AERIAL PHOTOGR“

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A Thesis Submitted to

The Faculty of
MICHIGAN STATE COLLEGE
of

AGRICULTURE AND APPLIED SCIENCE

by

Robert Owen Schaeffer
Candidate for the Degree of

Bachelor of Science

June 1938

THESIS

ACKNOYLEDSMEIT‘

lThe author wishes to acknowledge
the helpful COOperation of the staff
of Abrams Aerial Survey Corporation,
Lansing, Kichigan. It was from tie ex-
perience and knowledge of these gentle-
men that the material for this thesis
Was obtained.

Also Professor C. M. Cade, De-
partment of Civil Engineering, Michigan
State College for his willing and
helpful advice.

Robert O. Schaeffer
Class 1958

116398

II
III

IV

VI
VII
VIII

INDEX

Part I

.Aerial Photography

Introduction
History
Camera

Plane
Photographing
Index Map
Mosaic Map
StereoscOpe

Contour Finder

Part II

Page

10
10
12
12

Mapping from Vertical Air Photographs

Part I

Aerial Photography

(1)

INTRODUCTION

The development of the airplane was followed by
the development of aerial photography. The great rapidity
with which large areas can be photographed make the method
of photography of great value for commercial and military
purposes. Photographic maps can be made in a small fraction
of the time required by any other method of mapping, and
the airplane can reach terrain that for some reason may be
inaccessible to the surveyor or sketcher. Photographs are
timely, and portray many details of the terrain that are
lacking on the most complete topographical map. Thus
aerial photographic maps are of great value to the sur-
veyor in both the commercial and military.

I will try to describe in the following pages
the progress of this science and the type of equipment and
methods used in making maps from aerial photographs. One
should not get the impression from this thesis that the
following described methods are the best or most used, as
the material used here was gained from observations made at
Abrams Aerial Survey Labratories in Lansing, Michigan. It
also must be remembered that this science is now progressing
at such a rapid rate that what may be a good method today

will be obsolete tomorrow

(2)

HISTORY

The science of aerial photography began in 1861,
when two men made an oblique photograph of Boston from a
captive balloon at an altitude of 1200 feet. Although
much information was gained from this and other later
flights, aerial photography was slow in developing until
the development of the airplane. By 1911 the airplane had
been developed to the point where passengers and equipment
could be carried, thus photographing from the airplane
became possible.

The airplanes of this time were light in con-
struction and the pilot and passenger sat in open cock-
pits. It is known that the motion of the airplane in the
air causes wind of realitively high velocity. This wind
made aerial photography very difficult with this early
type of equipment. Clear pictures were hard to obtain be-
cause of this wind and the motion of the plane. To over-
come this, cameras were designed especially for aerial use.
Plates have been replaced by roll film which is much lighter.
Multi lens cameras are now used so as to cover a large area
in one exposure.

Though slow in development in the commercial
field, aerial photography has been used to quite some ex-
tent in the army. During the World War aerial photographs
were taken of enemy position and used to a large extent by

the artilleries in preparation of firing date. They have

(5)

developed it now so that an airplane can take a picture
of the enemy positions, develOp it in the air, and drOp
the print to the supporting artillery. This is all done

in a short space of time.

(4)

CAMERA

There are various types of cameras on the market
for use in aerial photographic work. These cameras can
be divided into three classes:

1. The single lens type, with focal §Ength varying
from 6 to 28 inches, with between the lens shutter or focal
plane shutter. Uses film.

2. The Brock Camera, which is a special type that
uses plates instead of film. This type is not very common.

5. The air service camera having one or multiple
lenses in a group. The focal length varies from 6% to 7%
inches. This type uses film.

A good example of the single lens camera is the
Fairchild, Type 4, general purpose, which is used by Abrams
Aerial Survey Corporation of Lansing. This is a fixed focused
type using an 8% inch lens. These lenses are especially
selected so as to get the greatest amount of accuracy. The
focal length of the camera is fixed but can be changed by
changing the cone and the lens. The distance from the lens
to the film is fixed and focused at infinity which will
allow for the small variations in altitude of the airplane
' while pictures are being taken. This type uses roll film
9% inches wide and 75 feet in length with 110 exposures.

Each exposure is 7 by 9 inches. Some of the principal ad-
vantages of this type of camera are: the large image area,

shorter focal length, greater amount of film. The large

(5)

image area gives a better and greater coverage, the short
focal length allows the plane to fly closer to the ground,
and the greater amount of film makes duplication of prints
possible.

The camera constructed of a light metal, such as
Dow Metal, is mounted in gimbals. (Some have a fixed mount
thus giving tilt to many of the exposures). Each camera
has a view finder so as to beable to check exposure. The
photographer levels his camera, so as to get a vertical shot
and checks his view finder beforeeach exposure. Most
American makes of cameras are electrically driven while some
foreign makes are driven by a wind mill on the outside of
the airplane. The mechanism driving the camera sometimes
causes static. Static isvery undesirable as it causes black
dots on the film.

Multi-lens cameras such as those used by the air
service have from one to ten lenses in a group. In this typ e
of camera the exposure is usually made with the axis of the
central lens vertical but the other inclined at a large
angle, the latter all taking oblique pictures. (Fig. 1).
These oblique photographs called wing pictures all have to
be rectified so as to give the equivalent of a vertical photo-
graph. The method of doing this will be described later.
The advantage of this type of camera is the great amount
of area photographed in a small amount of time. Its main
disadvantage is in rectifying the wing picture Which leaves

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PLANE

The type of airplane used in photography should
be designed for the work. It should have means of rapid
climbing so that time will not be lost in gaining the
required altitude (22,000 feet). There should be means
of good visibility for both the pilot and photographer.
The pilot requires visibility not only ahead but also below
so as to maintain his course. The photographer needs the
visibility to enable him to do a good Job of photographing.
The ship must also be as nearly stable as possible so as
to eliminate distortion due to tilt and maintain a constant
altitude and speed so as to give the correct scale to the
picture. An airplane containing these features has re—
cently been designed by Talbert Abrams of Abrams Aerial
Survey Corp. of Lansing, Michigan. This plane is of the
pusher type and the nose is all glassed in so as to give
maximum visibility. This plane is capable of gaining an
altitude of 20,000 feet in 20 minutes--200 miles per hour.

 

 

 

 

 

 

 

 

 

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PHOTOGRAPHING

In making the photographs the personnel must
be highly specialized. As the airplane flies at altitudes
between 20,000 and 25,000 feet the pilot and photographer
must wear oxygen masks. As this work requires that the
men be on their toes all the time, they do not depend upon
the thin air at these altitudes for their oxygen, as it is
insufficient.

There are two general classes of aerial photo-
graphs--vertical and oblique. Vertical photographs
(Plates I & II) are made from any altitude with the axis
of the camera lens perpendicular to the ground, that is,
with the camera pointed straight down. Oblique photographs
are made from relatively low altitudes with the axis of the
camera deviating from the vertical to a pronounced extent,
the best results being obtained when the axis of the camera
lens is inclined approximately 30 degrees from the hori-
zontal. The oblique photograph is the more artistic and
pleasing of the two, and gives one a splendid idea of the
terrain pictured. Buildings, roads, streams, etc. stand
out clearly in perspective, but the photograph is not to
‘ scale. It is because of this last reason that only vertical
photographs are used in mosaic and index maps which are to
be used as surveys.

The photographs are taken in flights. The pilot
sets a course and flies in a.straight line at a constant

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in the flights are taken at such an interval so as to give

a 60 per cent overlap (Plates 1 & 2 ). Each flight over-
laps the previous flight (side lap) about 50 per cent.

This overlap allows for the distortion at the edge of

prints and one can also use radial control when assembling
the map as each control point is on three prints. The over-
lap between flights also allows for a small amount of drift
the plane might make from its straight line course.

Each roll of film is labeled as to flight and
area after exposure. The film is then taken to the labora-
tory where it is developed by methods similar to other
photographic developments except more care is taken in the
process. After the film is dried and each print is labeled
it is ready to be printed. Each negative on the roll makes
a separate print. This is done by placing each print on
the printing machine in order and exposing it to sensitized
paper. The source of light for exposure in this machine is
from a group of light bulbs below the glass on which the
print is made. Each bulb is controlled by a rheostat,
thus enabling the operator to control the intensity of
light on different parts of the negative. This blends the
shade of the print, so as to give a clear cut photograph.

In exact mapping work a distortion corrector should
be used. The distortion corrector is an exclusive invention
of Talbert Abrams. Before this invention, it was impossible

to produce a photograph made with a lens where the distance

to the object was greater than the focal length of the
lens without multiplying scale error as the distance ex-
pands from the center of the negative. This is called
paralax or distortion due to change in elevation or tilt
of either the camera or the ground, and angular displacement
of ground objects due to the curvature of the lens. These
distortions are corrected in this machine by fastening the
print on the inside curve of the bowl and projecting it on
sensitized paper. Theczurve of this bowl matches and off-
sets the curve of the lens. Thus a map photograph can be
produced with accurate scale even at the extreme edges.
There are several types of projectors used to
make the finished print. The most common is the one that
makes the print the same size as the negative which has
been discussed previously. Other projectors used vary the
size of the print. The negative is put in the projector and
is projected on sensitized paper either larger or smaller

than the original.

INDEX MAP

After the prints are finished they are made either
into an index map or a mosaic. An index map is made by
laying the pictures in the order they were taken overlapping
both in line of flight and sidewise. This gives a map with
index numbers on each print. This index map is then photo-
graphed (Plates 1 and 2). The negative and prints of this
index map are then filed away for future use. When a
question arises about some portion of the map a look at the
index map will tell, from the index number which print is
needed. The original print is then found from the files

in a short time.

MOSAIC MAP

The mosaic method of mapping differs decidedly
from the index map. In a mosaic the series of photographs
are fastened together so as to form a composite picture.
When the prints are oriented by matching details, it is
called an uncontrolled mosiac; when they are oriented by
means of selected points of each print they are controlled
mosiac. These points are control points. A semi-controlled
mosaic is when the prints are laid on a straight that passes
through the principal points of each photograph. Radial
control is accomplished by locating a point in the center
of each photograph, one near each corner, and a point near
the center of each side of the print. The outer points are

connected to the central control point by radial lines.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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(11)

Then by use of slotted or scratch tempelets the prints are
oriented and ready for laying.

In laying the mosaic care must be taken to lay
to color, contour, and match. On different prints an ob-
ject might appear in two slightly different shades. The
two prints must be cut and lain so as not to show a dif-
ference of color from one to the other. The objects must
also be cut to match. In the case of a very irregular
terrian the photographs should be cut on the contour so
as to show distortion or displacement. At the same time
the color and the match should be controled.

The finished mosaic is then photographed to the
desired size. The negative of the mosaic is then touched
up so as shades out the lines where it was put together and
to fix slight color differences. The negative is then
printed giving a map all of one piece and with all the de4

tails complete without showing how it was put together.

(12)

STEREOSCOPE

The steresc0pe enables one to see the picture in
three dimensions. This consists of two photographs
of the same object or terrain, taken from slightly dif-
ferent positions. The area in the 60 per cent overlap
of two consecutive pictures constitute a stereoscopic
pair (Plates 1 and 2). The photographs can now be
viewed through a mirror stereoscOpe (Fig. 5). The ob-
ject seen will be in three dimensions. It will show not
only length and width but also the heighth. The ex-
planation is fairly simple. As our eyes are approxi-
mately 2% inches apart each eye views an object from a
different point of view and obtains a slightly different
image. The merging of these two images by the stereoscopic
power of our eyes gives us a distinct impression of
depth.‘ This is what is accomplished with the stereoscOpe.
The camera acts as the eye getting a picture of the image
from two different views. Then with the aid of the stereo-

scOpe we are able to get the third dimension.

CONTOUR FINDER
The contour finder is a stereoscOpe with a few
additions. The contour finder consists of the following:
a folding single lens magnifying stereoscOpe, micrometer
adjustment for measuring paralax, drafting arm for
scribing contours, and drafting board with swinging arm
adjustment to hold the contour finder in adjustment,while

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of two glass slices with a dot in the middle. These
slides fit in a frame anm the right one can be moved

by the micrometer adjusting screw. Place the two dots
on the same object on their respective print. Now

look through the stereoscope and adjust the right dot
until the dots appear as one and at the level of the
ground. Read the micrometer. Next move the rhiht slide
until the dot appears at the top of the object and again
read the micrometer. This difference in reading multi-
plied by a factor gives the height of the object. This
factor is obtained by actually measuring the height of
the object and dividing by the height found with the con-
tour finder. The drafting arm and drafting board are
used to scribe the contours on the same print or dup-
licate prints of the same size or even larger or smaller.
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Part II
Mapping from Vertical

Air Photographs

(l)

MAPPING FROM VERTICAL AIR PHOTOGRAPHS

The following pages give a brief description
of the method of mapping by means of the Topographical
Stereoscope, known as the "Arundel" method. As the
principle is the same in each Stereoscope, a description
of the steps necessary for the production of a map by one
type of Stereoscope will be sufficient.

The method of plotting consists of fixing a
control on the first overlap of a strip of vertical
air photographs and intersecting this control, this in-
tersected control being then used to resect the position
of the next photograph, and so on throughout the strip.
A strip of vertical air photographs consists of photo—
graphs exposed at regular intervals along a straight
flight, giving an overlap usually about 60%. Before
any system of intersection can be applied, it is necessary
to find some point on the photograph from which directions
can be transferred direct to the plot. Except in the
case of a perfectly vertical picture of flat country,
the photograph will not be of a uniform scale, so that
it is obvious that any point will not serve the purpose.

In the "Arundel" method the point chosen is the
principal point and it is assumed that angular measure-
ments from this point of a nearly vertical photograph
are equal to the horizontal angles between corresponding
points on the ground. This assumption is justified for
photographs of tilt not more than 2°; therefore it is

essential to have good level flying. It is also necessary

(2)

to have a rapid means of detecting accidental tilts, so that
any errors due thereto can be rectified by a closer ground
control. This is accomplished by drawing a "Minor Control
Plot" as described later.
The "Arundel" method is intended only for the

surveying of areas where differencgg of ground heights
do not exceed one-tenth of the flying height. Where
differences exceed this amount the "Clova" method is
used and in this method it is necessary to determine the
absolute vertical or plumb point, the "Clova" method
assuming that the angular measurements from the plumb
point are equal to the horizontal angles between corres—
ponding points of the ground.

- Accurate maps on a scale of l/20,000 can be pro-

duced by these methods.

follows.

E.
F.

G.

SUHMARY OF METHOD

The steps in the process may be summarised as

Details of each step are given later.

Mark principal points on all photographs to
be used.

By means of the Topographical Stereoscope draw
principal point base or line of flight.

Decide on minor control points, two per photo-
graph, and draw radial lines from principal
points to intersect minor control points. Each
minor control point should appear on three
successive photographs.

Draw a minor control plot on a suitable tracing
medium.

Analyse the minor control plot to find tilts.

Mark on the photographs information obtained
by ground parties.

Draw contours on every second photograph by
means of the Topographical Stereoscope.

Plot on the minor control plot tracing the
desired detail from the photographs.

Reduce minor control plot and added detail to
required scale by photography.

Make bromide prints for final fair tracings.

(4)

DETAILS OF STEPS

A. Mark the principal point on each photograph
throughout the strip, the principal point being the
centre of the photographic plate, or, more correctly,
the foot of the perpendicular from the rear nodal point
of the lens to the photographic plate. Some aerial cameras
record on the edges of each plate two small horizontal
and two small vertical lines, known as collimating marks,
and if these are joined, they will cross at the principal
point of the photograph. If these lines are not avail-
able or if the principal point is not otherwise indicated,
it can be obtained by intersecting portions of the diagonals.

B.l- Place the first two photographs of the strip
one over the other on a flat table, as if they were to be
joined in a mosaic, the overlapping portion common to the
two photographs being roughly in coincidence. Turn both
photographs round together until the run of shadows falls
as far as possible away from the source of light under which
they are to be examined. In this position the print to
the left willbe placed on the left turntable of the Topo-
graphical Stereoscope and will be seen with the left eye.

2- Adjust the left grid of the Topographical Stereo-

scope until one of the vertical lines on the base line
(that is, on the horizontal centre line of the grid) is
exactly in coincidence with the centre of the left turn-

table.(Fig. A) Accurately adjust the corresponding line
on the right grid to the centre of the right turntable with-

out moving the left grid.

(I‘

(5)

5- Without looking through the eye-holes of the
stereoscope, place the left photograph below the left
grid so that the principal point lies exactly below the
vertical line selected on the grid (1.6. so that the prin-
cipal point is exactly on the centre of the turntable),
and rotate the photograph until a point of detail which
appears close to the principal point of the right photo-
graph lies approximately on the left grid base line.

4- Similarly, fix the principal point of the right
photograph above the centre of the right turntable and
rotate the photograph until a point of detail which appears
close to the principal point of the left photograph lies
approximately on the right grid base line.

5- Fix both photographs by means of the clamps
supplied.

6- Choose a particularly distinctive object, which
appears on both photographs, and observe this object through
the eye-holes of the stereoscope. Adjust the two small
mirrors by turning the milled head at the front of the
mirror framework until the selected object can be seen
stereoscopically without straining the eye.

7- Observe the grid stereoscopically as well as
the ground, although not concentrating on the grid, and
it will be seen to rise and fall in relation to the ground
as the right grid is moved longitudinally. Displace the

left grid until one of the intersections of the N.E. and

(6)

N.W. lines lies over a point of detail at the principal
point, or slightly to the left of the principal point,
of the left photograph.

8- Displace the right grid until the intersection,
or what appears to be the lower line of the intersection,
lies just above the point of detail selected on the left
photograph.

‘,9- If both cross lines do not appear to touch
the ground at the point of intersection, the orientation
of the right photograph is at fault. To correct this,
rotate the right turntable very slowly until both cross
lines appear at the same depth when just above the point
of detail being observed.

10- Similarly, correctly orient the left photograph
by observing on a grid cross set on the right principal
point or slightly to the right of the principal point.

11- To mark the principal point base, raise the left
grid, place the steel straight edge against the stops pro-
vided, and draw a thin line from the centre of the photo-
graph to the right edge. This line is also drawn back-
wards to the left to form a tail at the edge of the photo-
graph. Raise the right grid and, again by means of the
straight edge, draw a line from the centre to the left edge
and also mark the tail on the right edge.

12- Transfer the right picture to the left turntable,
place the next picture on the right table and in a similar
manner obtain the principal point base - or line of flight,

(7)

as it is also called. Continue doing this until the line
of flight has been drawn along the whole strip.

C.l- Choose two points of detail A, B, in the small
overlap common to the first three photographs, so that
plp2, p2A, and p2B are all approximately equal, and such
that ApQB is nearly perpendicular to the principal point
base plp2. (Fig. B - 1)

Note: In this and subsequent paragraphs, the
principal point of any photograph is designated by the
letter "P" and a numerical suffix. With this notation
P5 is the principal point of the third photograph of a
strip. Similarly, the point of the third photograph, say,
corresponding to the princiapl point of the second photo-
graph is designated by P2.

2- Identify and prick with a fine needle the points
A and B on all three photographs. Similar points C and D
are then chosen on the second, third and fourth photographs
and so on throughout the strip. These points are known
as minor control points.

5- Draw short lines through the minor control points
from the principal point of the photograph on which they
appear.

4- Ink in red or orange the short lines passing
through the minor control points and the principal point
bases and their "tails". Any photograph, after the first

two at either end of the strip, should now contain six

minor control points, furnished with short radial lines
and two base lines complete with "tails."
D.l- Cut a strip of matt-surfaced drawing celluloid
to sufficient length and breadth to include the whole strip
of photographs.
2- Place the first photograph under the celluloid
in such a position and orientation as to confine the whole

strip plot to within the limits of the sheet of celluloid.

3- Fix the first photograph of the strip and celluloid.

down flat by drawing pins.
4

Prick through the points A and P1.

5- Trace in light blue ink the radial directions of
B and the first principal point base P1P2. (Fig. B)

6

Remove the first photograph and replace by the
second photograph, placing it so as to make P1P2 coincide
in direction with the tracing PlP2. (A good fit is ob-
tained as the red or orange lines on the photographs show
an unmistakable heliotrope when in perfect coincidence
with the blue lines on the celluloid). Still maintaining
this orientation, move the tracing over the print until
the radial direction to A passes through the pricked position
of this point.

7- Fix down the tracing securely, prick through the
principal point P2 and trace the radial directions of B,
C, D, and of the next principal point base P2P5. B has

thus been intersected on the scale of the plot

(9)

8- Place the third photograph under the tracing,
orient along the principal point base P2P5 - P2P3 and
slide along this common direction until the radial direction
to A passes through the plotted position of this point.
The radial direction to B should now pass through the plotted
position of B.
9- Trace in the radial directions and principal
point base and continue until the whole strip has been
traced.
The minor control plot thus drawn is a plan of
the minor control points and principal points on an unknown
uniform scale. The scale of the plot is actually the
local scale of the first photograph in the vicinity of the
minor control point A. It is best to commence the plot
at approximately the same scale as the area of heaviest
detail. As such an area may occur at any part of a strip,
the minor control plot need not therefore be commenced at
the first photograph but could be commenced at any part of
the strip and built up by working in both directions.
It will be seen that no check is available on
the position and orientation of the second photograph, while
the third and succeeding photographs are each checked from
two other photographs.
E. As the photographs are taken on a fixed time
interval, the plotted positions of the principal points
will be equally spaced along a straight line, provided the
flying has been good.

(10)

1- If the principal points are not equally spaced
this is probably due to fore-and-aft tilt.

2- If the principal points do not lie on a straight
line, this may be due to departure from a straight course
or due to lateral tilt.

The presence of tilt in photographs of compara-
tively flat.country can be checked in the Topographical
Stereoscope by means of the grids. If tilt is present,
the grid will appear deformed and its deformity relative
to the ground at any point and in any direction affords an
indication of the tilt.

Where tilt is found, it is necessary to have a
good ground control.

F. If no tilt has been found from the minor control
plot, a single strip of vertical photographs may be plotted
without ground control. Relative errors of position and
orientation may occur within the strip plot, but the de-
tail will be true to shape and the result may often be
all that is necessary for many purposes.

For more accurate work and to prevent an undue
accumulation of error, it is necessary to provide a net—
work of fixed positions surveyed on the ground.

From information obtained from the ground
parties:

1- Locate and mark, with a green dot on the photo-
graphs, the position of all measured heights. Mark the
actual height beside the dot.

<11)-

2- Locate and mark on the photographs all other
positions Which have been surveyed on the ground.

3- Draw short colored radial lines through these
positions in the same manner as for the minor control
points.

(On very steep slopes the shape of the ground
detail will appear quite different on the two photographs.
Corresponding points for intersection should in that case
be identified and marked in the Topographical Stereoscope).

G. Once a network of heights has been fixed by
measurement, the actual contours are best interpolated
in the Topographical Stereoscope, the whole area of an
overlap being examined at one time. Further heights can
be obtained from the photographs by taking parallax
measurements with the Stereoscope. Where tilt is not
considerable and an unknown height lies between two known
heights, it may be assumed that differences in parallax
are directly proportional to differences in height.

Parallax differences are measured in the Preci-
sion Topographical Stereoscope as follows:-

(a) Take one of the vertical lines of the left grid
alongside, but not actually touching, one of the points
whose difference of parallax is to be measured and then
move the right grid until the fused line appears to lie
at the same depth as this point.

(b) Note the reading of the scale.

(c) Turn the left working head until the line on the

left grid lies alongside the second point to be observed
and then move the right grid until the fused line appears
to lie at the same depth as the second point.
(d) Again note the reading of the scale. The differ-

ence between the two readings is the required difference
of parallax.

With a little practice parallax readings to 0.02-mm
can be obtained.

This system can also be applied to photographs
which are considerably tilted, subject to certain limitations.

A modified and somewhat complicated procedure may
be used where the unknown heights do not lie between known
heights, by making a calculated allowance for the influence
of tilt on parallax.

 

Having obtained additional heights, as described
in the foregoing, contouring can be done my means of the
Stereoscope.

The contours are drawn on every second photo-
graph only, the intermediate overlapping photographs being
used only to provide the necessary stereoscopic impression.

H. Detailed plotting can be done on the minor control
plot between the intersected positions of the control points
or between a closer intersected control. Tracing of de—
tail between the intersected control is carried out by
fitting triangles, that is, the areas within sets of three

control points.

As a check, the points surveyed on the ground
should be marked on the photographs and short radial
lines drawn through them as previously described. These
points should also be drawn in their correct positions on
the minor control plot to the mean scale of the photographs.

The plotted positions of the ground control
points should be compared with their true positions. Any
discrepancy between them will indicate the amount by which
the plotted positions have to be shifted and the detail
can then be traced from point to point.

Cases may arise where sufficient ground control
already exists to enable the photographs to be used for
detail plotting without drawing a minor control plot. The
ground control points (which should not be fewer than four
per photograph) are then plotted at the mean scale of
photography on a sheet of drawing cellulOid.

The lateral strips are usually flown so as to
give a lateral overlap of about 50% and in compiling a
map the lateral strips are Joined through this overlap.

I. By photography reduce the minor control plot and
added detail to the desired scale, due allowance being
made for the shrinkage of the paper prints.

J. Make fair tracings from the bromide prints
supplied from the photographs.

Blocks for printing the various colors required

on the final map are made from the fair drawings and a

blue "pull" is taken from the block containing the out—
line of the country, rivers, roads and position of towns.*
To this "pull" are added the names of the various towns,

rivers, etc.

 

 

 

 

 

 

 

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