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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 26 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 26 of 33)
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cides with the base of an embankment at an apparent distance
from S=A = 4ooo m., while the crown of the embankment may
bisect the iudex line at one third of its length, the absolute
height of the embankment would then be

4000X0.003

= 4 m.

It is very essential that each pair of plates be exposed in a
vertical plane containing the base line or being parallel with it.
If such be the case, points lying at infinite distance in the ver-
tical planes of the objectives should appear pictured in the prin-
cipal lines of the plates. If either of the plates, say P, includes
an angle = d with the vertical plane of the other, the distant
point will be pictured to one side of the principal line (see Fig. i,
Plate CVI). The distant point A will be pictured at ai in
plate pn, but the principal point will be to one side, at #n.



326



PHOTOTOPOGRAPEIC METHODS AND INSTRUMENTS.



The plates after being placed on the holders ot the stereocom-
parator are adjusted by means of the horizon and principal
lines, and in this case all parallax values will be measured too
small by



A correct measurement of the length of the base in rough
mountain regions often offers serious difficulties, telemeter read-
ings generally being the only available means for measuring these
base lines. Any error made in the base will affect all distances
determined from its left station, and such being the case it would
appear advisable to select relatively long base lines. The lengths
of the latter, however, are controlled by the fact that picture
pairs can no longer be viewed stereoscopically in their full extent
when the length of the base exceeds a certain limit. Pictures
obtained from the ends of too long a base will have but limited
distance zones that may be examined stereoscopically through the
binocular microscopes; areas outside of these, both near and far,
will appear blurred and indistinct. The examination of such
plates through the microscopes is not only very trying to the
eyes, but the observer also loses the general view of the terrene
and he will have to refocus the microscopes for every change in
distance.

For a constant focal length of 241.5 mm. and an error in parallax
of o.o i mm., errors in distances may be made, for base lengths
of 50, 100, 200, and 300 meters, as listed in the following table:



Distance of


Length of Base Line in Meters.


Bisected Point




in Meters.












50 m.


ioo m.


200 m.


300 m.


IOOO


0.8


0.4


0.2


O.I


2000


3-3


*-7


0.8


0-5


3000


7-4


3-7


1.9


1.2


4000


13.2


6.6


3-3


2.2


5000


20.7


10.3


5-2


3-4


7500


46.8


23-4


11.7


7-7


IOOOO


82.8


41.4


20.7


13-8



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 327

If errors in position of 15 m. be permissible in rough moun-
tain work plotted in 1 125000 scale, a mean error of 3 m. may
be accepted for the same kind of work plotted in i : 10000 scale.

For a parallax error not exceeding 0.0 1 mm. distances to
6000 meters from the base stations should be controlled, and
for the 1 125000 plotting- scale base lines of 100 meters preferably
should be selected. For the i : 10000 scale a loo-meter base
should be selected for distances up to 3000 meters and a 200-
meter base for 4000 meters, etc. If the objective has a focal
length shorter than 240 mm. the base should be made propor-
tionately longer. For instance, for a focal length of 180 mm.
the base lines as given above should be increased by one quarter.

The terrene pictured on a pair of stereoscopic plates, when
examined through the binocular microscopes, appears very much
like a relief model of the country, the changes in the surface for-
mation being far more clearly shown than in the landscape itself
when viewed from either of the two base stations.

For the best iconometric results each plate should contain
from 6 to 12 control points of known elevations and positions
(tertiary triangulation points). After the left base station and
all the control points have been plotted, the two stereoscopic
plates are placed on the comparator frame to be adjusted in
the manner already described. After the parallaxes, abscissae,
and ordinates of all the control points that are pictured on the
plates have been measured and tabulated, the picture trace of
the left picture is plotted, based on the computation of the radials
drawn to two control points. It is preferable to select two extra
axial points, one near the left and one near the right margin of
the plate, for plotting the picture trace. The position of the
latter is checked by means of the abscissae of the other pictured
control points. It would not be sufficiently accurate for our
purpose (" stereophotogrammetry ") to plot the picture trace
by means of a paper strip, as generally used in the plane-table
or intersection method.

Parallel with the picture trace and from ij to 2 times its



328 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

distance from the station point a scale is drawn having the
same graduation and numbering as the scale (of abscissas) A
of the comparator, the divisional parts of course being enlarged,
according to the selected distance, ij to 2 times. A ruler having
the plotting-scale along its fiducial edge may be secured to the
station point in such manner as to revolve about the station
with the zero mark of the scale as pivot. With these means
the pictured points may be quickly plotted without actually
drawing their lines of direction, which radiate from the station.

The next step is to check the position of the horizon line by
means of the ordinates of the pictured control points. Any cor-
rection affecting all points alike is made by changing the zero
mark of the scale B on the comparator. If the horizon line
has to be raised or lowered on one side, the plate will have to
be turned correspondingly on the holder of the comparator. In
the latter case the adjustments of both plates on the comparator
will have to be repeated to allow for the change just made.

With the measured parallax values a the distances A
of the control points are computed and compared with those
of the plotted points. Discrepancies A between these exceed-
ing the amount due to errors in parallax of 0.01 mm. would
point toward an error in " swing " (J) during the exposure
of the plate, errors in base measure (J 6 ), or toward errors
due to both. We may, therefore, express these discrepancies
by the equation



If the error in base measure equals b and the error in
parallax, due to the "swing of plate," d, equals s=jd, we
will have the equations



^
B



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 329

We can now compute the errors J and A\ (discrepancies
between the computed and plotted distances A) from the
parallaxes of two pictured control points and substitute these
values for A and Ji in the equations

A A 2



after which the values for the base-line error (b) and the
error in parallax (s) may be computed and applied to the
base-line and parallax values.

A better way would be to use all the control points, tabulate
the errors (J) graphically, and find the values for J 6 and J, by
interpolation, as shown on Plate CVTI.

The abscissae of the control points are plotted in their true
lengths and the corresponding errors J are plotted as ordinates,
giving the points 47, 48, 44, 28, 38, 32, 34. A curve passing
through the initial point O is laid through this series of points.
To separate the ordinates, J, of this curve, OC, into the
component parts, A b and 4,, a tangent OG through O to the
curve OC is to be drawn in such manner that the upper sec-
tions m, n, o, etc., increase in length with the squares of A
(0=41*1, p=4n, q=4o, etc.), as the increase in the errors A 6 is
directly proportional to the squares of the distances A and
the errors J& increase in the same ratio as the distances A.

With a pair of dividers and a ruler the position of OG
may be located tentatively. For an error in the base line, b = O,

~D t

the curve OC will be a parabola, having the parameter = - ;

for an error in swing, s=O, a straight line will replace the curve,

b
and J = A -=-.



330 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The lower ordinate section serves for the determination of

7?

the' base-line correction, & = -r-J 6 , and the upper section gives

A.

B'f

the correction for the parallaxes s=~-r^A s .

The general course of the curve OC will be a criterion
of the errors affecting a pair of plates, showing whether they are
due to regular causes or whether errors of level adjustments,
errors in computation, etc., have crept in also. If no smooth
compensating curve may be drawn to harmonize with the series
of plotted points, errors outside of those referred to in the pre-
ceding paragraphs should be looked for.

A serious error in the swing of the plate may affect the curve
in a marked manner. The correction applied to the parallax,
as referred to in the preceding, neutralizes only the constant
dj. It corrects the position of the principal line with refer-
ence to the pictured points, but when there is a decided swing
in the plate the parallax, for points to either side of the principal
line, will be in error, even after the correction 5 for the paral-
laxes has been applied.

After the correction df has been applied, the plates may
yet have the relative positions indicated in Fig. i, Plate CVI,
where points at infinite distance and situated in the principal
plane will be pictured in the principal lines of both plates, whereas
the images a of a distant point A, lying to one side of the
principal plane, will be pictured at a and 0n, instead of at a
and a\. In lieu of the correct parallax (x\x) we obtain
the smaller value (#n x), referring to Fig. 2, Plate CVI. The

oc 2
error thus remaining, which may be expressed as J = -r, increases

rapidly with an increase in the length of abscissa; it is positive
on one side of the principal line and negative on the other, being
O for points on the principal line. It is a prime requisite,
therefore, to expose pairs of plates as near as possible in a ver-
tical plane parallel with the base.



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 331

After a plate pair has been tested and after the corrections
found necessary have been applied the iconometric mensuration
may be commenced. The pictured points may be plotted by
means of their lines of direction, based on the abscissa values,
recorded on the scale A, and its horizontal distance from the
left station, based on the measured parallax as given on scale a
of the stereocomparator. The difference in elevation between
the station and the plotted point may be computed from the
reading of scale B. To ascertain the parallaxes of the pictured
points the index mark is moved from point to point in the stereo-
scopic image field, very much in the same way as the telemeter
is caried from point to point in the field when reading distances.
It is evident that the index mark may readily te moved to bisect
points in the image field that would be inaccessible for the ordinary
telemeter in the field. The distances, obtainable by moving
the index mark in the stereoscopic field, considerably exceed
those measured with the telemeter and the time required for
obtaining these distances stereoscopically is so short that the
advantages of the stereoscopic method over the plane-table and
tachymetric methods are out of question for topographic recon-
naissance work in rough mountains.

The stereocomparator, furthermore, is peculiarly well fitted
for a quick location of points having the same elevation; the
index mark may be used to trace out the contours in the stereo-
scopic field. Points may also be readily located that are in
the same frontal plane, in the same plane parallel with the base
line. Actual profiles parallel with the picture traces may thus
be run out and by locating points to either side of the profile,
using the micrometer screw of the binocular microscope for this
purpose, terrene strips of 150 to 250 meters width (scale 1 125000)
may be developed, which will form the base for the subsequent
orographic development of the topography.

The positions of points that have been plotted by the usual
method of direction and distance may be checked by referring
them to the plotted positions of near-by pictured control points.



332 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The stereoscopic photogrammetric methods evidently offer a
wide field for application to ascertain changes that may have
occurred during periods of time that are allowed to elapse before
taking a new set of pictures from the same base line (or at least
from the same vicinity). The examination of two stereoscopic
plate pairs of a glacier, for instance, would at once show any
change in form or location if the left plate of one pair be examined
in the stereocomparator with the right plate of the second pair,
both pairs being obtained at different times from the same base
line.

Good results may be expected from this method, if applied
by the navy, for mapping coast lines without making a landing,
by taking simultaneous views of the coast, fortifications, etc.,
from a vessel (a base line being measured on the deck between
the camera stations), noting the position of the vessel on the
chart at the time of exposure.

The use of the stereocomparator may also be recommended
for recording the positions of moving bodies (army corps, fleets
during maneuvers or in time of war), making plans of inaccessible
objects, for the mapping of cities, for making profiles, plans, and
relief models of areas to be studied for comparative locations of
roads, railroads, irrigation plants, etc.

To recapitulate, the actual mapping of the terrene details,
based on the examination of stereoscopic picture pairs, may be
made:

(1) With relation to a series of control points plotted from

data obtained directly with the stereocomparator;

(2) With relation to a series of contours obtained directly

from the picture pairs, or

(3) By means of profiles composed of points having the same

parallax, i.e., points in frontal planes.

Of the many-fold uses to which the stereocomparator is adapted
we may mention stellar surveys, the testing of banknotes, the
comparison of scales and their prototypes, comparing facsimiles
and replica of various kinds, the study of animals in motion^



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 333

migrator}' bird flights, changes in the northern lights, cloud ele-
vations, terrene changes due to landslides, volcanic eruptions,
inundations, forest fires, etc., for the study of effects produced
by bombardments and explosives, changes in sand dunes, etc.

In the preceding paragraphs it was assumed that each pair
of plates was not only exposed in the same plane (the plane con-
taining the base line or laid parallel with it), but this plane was
also supposed to be vertical. The vertically, of course, greatly
facilitates and simplifies the iconometric constructions, yet it is
not a sine qua non. If a plate pair be exposed in the same inclined
plane, all that has been said about the stereotopographic method
still holds good if the angle of inclination of the plane containing
both plates during their exposures be measured and taken into
account.

If the landscape pictures on the inclined plates could be trans-
ferred to vertical plates by photography, the latter could be used
on the stereocomparator just as if the plate pair had been exposed
in the vertical plane originally.

The inclined -plate position will often be unavoidable in making
stereophototopographic surveys from the decks of vessels, and
even in mountain work a suitable location for the base line will
sometimes necessitate exposures to be made on inclined plates
in order to control deep valleys or high elevations from the two
base stations.



CHAPTER XT.
PHOTOGRAPHIC OPERATIONS IN THE FIELD.

UNTIL now the principal operations have been considered
for obtaining the so-called " latent " or invisible image on the
exposed plate, which is still to be converted into the " negative, "
in which form the terrene image is used, either directly or indi-
rectly, for the iconometric plotting of the pictured topographic
features.

The work of developing and fixing the negatives of an exten-
sive photographic survey is best done by a photographic expert
who has made special studies and experiments for this purpose.
He should be thoroughly familiar with the laws that control the
changes, both chemical and physical, which take place in the
compositions of the sensitized coatings of the photographic plates,
when they are exposed to the action of light, as well as those
which control the changes in the chemical compositions of the
sensitized films when the plates are immersed in the developing,
toning,, and fixing baths.

Still, every phototopographer should be sufficiently familiar
with the general routine practice of photography to develop
some " trial " or " test " plates understandingly and successfully
while he is yet in the field.

At least a few plates taken at random from every batch
originally packed together and which are likely to have passed
through the same conditions during transportation should be
developed, while still in the locality where the exposures were
made, to feel satisfied that no plates were spoiled and also to
feel reasonably assured that the exposures were correctly timed.

334



EXPOSURE OF A PHOTOGRAPHIC DRY-PLATE. 335

The wisdom of developing test-plates, to avoid loss of valuable
time and material by incorrect exposures or by the use of spoiled
plates, is beyond dispute. If all development of plates be post-
poned until after the return of the expedition, defective plates
cannot be replaced without expending large sums of money, and
the results of the expedition may be robbed of much, if not of
all, practical value.

Whenever there is danger of losing undeveloped plates through
careless and ruthless inspection of baggage on frontiers, or through
the inquisitiveness of packers, to whom the transportation of the
plates must be intrusted, it is of course advisable to develop all
the plates of the survey in the field, pari passu with the progress
of the survey.

The principal records of the season's work, regarding the
topography at least, consist in a series of undeveloped plates,
and the phototopographer should feel reasonably certain that
they are of as good a quality as could be obtained under the con-
ditions of climate and surroundings prevailing at the time of
their exposures.

We will give in the following a cursory review of those opera-
tions to which an exposed plate is to be subjected before it is
converted into the permanent negative, and with which the
phototopographer should be familiar to enable him, for the rea-
sons just stated, to develop some test-plates while he is still
in the locality where the exposures were made.

I. General Remarks on the Exposure of a Photographic

Dry-plate.

When the sensitized coating of a photographic plate is exposed
to the action of the rays of so-called white light solar light
certain effects upon the chemical composition of the coating
will be produced, consisting primarily in a reduction of the silver
haloids that are embodied in the gelatine coating of the dry-plate
into an unstable condition, permitting a deposit of metallic (black)



336 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

silver to be readily made upon the plate when the latter is immersed
in the so-called " developer " (reducing bath), which converts
the light-sensitive " latent image " of the exposed plate into the
" negative " of a permanent and stable character.

The greater the intensity of the light that reaches the plate
in the camera, or the longer the exposure of the plate to the action
of the light-rays, the greater will be the amount of reduced silver.

The quantitative effect, in a given time period, of white
light upon the sensitized coating of a photographic plate may
differ perceptibly from the quantitative effect of chromatic or
color rays, although their qualitative effect upon the silver haloid
(bromide of silver) is essentially the same. For short exposures
the quantity of reduced silver may be regarded as directly pro-
portional to the duration of the exposure. The " density " of a
negative is more or less great according to the larger or smaller
amount of reduced silver that has been deposited; density increases
directly with the length of exposure.

Photographic dry-plates differ materially regarding their
" speed," or their sensitiveness to light action. The speed is
generally indicated by the so-called " sensitometer number,"
ascribed to each emulsion. The same density for two different
plates, when photographing the same object under identical con-
ditions, may be attained by giving each plate a different length
of exposure, corresponding to its sensitometer number, the less
sensitive plate, of course, being given the longer exposure.

Under ordinary conditions, three different stages of exposure
may be considered in practical photography:

1. Underexposure;

2. Correct exposure;

3. Overexposure.

A fourth stage, the so-called " period of reversal," may pos-
sibly be reached, but this requires so lengthy an exposure that
it will rarely be attained, inadvertently, when exposing plates
for phototopographic purposes.

An underexposed plate may be recognized by the marked



EXPOSURE OF A, PHOTOGRAPHIC DRY-PLATE. 337

contrast in the negative between the lights and shadows and a
general deficiency in details. Such plates will be of little or no
value for iconometric plotting.

An overexposed plate shows little contrast between the lights
and shadows and the general details will be weak and flat.

When a plate had been exposed correctly, its scale of grada-
tion in tint, after proper development, will embrace the widest
range possible, from pure transparency (white) to black. A
negative appears transparent where the photographed object
was dark and vice versa. The negative should be a true inverse
of the original regarding the light gradations.

The source of the light- rays which are emanated by any
object in nature may be a threefold one, comprising:

1. Rays of direct sunlight;

2. The less intense rays of diffused skylight;

3. Rays originating from the foregoing two sources, but

reaching the subject indirectly after having been re-
flected from surrounding objects.

The intensity of the light- rays, generally summarized as>
daylight, is subject to many variations. The sunlight alone will
have a variable intensity at different altitudes and under differ-
ent atmospheric conditions, irrespective of the geographic latitude.

The tendency of aerial perspective is in the direction of diffu-
sion of sharp outlines of distant objects and toward obliteration
of details. The skyline of distant mountains becomes merged
into the so-called " blue haze." The nearer sea-level the observer
is stationed the more indistinct will distant objects become,
while in high altitudes, with a relatively dry atmosphere, objects
will be discernible, as to form and color, at far greater distances..

Some of the poly chromic rays of sunlight, on their passage
through the atmosphere, intervening between the observer and.
the object, will become diffused or absorbed, while others wfll
transverse the same without suffering any perceptible modi-
fications. Color rays near the violet end of the solar spectrum,
rays of short wave-lengths, are more largely absorbed by the



338 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

atmosphere than those of longer wave-lengths near the red
end of the spectrum.

We had seen (Chapter VII) that the component rays of so-
called white light after transmission through a lens will be differ-
ently refracted and the actinic effects of such refracted rays
upon the sensitized films will differ according to their colors
or wave-lengths. Those of short wave-lengths, the ultra violet
to blue, between the Fraunhofer lines HI and F, have by far a
more pronounced chemical action upon the silver haloids of the
plate coating than light- rays with longer wave-lengths, the green,
yellow, orange, and red rays, between and beyond the Fraun-
hofer lines E and A.

The luminous (optical) effects of the component colors of a
landscape upon the eye are not identical with the actinic (chem-
ical) effects upon the photographic plate. The optical effect is
governed by the various degrees of tint, hue, or shade that the
several parts of the landscape convey to the eye, some parts



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 26 of 33)