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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 11 of 33)
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cllo will be the trace of picture I and c! will be the picture
trace of II oriented at station // with reference to the base line
/ // (Plate XXX).

Fig. 54, Plate XXX, illustrates a more simplified way of orient-
ing the picture traces.

After the base line / // has been plotted the horizontal angles
a 1 and a 11 (azimuthal deflections of the optical axis from the



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 131

base line / // for the negatives I and II) are plotted at / and //
with reference to their positions in regard to the principal planes
at the stations / and // as shown in the negatives I and II (whether
the station's image falls to the right or to the left of the principal
line).

The constant focal length = / of the negatives is laid off on
the principal line = c'O for negative I and = c"O for negative II.
The images of the stations are projected upon the horizon lines,
Sn upon HiHi (Plate I) and Si upon H U H U (Plate II), when



d 'OS u= a' = horizontal angle included between the principal

plane and base line / //, and
= a" = corresponding horizontal angle for station //.



These angles, a' and a", are transferred from the negatives I
and II to their corresponding ends of the base line / //, as indi-
cated in Fig. 54, Plate XXX. Now lay off the focal length /
from 'the base stations / and II upon the sides of the angles a!
and a"=Ic' and = //c" respectively, and erect perpendiculars
H'H' and H"H" to Ic f and He' in c' and c" respectively. They
will represent the traces in horizontal plan of the vertical picture
planes / and // in correct position and orientation with reference
to the base line / II. The remaining two sets of five plates
each of the panoramas at the stations / and // are easily oriented
and plotted ; the next plate hi order at station //, for instance,
would have the principal ray (optical axis) in the direction
(a" +60), the third (a" +120), etc., Fig. 55, Plate XXXI.
Every plotted camera station will be surrounded by a regular
hexagon the sides of which represent the picture traces of the
six negatives forming the panorama set for the station.

3. LOCATING POINTS, IDENTIFIED ON SEVERAL PHOTOGRAPHS, ON THE
PLOTTING-SHEET.

The horizontal locations of all points identified on two or
more plates are plotted by locating the intersections T of the
lines of horizontal directions It, II t" , III if" . . . , Fig. 54,



132 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Plate XXX, in the same manner as has been described for CoL
Laussedat's method.

4. THE ICONOMETRIC DETERMINATION OF ELEVATIONS OF PICTURED
TERRENE POINTS.

The elevations of points iconometrically plotted are found
in the same way as described for Col. Laussedat's method. If

the scale of the map is r? we will have, Fig. 54, Plate XXX, the
elevation of station I above II=H=I(Si).



The values of y. = Si(Si), I II and SiII=l" are found by
direct measurement with a small ivory beveled scale divided
into 0.5 mm., of which o.i mm. may well be estimated after some
little practice.

C. Capt. E. Deville' s Method (Canadian Method).

This so-called Canadian method has been in use under the
auspices of the Canadian Department of the Interior since 1888.
Capt. Deville, Surveyor- General -of Dominion Lands, has given
a detailed account of his methods in " Photographic Surveying,"
published at the Government Printing Bureau in Ottawa in 1895,
and the following paragraphs have been largely taken from
Deville 's book.

i. GENERAL REMARKS ON THE FIELD-WORK.

The area to be surveyed is covered with a triangulation net,
preferably before the phototopographic work is begun, and a
secondary or tertiary triangulation, if needed, is carried along
with the phototopographic work to locate the camera stations,
in both the horizontal and vertical sense, with reference to the
primary triangulation stations already established.



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 133

The surveyor makes a plot of the entire triangulation covering
his territory in the field, and he locates on the same all the stations
that he may occupy to enable him to recognize the weak points
in his scheme and to plan his operations with a thorough under-
standing and to secure a good assurance of success. The instru-
mental work in the field is done merely to locate the camera
stations and certain reference points (used for the subsequent
orientation of the picture traces), all topographic details being
deduced from the pictures.

The camera stations are located either by angles .taken from
the station to surrounding triangulation points (resecting, three-
point problem), or by angles observed from the latter to the
camera station (intersecting, concluding), or by both methods
combined.

The strength or value of the work depends very much upon a
judicious selection of the points that are to be used as camera
stations, in order to bring the enitre terrene under proper control
and to be enabled to construct the map by the method of inter-
sections of lines of direction. Other methods for plotting topo-
graphic features and details being employed only when the method
of intersections fails on account of insufficiency of data to give the
requisite number of horizontal directions (the camera stations
not being well situated) for a good location of points by intersec-
tions.

Each camera station should be marked with a signal before
leaving it, not be shown on the pictures, but to be observed upon
with the transit or altazimuth from the triangulation stations,
in order to locate the position of the camera station upon the plot-
ting-sheet.

Frequently it will be of advantage to set the camera up eccen-
trically over a triangulation station, in order to include certain
parts of the landscape in the views. The position of the eccentric
camera station with reference to the triangulation point can
readily be ascertained (by azimuth and distance) and should always
be recorded.



134 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Complete panorama sets are not taken at every station ; it is
preferred, rather, to increase the number of stations, often occupy-
ing a special station to obtain a single view only, if by doing so
better intersections for the location on the plan of some special
feature may be obtained.

Multiplicity of stations demands but a small increase in labor,
either in the field in the extra observations of horizontal direc-
tions for their location or for plotting them in the office, and
enough stations should always be occupied to give a full control
of the relief of the area to be surveyed.

A certain section of the terrene may be so located that it will
be a difficult matter to select more than one station, whence it
may be seen. In such a case the method of "vertical intersec-
tions" may often become useful; two or more views of such area
may be taken from stations at different elevations: the greater the
difference in altitude between such stations the longer will the
base line be, and the better will be the intersections which
locate the features in question (if the latter are not too distant).

As enough plates should be exposed to completely cover the
ground, the camera stations will have to be distributed in such
a way that all valleys, sinks, and depressions that may be repre-
sented in the scale of the map are well controlled (i.e., are seen
from different camera stations). It is evident, therefore, that
the number of stations to be occupied for the topographic develop-
ment of a certain area will depend upon the character of the
terrene and upon the scale of the chart.

Two or three well-defined points (" reference points ") in
each panorama section (covered by one plate) are observed
with the transit or altazimuth, noting the vertical and horizontal
angles upon the outline sketch that is made for every exposed
plate.

These sketches serve far better to identify points with cer-
tainty than a mere designation (by name or symbol) or descrip-
tion of the points observed upon. The general triangulation
notes are kept in the usual manner.



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 135

Vertical angles are observed to check the position of the
horizon line on every photograph and to correct errors due to
small changes in the level adjustments of the camera that may
arise during the transportation of the instruments over a rough
trail.

The horizontal angles are needed, both for the location of
the camera stations and for the orientation of the pictures (pic-
ture traces) on the plotting-sheet for the subsequent map con-
struction.

2. GENERAL REMARKS ON THE ICONOMETRIC PLOTTING OF THE SURVEY.

The field notes of the phototopographic surveys made in
the Northwest Territory of the Dominion of Canada by the
Topographical Surveys Branch of the Department of the Interior
under Capt. E. Deville, Surveyor- General of Dominion Lands
are plotted on the scale of i : 20000, but the maps are published
on the 1:40000 scale with equidistant contours of 100 feet.

The phototopographic reconnaissance in southeast Alaska exe-
cuted by Dominion Land Surveyors under W. F. King, Alaskan
Boundary- Commissioner to Her Majesty was plotted on the
scale of 1:80000, with a contour interval of 250 feet, and it
was published on the i: 160000 scale.

After the triangulation has been computed and the points
have been plotted, and after the computed elevations have all
been affixed to the marked points on the plotting-sheet, the tri-
angle sides of the secondary triangulation scheme executed
during the phototopographic survey are computed, the cor-
rections to the horizontal angles, indicated by the closing errors,
having been applied. The latitudes and departures from every
secondary point to the nearest primary station are then com-
puted and the secondary stations are plotted by their latitudes
and departures (unless the primary sides are too long).

All camera stations not already included in the secondary
triangulation scheme are now plotted with reference to the tri-
angulation points, using either a table of chords or a station-



136 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

pointer (vernier protractor). If many points had been observed
upon from the camera station, the horizontal angles are pref-
erably laid off on a piece of tracing cloth or paper and this
improvised multiarm protractor is used, like a station-pointer,
to locate the plotted position of the camera station.

The surveyor should endeavor to obtain at least one direction
from a triangulation station to every camera station; the plotting
will then be less troublesome and far more accurate. Photo-
graphs should not be used for plotting the positions of camera
stations; enough angles should always be observed in the field
to locate (trigonometrically) every occupied station in the manner
just indicated.

From the original negatives copies are made, enlarged to
9i by T 3 inches, on heavy bromide paper (more recently so-
called " Platino Bromide " paper has been used by Capt. Deville).
The enlargement adopted in Canada for these bromide prints
is about 2.1 times, which ratio was selected to utilize the full
width of the paper found in market. These enlargements,
being extensively used in the map construction, should be made
with great care to reduce distortion to a minimum.

After the prints have been developed (with iron oxalate),
well washed in acidulated water and fixed, they are again thor-
oughly washed and dried in a flat position, under special pre-
cautionary measures to control the contraction or expansion, in
such a way that the final size of the dry-prints have uniform
dimensions. Slight distortions that would arise from a play oi
the negative carrier in the enlarging camera or from the bromide
paper not lying perfectly flat on the surface of the copying- screen
are best reduced by using a copying-lens of long focus.

Before using the prints for the map construction any distor-
tion due to the enlarging process should be ascertained, which
is best done in the following manner:

Fig. 56, Plate XXXI. Join the middle notches H and H',
P and P f > and with a set-square test these two lines for perpen-
dicularity. Take with a pair of dividers the distance of the



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 137

two notches A and B, which should be one half of the enlarged
focal length and equal to the distance between the two notches C
and D. Apply one of the points of the dividers in P and the
other should come in E and F. Transfer the point to P' and
check P'G and P'J. If the print stands all these tests, it may
be used iconometrically ; if it does not, it is returned to the pho-
tographer with the request for a better one.

3. ORIENTING THE PICTURE TRACES ON THE WORKING-PLAN.

Every photograph contains at least one, generally several,
of the triangulation points plotted on the working-sheet and
the traces of the picture and principal planes are oriented and
plotted on the plan as follows:

The distance or principal line PS, Fig. 57, Plate XXXI,
is made equal to the focal length and the pictured point a of
the reference point A is projected upon the principal line ( = a')
and upon the horizon line ( = #).

If Si t Fig. 57, Plate XXXI, is the plotted position of the
camera station on the plan, and if SiAi represents the horizontal
direction to A from the stafton 5, make Sidi equal to Sa (taken
from the photograph on the " photograph-board ") and from di
as center, with aa' = Pa as radius, describe an arc to which
Sip is drawn tangent. Sip will be the trace of the principal
plane (or the distance line) and the perpendicular to Sip through
a l= p ai will be the picture trace. Instead of making this con-
struction on the " photograph-board " (which will be described
under section i) it may be made on the plan.

On SiAi take SiB, Fig. 58, Plate XXXI, equal to the focal
length, erect BC perpendicular to SiAi and make it equal to
the abscissa (=aa f , Fig. 57) of the reference point. Join
SiC and take Sip equal to the focal length; at p erect a per-
pendicular to SiC and it will be the trace of the picture plane,
SiC being the trace of the principal plane.

Another simple method for orienting the picture trace of a
photograph having the image of a reference point C is as follows



138 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

(Fig. 59, Plate XXXII) : Take a triangle of hard rubber or wood
and mark off along one side the focal distance SP, Fig. 57, Plate
XXXI, equal to ab, Fig. 59, Plate XXXII, from the right-angle
corner a. Carefully notch the triangle side at b so that the cen-
ter of a fine needle marking the plotted station point will fit
into the notch. From the photograph take the abscissa (a' a = +a,
Fig. 57, Plate XXXI) of the pictured reference point (a) between
the points of a pair of dividers, move the triangle about the needle,
marking the plotted station b with the left hand until ac, Fig. 59,
Plate XXXI, is equal to the distance a' a (abscissa of reference
point), held between the points of the dividers. The triangle is
securely held in this position and lines are drawn along the tri-
angle sides ab and ac. Produce ac beyond a and check the dis-
tance ac again to be equal to a' a. The line be represents the
horizontal direction from the plotted station b to the plotted
reference point C (its image on the negative, Fig. 57, Plate XXXI,
was a) and we will now have:

ba = trace of the principal plane ;

ac = trace of the picture plane ;
a = projection of the principal point on the plotting-sheet.

The trace of the principal plane = ab is preferably marked
by a short line only, bearing an arrow pointing toward the plotted
station whence the picture was taken, and the principal point a
is marked to correspond with the designation of the print, and
it may be remarked here that as few constructive lines as pos-
sible are drawn on the working-sheet to avoid confusion and
mistakes (see photograph-board, section 10, page 148).

4. THE IDENTIFICATION OF PICTURED POINTS IN PHOTOGRAPHS
REPRESENTING IDENTICAL POINTS OF THE TERRENE.

The survey being plotted, analogous to a plane-table survey,
mainly by intersections of horizontal lines of direction, points
controlling the same area must be identified on pictures taken
from different stations. When selecting such points on a photo-
graph, preference should be given to such that best define the



GRAPHICAL ICONOMETBJCAL PLOTTING METHODS. 139

surface or terrene like characteristic points of mountain ridges,
peaks, saddles, points at the changes of slope, bends in streams, etc.,
each point being marked by a dot in red ink on the photograph
and affixing a number or symbol to it. It will now be necessary
to identify as many of these points as possible on other photo-
graphs covering the same area, marking these also by dots and
giving identical points the same designation, by number or by
symbol in red ink, as on the first photograph.

The identification of geodetic points on several pictures offers
no serious difficulty, and with some practice as many points
as may be needed for a full development of the terrene, even
under different illumination of the pictured areas, may be picked
out with rapidity and precision. In case of doubt, beginners
may resort to Prof. Hauck's method, which has already been
mentioned several times in the preceding pages'.

5. APPLICATION OF PROF. HAUCK'S METHOD FOR THE IDENTIFICATION OP
TERRENE POINTS PICTURED ON SEVERAL PHOTOGRAPHS.

The two photographs, picturing the same areas as seen from
different stations or points of view, are pinned side by side on a
drawing-board. The images of the camera stations whence the
pictures were obtained are kernel points; if they fall outside
of the limits of the pictures their projections on the picture
traces may be determined from the plotting-sheet or working-
plan. The parallels to the principal lines, on which the scales
are to be placed, are drawn as explained in Chapter IV, para-
graph VII, section 2, and the scales are fixed in position.

A fine needle is inserted into the drawing-board through each
of the kernel points and the loop at one end of a fine silk thread
is dropped over the needle, the other end of the thread being
fastened to a small weight by means of a slender rubber band
(see Fig. 60, Plate XXXII).

A well-defined point is now identified on the two photo-
graphs sufficiently far from the kernel points, and one thread
is moved (by taking the small weight in one hand) to pass



140 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

through the point identified on the photograph; the weight
is deposited on the drawing-board, holding the thread in this
position under slight tension of the rubber band b.

The same operation is repeated with the other thread and
the other photograph, when the two threads should intersect
the scales at the same division mark; if they do not, one of the
scales is to be moved until identical division marks are bisected
by the two threads. The identification of other geodetic points
pictured on both photographs may now be proceeded with:

Having selected a characteristic geodetic point on one of
the photographs, the corresponding thread and weight are moved
until the thread bisects that point. Noting the point of intersec-
tion on the scale by the thread in this position, the other thread
is now moved to bisect the corresponding graduation mark on
the second scale. The second thread will then also bisect the
corresponding image of the same geodetic point on the second
picture.

6. PLOTTING PICTURED TERRENE POINTS AS THE INTERSECTIONS OF LINES
OF HORIZONTAL DIRECTIONS. ICONOMETRIC PLOTTING OF TERRENE
POINTS IN GENERAL (" HORIZONTAL INTERSECTIONS").

After enough pictures have been selected to develop a cer-
tain area, and the identification and marking of the images of
corresponding geodetic points have been completed, projec-
tions of all these points on the horizon lines of the pictures are
marked and transferred to the straight edge of a strip of paper,
including in this transfer the marking on the strip's edge of the
principal point of every photograph.

The strips are given the same designation as the pictures
to which they belong (by number or symbol) and they are
then placed upon their picture traces on the plotting-sheet in
such a manner that the principal points of picture trace and
paper strip coincide. They are secured in this position on the
working- sheet by means of small weights or fine thumb-tacks.

To plot the horizontal projection of a geodetic point, shown
. i d marked on two photographs and marked on the correspond-



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 14!

ing paper strips, two fine needles are inserted into the plotted
stations / and //, Fig. 60, Plate XXXII, of the two photographs.
A fine silk thread is attached to each needle. The other end
of the thread is connected with a small weight W by means of
a fine rubber band b.

The thread looped over station needle I is now moved over
the paper strip (indicating the picture trace on the plan) until
it bisects the projection a' of the geodetic point's image. The
weight is now placed upon the paper, holding the thread under
slight tension of the rubber band in this position.

The second thread, connected with the needle in station //,
is placed over the horizontal projection a" of the image of the
same point A. The point of intersection A of the two threads
will be the desired position on the plan of the point A.

After this position of A upon the plotting-sheet has been
checked by means of another photograph taken from a third
station ///, and containing the image a'" of the point A, its
plotted position is marked by a dot in red ink and its designa-
tion, corresponding with that given on the prints, is also affixed
in red ink. A sufficient number of points having been plotted
in this manner, and all having been supplied with the same let-
ters or numerals (in red ink) that had been given their images
on the photographs, their elevations are determined and also
affixed to the points in red ink. Frequently the designation
of the point by letter or numeral is added hi pencil on the
working-sheet, to be erased after the elevation of the point has
been affixed to it in red ink.

In case the strips of paper on the picture traces should over-
lap, as shown in Fig. 61, Plate XXXII, the part CD of the pic-
ture trace PQ is marked off on the strip MN lying under it,
the band of paper PQ is then placed in proper position and the
marks on its edge are transferred to the line CD. The strip PQ
is now placed under MN, the marks on the latter, along CD,
serving the same purpose as those of PQ.

When a station, A, Fig. 62, Plate XXXII, falls so close to



142 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

the edge of the working-sheet that the trace QR of the picture
plane falls outside of the limits of the plan, then the trace AC
of the principal plane may be produced to B, AB being equal
to A C = focal length of the picture, and MN, drawn perpen-
dicular to BC or parallel to QR, will occupy with reference to QR
the same position as the focal plane of the camera does to the
image plane of the perspective.

The direction of a point of the photograph projected in Q
on the picture trace is found by joining NA and producing
to the opposite side of A.

As already mentioned, the intersection of two lines of direc-
tion, establishing the plotted position of a geodetic point, pic-
tured on two photographs, should be checked either by a third
line or otherwise, before the position of such point should be
accepted on the plan as correct.

Such intersections may, for instance, be checked by deter-
mining the elevation of the point from both photographs. Unless
the point has been correctly plotted, these two heights will not
agree. This check, however, does not guard against slight
errors in position.

A check may also be obtained by drawing a line, on which
the point may be situated (for instance, the shore line of a river
or lake), with a perspectograph or perspectometer; still, the best
check will always be afforded by a third intersecting line of direc-
tion obtained from a third photograph.

7. ICONOMETRIC PLOTTING OF PICTURED TERRENE POINTS BY SO-CALLED

" VERTICAL INTERSECTIONS."

We have seen how the base line between two stations is pro-
jected into horizontal plan when using the method of intersec-
tions of horizontal lines of direction, hitherto considered, but
when two camera stations are occupied at different elevations,
and not far apart horizontally, to locate geodetic points by inter-
sections of lines of direction, the so-called " method of vertical



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 11 of 33)