintained
undisturbed for a long time and the horizon wire will now coin-
cide with the horizon line of the photographic perspective for
all positions of the optical axis, when the camera is rotated about
its axis during the exposures of the plates comprising a pano-
rama, provided, of course, the phototheodolite's axis of rotation
has remained vertical.

The final adjustments to be made in the field after the
phototheodolite has been placed in position at a carefully
selected phototopo graphic station may now be summed up as
follows :



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 209

First. Adjust the camera's objective to make the principal
focal length, previously ascertained and recorded in the
field-book, agree with the reading on the " objective
scale."

Second. Verify the position of the rotary diaphragm in order to
have the particular aperture in position that may be required
for the particular brand of plate under the particular
conditions of the atmosphere at the place occupied. Also
verify the setting of the time-shutter to see that the expos-
ure will be correct for this particular diaphragm aperture,
plate, illumination, altitude, and subject, although the
latter plays a minor role here, as it seldom varies much in
phototopography.

Third. Verify the level adjustments and see that the axis

of revolution of the instrument is vertical, as the hori-

zontality of the optical axis, the verticality of the image

plane and of the " principal " wire, as well as the hori-

zontality of the " horizon " wire, depend thereon.

The preceding descriptions show that much has been done

in Italy towards pushing photographic surveying to a high state

of perfection, and we are particularly indebted to Paganini for

numerous improvements in phototopographic and iconometric

instruments, including methods of their use for topographic and

hydrographic surveys. Paganini's good results, his experience,.

and advice have materially aided in the decision in favor of the

phototopographic method in a number of surveys, particularly

for the survey of the mountains of the Caucasus (and Tiflis)

under the direction of Baron von Steinern.

m. Surveying-cameras Combined with Geodetic Instruments.
(Phototheodolites, Phototachymeters, Photographic Plane
Tables, etc.)

The data obtained in the field with the photographic-sur-
veying cameras considered in the preceding paragraphs had to



210 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

be supplemented with instrumental field observations to gain a
complete topographic control of the territory traversed by the
phototopographic surveying party.

The idea of combining surveying instruments with a camera
into a compact and serviceable apparatus originated very early
with phototopographic workers. Refined phototheodolites are
to this day the favorite photographic-surveying instruments not
only in Europe, but they are also widely in use in other countries,



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 16 of 33)