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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

. (page 17 of 33)
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 17 of 33)
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particularly when these are dependent on European mechanicians
to supply the demand for instruments of precision.

Phototheodolites have been devised to secure great precision
in the results obtained with them; refined methods are largely
employed in the field observations in the culling of data from
the photographic perspectives, and in the computations.

Generally speaking, the best results for topographic pur-
poses are obtained by methods that have been devised with
due reference to the fact that phototopography essentially
and primarily is a constructive or graphic art, based on graphic
or pictorial records, in the form of perspectives, that are to
be transposed into orthogonal projections in horizontal plan,
instrumental observations being required only to furnish such
elements as may be needed to facilitate the graphic trans-
position of lines of direction and distances, to insure accu-
racy, and to obtain certain checks or a proper control for the
work in its entirety.

It has already been pointed out in the preceding chapters
that phototopography is based essentially on the same methods
which are followed in topographic plane-table surveys, and the
best results may be expected when the surveying-camera is used
with the same object in view which the plane-tabler essays to
obtain. To increase the degree of precision the plane-tabler
will occupy a greater number of stations, and similarly in photo-
topography any degree of accuracy may be attained by increasing
the number of camera stations for any given area.

Photographic surveys have been conducted principally in



SURVEYING-CAMERAS. AND GEODETIC INSTRUMENTS. 21 1

regions where other surveying methods are either precluded or
where their application would entail great cost and consume
too much time and such regions are characterized chiefly by
a rugged and broken topography. The necessity, therefore,
lies close at hand to devise instruments which will not easily get
out of order or drop to pieces when transported over rugged
mountain trails; the more simple their structural composition
the better adapted they will be for the production of rapid and
accurate work.

In Europe phototopography has generally been employed
for surveys plotted on large scales, necessitating the occupation
of numerous stations, with a resulting slow progress from one
locality to another. Then too the instrumental outfit could
readily be brought very near, if not actually to the very place,
where the work was to be done, by convenient and safe means of
transportation. The instruments are very seldom exposed to such
primitive and rough " methods of transportation over long dis-
tances, as generally has been the case on our continent when
surveying- cameras have been used.

It is evident that the combination of a camera and a surveying-
instrument into a well-united, well-balanced, easily manipulated,
and essentially light and withal rigid instrument is not easily
accomplished. It is not surprising, therefore, that we meet
with a great number of types of phototheodolites and other photo-
grammeters in which the difficulties in construction have been
overcome, more or less successfully, by various devices. In
the following we shall describe the principal types of photo-
graphing-surveying instruments that are either of historical
interest or are in use at this date.



A. L. P. PaganinVs Photo grammetric Instrument (Model o] 1884).

The Italian photogrammetric apparatus devised by L. P.
Paganini, model of 1884, is illustrated in Figs, in and 112,
Plates LIII and LIV. It is supported by a tripod which may



212 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

be dismembered into the tripod head H and three " alpenstocks "
A. The instrument proper may be separated into two parts,
the camera-box C and the Y supports with eccentrically located
telescope T.

S', S f , S' indicate the 3-foot screws only two are visible in
the illustrations on Plates LIII and LIV which form
parts of the tripod head H and which serve to level
the theodolite.

Sit $2> $3 represent three leveling-screws which support the
camera proper and which serve to adjust the position
of the cross-wires affixed to the rear frame of the
camera-box. The camera C is connected with the
upper limb of the theodolite by means of a catch-
lever K in such manner that the azimuthal revolu-
tion of this limb will also rotate the camera hori-
zontally.

L is a spirit-level attached to the telescope T, both being
supported by an upright or Y support U secured at
right angles to the horizontal limb of the theodolite
and at one side of but close to the camera.

T is an ordinary surveying-telescope (astronomical)
and it is provided with the usual cross-hairs (one
vertical and the other horizontal), adjustable in the
customary manner.

C represents the camera-box. It is made of hardened
pasteboard, which is strengthened by a metal skele-
ton frame or casing B.

The camera is supplied with an aplanatic objective
(" antiplanat "), made by Steinheil, having a focal
length of 244.5 mm -
The aperture in the diaphragm has a diameter of 5 mm.

Regarding the general arrangement of this instrument it may
be said that:



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 213

First. The optical axis of the photographic lens (objective)
is parallel with that of the telescope T and it always is
perpendicular to the picture or image plane.
Second. The intersection of the optical axis of the camera
and picture plane the principal point of the photographic
perspective is marked by the point of intersection P,
Fig. 113, Plate LV, of two very fine and adjustable plati-
num wires OO r and ff securely fastened to the rear frame
of the camera-box, very close to the image plane.
When the instrument is leveled and in adjustment one
of these fine wires (OO') will be horizontal, while the other
(//') will be vertical, and both will be in a (vertical) plane
parallel to the image plane.

The optical axis of this camera may be brought into hori-
zontal plane by rotating the same about the horizontal axis CC y
Fig. in, Plate LIU, and clamping the screw h. In this position
the image plane and plane containing the platinum cross-wires
OO' and jf will both be vertical.

The horizontal wire OO' may be adjusted into horizontal
plane, after the instrument has been carefully leveled, by find-
ing some easily identified and readily recognized point on the
ground-glass plate, which is bisected by this wire OO', and by
gently revolving the camera in azimuth. If the wire OO' is in
horizontal plane, the observed point will be seen to move over
the entire length of the wire while the revolving motion is given
the camera. Should the bisected point, however, appear above
or below the wire OO' at any time during the azimuthal revolu-
tion of the camera, the same will have to be adjusted into hori-
zontal plane by aid of the two front screws S 2 and 5 3 , Figs, in
and 112, Plates LIU and LIV.

The camera is provided with a short tangent or slow-motion
screw /, by means of which the same may be slightly moved
in azimuth, while the telescope T and horizontal limb of the
theodolite remain stationary. This arrangement will enable
the observer to place the optical axis of the camera parallel to



214 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

that of the telescope T, provided both had been adjusted in hori-
zontal plane. This correction is made by " pointing " the tele-
scope to some well-defined distant point and clamping the the-
odolite in this position. The camera is now moved by means
of the tangent-screw t to the right or left until the same point
appears in the intersection P of the two camera wires OO'
and //'.

The prints of the camera cross- wires OO r and /^ appear on
every negative taken with this instrument, and as their plates
were exposed while vertical to the optical axis of the camera,
the perspectives that are obtained (after the instrument had
been adjusted as described) are in vertical plan, each showing
the principal point of view P, as well as the principal and horizon
line /'/ and OO', intersecting each other in P at right angles. The
horizon line OO' on the picture represents the intersection of the
horizon and picture plane. All points on the picture bisected
by the horizon line have the same elevation (disregarding the
error due to curvature and refraction) as the optical axis of the
camera at the station whence the picture was taken.

In place of the fixed platinum wires some photographic-
surveying instruments have four sets of teeth (or a series of notches)
attached to the rear frame of the camera-box, close to the picture
plane (Fig. 114, Plate LV). If prints are used for the map
construction instead of the plates, this arrangement is preferable
to the fixed wires, as the latter often obscure details and as the
prints may be distorted to such a degree that the lines OO' and
//' may have to be corrected, thus giving two sets of lines across
the face of the print. When only the ends of the cross-wires are
indicated on the pictures by means of the teeth, the correct posi-
tions of the cross- lines may be ascertained or checked experi-
mentally and the lines are then drawn across the face of the
picture by very fine lines in red ink.

Great care should be exercised in the proper location of those
lines, as they form a rectangular system of coordinates to which
every pictured point that is to be mapped must be referred. They



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 215

also play an important part in ascertaining the value of the focal
length of the picture, which is one of the principal elements
required in iconometric plotting.

Fig. 112, Plate LIV, shows the camera in a position to take
a picture of terrene so far below the camera horizon that the
plate when exposed in vertical plane would not "take it in."
The construction of the instrument will permit a depression
(or an elevation) of the optical axis of 30 below (or above) the
horizon by loosening the clamp-screw and revolving the camera
about the secondary axis of rotation. (See paragraph on in-
clined picture plates.)

Constant Focal 'Length of the Italian Cameras. Fig. 115,
Plate LV, shows the longitudinal section of a surveying-camera
with the diaphragm AB in position between the lens doublets.
The aperture of the diaphragm is taken as 5 mm. in diameter.
Only such rays of light emanating from a point N in nature
will reach the point n on the image plate // that form a cone
about the central ray nON as axis, with apex in n and base in O.
For the case illustrated in the diagram (Fig. 115, Plate LV),
that base will be an ellipse with 5 mm. length for the short axis,
while a pencil of light emanating from a point C on or very close
to the optical axis of the objective would be intercepted by the
plane of the diaphragm AB in a circle of 5 mm. diameter.

The Italian lens is so focused that even for the largest aperture
of diaphragm used, all points from 10 meters to infinite distance
from the camera-lens O, Fig. 115, Plate LV, will be clearly
photographed with a maximum error in definition of 0.06 mm.
(for 10 m. distant objects).

If a = distance of object from the point O (10 meters to infinite

distance);

/ = principal focal length of the camera (240 mm.);
6 = focal distance, variable for different lengths of a;
we find from the well-known relation



2l6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

a}



By adopting 240 mm. as value for /, and substituting different
values, from i meter to 300 meters, for a in the preceding formula,
we obtain the following values for b :

a (in m.) = i 10 20 30 40 50 75 100 200 300 oo
b (in mm.) = 315.8 245.9 242.9 241.9 241.4 241.1 240.7 240.5 240.2 240.02 240.00

The error, therefore, in maintaining the focal distance constant is
6 mm. if the object is 10 meters distant from the nodal point;
it is i mm. if the object is 50 to 100 meters* distant and it is in-
appreciable if the object is 300 m. or more distant from the nodal
point of the camera-lens.

The value f-j of the error (lack of definition or distortion)

produced in the photograph for points or objects at different
distances (a), when maintaining a constant focal length, may be
seen from the following : Assuming again that the image plane //,
Fig. 115, Plate LV, be held in a fixed position and 240 mm.
distant from the nodal point of the lens, it will be evident that the
image plane // (Fig. 115, Plate LV) will intersect some of the
light pencils or cones of rays (passing through the aperture O of
the diaphragm) in a circle (or in an ellipse) instead of intercepting
their apex. We see from an inspection of the foregoing table that
this circle of diffused light will increase in size with a decreasing
distance (a) of the object to be photographed. The true point
would be the center of the circle and the length of its diameter
x may be ascertained from the following relation (Fig. 116,
Plate LV):

x:O=f:a,



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 217

Again assuming (Fig. 116, Plate LV)

) = 24o mm. (principal focal length),

a = i meter to infinite distance, and

O= diameter of aperture in diaphragm = 5 mm.,

we find the following values for x from the preceding formula:



a


(in


m.)


=


i


IO


2O


30


40


50


75


.\


(in


mm.)





1.20


O.I2


O.O6


O.O4


0.03


O.O25


0.016


a


(in


m.)


=


2OO


3 00


4 00


500


700


IOOO


00


X


(in


mm.)





O.OO6


0.004


0.003


0.003


O.OO2


O.OOI


o.ooo



IOO
O.OI2



The diameter x of the circle (or ellipse) is evidently quite
small, and a constant focal distances may well be maintained for
all phototopographical work without producing any appreciable
error.

In order to enable the observer to obtain good definition
in the pictures of objects not very distant from the camera the
Italian apparatus was devised with a movable objective and pro-
vided with a metal scale (a, Fig. in, Plate LIII) extending in
the direction of the camera axis, which reads zero (or rather 240
mm.) when the camera has been focused upon objects at infinite
distance. The millimeter graduation of this scale, extending
in the direction toward the sensitive plate of the camera, enables
the observer to measure the focal length directly if the same
had been changed at any time from the principal focal distance
( = 240 mm.). The objective cylinder M, Fig. in, Plate LIII,
may be moved in the direction of the camera axis by revolving
it within N, both tubes N and M being connected by means
of a screw, the rise of its thread being i millimeter.

The circumference of N is divided into ten equal parts, and
the position of the metal scale a, passing over this graduation,
when the objective tube M is screwed into TV, will indicate the
tenths (and estimated hundredths) of millimeters which it had
been moved beyond the number of millimeters read off on scale a.

The focal length plays a very important role in all photo-



2l8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

topographic work, and it is advisable to verify, at the beginning
of operations, the reading of the metal scale, and if the principal
focal length has been changed, the difference must be entered
into the note-book, so that the proper correction may subsequently
be applied.

The distance of the point of view from the perspective plane,
the position of the principal line, and the correct position of the
horizon line can always be ascertained or rectified by instru-
mental observations and computations, or graphically (if the
picture plane has been exposed in vertical plan or if its deviation
from that position be known) as has been indicated, and as will
be shown more fully later.

It has been described how the optical axes of the telescope
and of the camera are brought into two vertical and parallel
planes. Both may be kept in this position and yet be revolved
about the vertical axis of the instrument in order to successively
expose the plates covering the entire panorama. The horizontal
limb of the theodolite is divided into 360 with subdivisions reading
to 20', and by means of two verniers 30" may be read off. The
vertical circle is provided with the same graduation and similar
verniers. Thus the means are provided to ascertain the azi-
muthal positions of the camera axis (the principal ray) for each
perspective, or the means of " orientation " are thus provided
for. The magnetic azimuth of the principal ray of the perspec-
tives (i.e., direction of optical axis for each exposure) or the hori-
zontal angle which is included between said ray and any other
line passing through the station and some known point on the
photograph (e.g., trigonometrical point) may readily be ascer-
tained by observation.

All perspectives that are to be used for mapping must be
obtained from stations with known geographical positions. Gen-
erally trigonometrical points are selected for the camera stations,
but if points beyond these have to be occupied to better con-
trol the topography, the elements needed (horizontal and ver-
tical angles) to determine their positions with respect to sur-



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 219

rounding triangulation points may readily be observed with
the theodolite before leaving the detached camera station.

B. L. P. Paganini's new Phototheodolite (Model of 1894).

The following description of PaganinVs new phototheodolite
has been extracted from his "Nuovi Appunti di Fototopagrafia,"
Roma, 1894.

Paganini's new phototheodolite, model of 1890, differs from
the one just described, although the general form and the dimen-
sions of the camera-box, as well as the focal length of the lens,
remain about the same as with the older model. The principal
change rests in the omission of the eccentric telescope, which
has been replaced by the centrally mounted camera, which may,
at will of the observer, be converted into a surveying-telescope.

The telescope which we generally find attached to surveying-
instruments consists of a tube, slightly conical in shape, having a
positive lens (or a system of convergent lenses) at one end, known
as the " objective," which produces within the telescope a real
and inverted image (the same as the camera-lens) of any object
towards which this telescope may be directed. The other smaller
end of the telescope-tube has a still smaller tube inserted into it
which may be moved in the direction of the axis of the tube.
This second tube also contains a system of convergent lenses
it is the so-called " ocular " lens set or " eyepiece " of the telescope
which serve to project an enlargement of the image formed
in the telescope upon the retina of the observer's eye.

In the image plane of the objective the so-called diaphragm is
placed; it is a ring-shaped metal disc to one side of which a pair
of cross-hairs is attached in such a way that the hairs (spider
webs or lines cut into the surface of a thin piano-parallel glass
plate) will coincide with the image plane. One hair is horizontal
and the other vertical, their point of intersection falling in the
optical axis of the telescope.

A suitable eyepiece had only to be combined with the objec-
tive of the older camera model to convert the camera into a



220 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

telescope. The eyepiece of the camera telescope, camera model
of 1890, consists of a positive lens set, known in optics as Rams-
den's ocular lens.

The inner wall surfaces of the camera-box should be well
blackened to avoid side reflections and a consequent dimness in
the appearance of the cross- wires of the camera telescope.

The camera proper consists of two parts, a truncated pyramid
A, Figs 117-119, Plates LVI-LVIII, and a cylindrical attachment
B containing the tube /. A second tube, placed within the tube
/, may be moved in the direction of the optical axis by means of
a screw the threads of which have a rise of i millimeter. By
rotating this inner tube the lens may be brought nearer to or
farther from the image plane, the lens remaining parallel to the
image plane at any position that may be thus given it.

A scale a, Fig. 117, Plate LVI, graduated to millimeters, is
permanently attached to the tube / and it lies very close to the
ring n t the circumference of which is divided into ten equal parts
(this graduated ring n is soldered upon the cylinder u encasing the
camera-lens). This scale a (extending in a direction parallel to
the optical axis of the lens) has a mark coinciding with the index
rim of the ring n, thus indicating the focal length of the camera-
lens when focused upon objects at infinite distance. The milli-
meter graduation of the scale a, extending from the zero mark
towards the ground -glass, serves to ascertain the focal lengths
for objects nearer the camera. The graduation on the ring n
serves to read one tenth of one revolution of the tube u, which is
equal to an axial motion of the lens of o.i mm., hence the focal
length for any object focused upon may be read to single milli-
meters on the scale a and to tenths of a millimeter on the graduated
ring n.

The construction of this phototheodolite is such that the optical
axis of the camera-lens is always at right angles to the picture
plane (the ground -glass surface or sensitive film of the photo-
graphic plate). The intersection of the optical axis and the
picture plane (the so-called principal point of the perspective) is



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 221

marked by the intersection P, Fig 113, Plate LV, of two very
fine platinum wires, OO' and //', one horizontal and the other
vertical. They are stretched across the back of the camera-box
as close as possible to the picture plane. The buttons &, Figs. 117
and 1 1 8, Plates LVI and LVII, serve to impart tension to the
wires. The horizontal line OO' corresponds to the horizon line
and the vertical line //' corresponds to the principal line of the
perspective represented by the image on the ground-glass surface.

Fig. 119, Plate LVIII, shows the rear view of this instrument,
the ground -glass or focusing plate having been replaced by an
opaque plate stiffened by a metal frame, which supports the
Ramsden eyepiece in the center in such manner that its optical
axis coincides with that of the camera-lens. The cross-wires OO'
and //', at the rear of the camera-box, serve also for the astronom-
ical telescope, into which the camera may be converted by attach-
ing the opaque plate with central eyepiece as shown in Fig.
119, Plate LVIII.

The fitting of the eyepiece allows for axial motion to adjust
its position with reference to the cross-wires to avoid parallax.
The opaque plate supporting the eyepiece is composed of a thick
cardboard impregnated with chemicals to harden its fibers and to
render it impervious to moisture. The camera-box is made of the
same material and both are strengthened by a frame and ribs of
metal, as indicated in Figs. 117 and 118, Plates LVI and LVII.
The cylindrical tube B is inclosed by a metal collar C which is
held in position within the metal ring II' by four screws R, R, S, S'.
The ring //' is connected with the frame gg f by means of two arms
Ig and Vg, all being cast in one piece. The pivots q attached
to the frame gg' serve as horizontal axis of rotation for the
camera.

This instrument is provided with a vertical circle, horizontal
circle, verniers, reading-microscopes, levels, clamps, and tangent-
screws, forming a complete transit with centrally located camera
telescope.

A cross-section of this instrument is illustrated in Fig. 120,



222 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate LIX. The scale 0, already described, is here placed on top
of the tube w, to better illustrate its use.
yy = uprights forming the supports of the horizontal axis of

rotation for the camera telescope;

h = upper horizontal limb, or alidade, supplied with two verniers j
H = lower limb, or horizontal circle, bearing the graduation ;
TT= tripod head, supporting the instrument by means of three

leveling-screws (W)\
a = casing for conical center;

^ = central clamp -screw entering a ball which is supported by
the hemispherical socket aj of the lower part of a. (f
secures instrument to tripod-head and it guards against
an accidental falling off of the instrument.
The horizontal circle, having a diameter of 17 cm., is grad-
uated into 20 minutes and suitable verniers are supplied to read
horizontal angles to 30 seconds.

The vertical circle, with a diameter of 10.5 cm., is graduated
into 30 minutes and its verniers read to single minutes.



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 17 of 33)