MAP MAKING
=Technique of Negative Making.=—Stated in its simplest terms, the whole
problem of making a photographic map from the air consists in taking a
large number of slightly overlapping negatives, all from the same
altitude, with the plane flying uniformly level. When trimmed and
mounted in juxtaposition, or pasted together so as to overlap in their
common portions, the prints from these negatives constitute a complete
pictorial map. There is thus furnished by a few hours' labor topographic
information which would be the work of months to obtain by other means.
The making of map photographs involves all the special technique of
spotting, with much in addition. The pilot's task is not merely to go
over one object; he must navigate a narrow path, at a constant altitude,
on an even keel. If he is to make not merely a ribbon, but a map of
considerable width, he must take successive trips parallel to the first,
each displaced just far enough from the previous course to insure that
no portion is missed—a difficult task indeed.
It is the observer's duty to so time the intervals between exposures
that they overlap enough, but not so much as to be wasteful of plates or
film. He must also change magazines or films so quickly as to miss no
territory, or if some be missed, his is the task of directing the pilot
back to the point of the last exposure, where they begin a new series.
Level flying is entirely a pilot's problem. Its importance will be
realized when we consider the accompanying diagrams (Figs. 134 and 135),
where the effect on the resultant picture is shown of climbing, gliding,
or banking to either side. Prints from negatives distorted in this way
neither will be true representations of the territory photographed, nor
will they match when juxtaposed. In fact, they can be utilized only if
special rectifying apparatus is available for printing. Flying at a
constant altitude is similarly necessary if the prints are to be
utilized without enlargement or reduction in order to make them fit.
[Illustration:
FIG. 134.—Diagram showing effect of banking on aerial photograph.]
[Illustration:
FIG. 135.—Diagram showing effect of climbing and diving on aerial
photograph.]
Assuming a skilled pilot who will do his part, the next step is to
calculate the exposure intervals in order to insure an adequate overlap.
If a negative lens is installed which has been marked with a rectangle
the size of the camera field, the simplest method is to estimate the
proper instant for exposure by watching the progress of objects across
the lens face. This of course requires constant attention, and it is
easier to do this only occasionally, in order to determine the ground
speed in terms of camera fields traversed per minute. Thereafter
exposures are to be made by time, as determined by a watch or clock. Any
desired degree of overlap can be chosen, and either estimated, or more
or less accurately fixed by lines marked on the negative lens at a
shorter distance apart than the edges of the field. The most usual
overlap is 20 per cent., except for stereos, which call for 50 to 75 per
cent.
In the absence of a negative lens or some other sight to show the whole
camera field, it is necessary to resort to calculation from the speed
and altitude of the plane, the focus of the lens and the dimensions of
the plate. If _A_ is the altitude, _a_ the focal length of the lens, _d_
the diameter of the plate in the direction of travel (usually the short
length is chosen for economy of flights to cover a given width), _f_ the
fractional part by which one negative is desired to overlap the next,
and _V_ the ground speed of the plane, then we have, by simple
proportion, that the interval between exposures, _t_, must be—
_Ad_(1 - _f_)
_t_ = —————————————
_aV_
If _A_ = 2000 meters, _d_ = 18 centimeters, _f_ = ⅕, _a_ = 50
centimeters, and _V_ = 200 kilometers per hour, this relation gives—
2000 × .18 × .8 × 3600
_t_ = —————————————————————— = 10.3 seconds
.5 × 200,000
The principle of overlapping map exposures is shown in the accompanying
diagram (Fig. 129), together with data calculated as above for a 4 × 5
inch plate.
It is particularly to be noted that it is the _ground speed_ of the
plane that is used. This may be calculated by knowing the air speed and
the wind velocity and direction. Fig. 136 shows the method of doing this
graphically. First an arrow is drawn representing the direction it is
desired to fly. Next a second arrow is drawn of length to represent the
wind velocity. This must be inclined toward the first arrow in the
direction of the wind, and its head is to touch the head of the first
arrow. Then with the farther end of this second arrow as a center,
describe a circle of such a length as to represent the air speed of the
plane, in the same units as the wind velocity. Connect the point where
this circle cuts the arrow of flight direction to the center of the
circle by a straight line. This line constitutes the air speed arrow,
giving the direction it is necessary to fly, at the given air speed, to
make the course desired. The length of the flight direction arrow
between its head and its point of intersection with the air speed arrow
gives the ground speed.
[Illustration:
FIG. 136.—Diagram showing method of calculating ground speed from air
speed and wind velocity.]
When the wind is ahead or astern this calculation reduces to the simple
subtraction or addition of the wind velocity to the air speed of the
plane. Whenever possible, mapping should be done up and down the wind
(Fig. 137). If the plane is “crabbing,” the above calculations for
overlap are only valid if the camera can be turned normal to the
direction of travel over the ground. If the camera cannot be so turned
the corners of the successive pictures overlap instead of their sides,
with quite unsatisfactory results (Fig. 138).
Calculation of the distance apart of the parallel flights necessary to
make a map of any width is done by the use of a formula similar to the
longitudinal overlap formula above, distance figuring instead of time.
Using the same symbols, and denoting the distance by _D_, we have—
_Ad_(1 - _f_)
_D_ = —————————————
_a_
With the same figures as before, but substituting 24 centimeters for the
plate dimension, this relation gives—
2000 × .24 × .8
_D_ = ——————————————— = 768 meters
.5
[Illustration:
FIG. 137.—Overlaps made when flying with or against the wind.]
[Illustration:
FIG. 138.—Unsatisfactory overlaps made when plane is “crabbing.”]
It is of course largely a pilot's problem to steer the plane over
parallel courses at a given distance apart, although the observer,
noting conspicuous objects through a properly marked negative lens, may
direct the pilot by any of the means of communication already mentioned.
An alternative method of securing parallel strips, which is to be highly
recommended where enough photographically equipped airplanes are
available, is for several planes to fly side by side, maintaining their
proper separation (Fig. 139).
=Cameras and Auxiliaries for Map Making.=—Mapping can be done quite
satisfactorily by hand operated or semi-automatic cameras, provided the
observer has not too many other duties. On the other hand, the operation
of exposing at more or less definite intervals of time, irrespective of
the object immediately presented to the camera, is a largely mechanical
one. It naturally suggests the employment of an automatic mechanism,
whose speed of operation only is it necessary to watch.
If a non-automatic camera is used the timing of exposures may be done by
watching a negative lens, as described above, or by reference to a
clock, assuming that the ground speed is known through calculation. A
very practical advance over the ordinary use of a clock is to attach a
stop-watch to the shutter release, so that it is turned back to zero and
re-started at each exposure (Fig. 70). In passing, it may be noted that
if the stop-watch hand makes an electric contact which throws the
shutter release, then the device constitutes an attachment for turning
any semi-automatic camera into an automatic. The most suitable cameras
for mapping are unquestionably those of the entirely automatic type. The
use of such cameras always demands a knowledge of the ground speed. This
demand has led to many suggestions for _ground speed indicators_. The
common idea of these is to provide a moving part on the plane—either a
disc of large diameter, or a chain, or a revolving screw—whose speed may
be varied until any point upon it appears to keep in coincidence with a
point on the moving landscape below. The ground speed is then to be read
off a properly calibrated dial. Or, as a further step, the frequency of
the exposures may be directly controlled by the ground speed indicator
mechanism. The entire control of the camera would then consist merely in
occasional adjustment of the ground speed indicator.
[Illustration:
FIG. 139.—Planes starting out to make a map by flying in parallel.]
While entirely possible in theory, these devices are not easy to work
with in practice, because the plane is always subject to some pitching
and rolling, which make it difficult to hold any object constantly on
the moving point. This is especially true at high altitudes, where the
apparent motion of the earth is quite slow compared to the swervings of
the plane. This objection is in part removed if the ground speed
indicator is carried by a gyro stabilizer.
Ordinary mapping does not demand such exquisite rendering of detail as
does trench mapping. Nor is it necessary to fly in peace-time at such
high altitudes as in war. In consequence, mapping cameras are preferably
of the short focus, wide angle type, say, 25 centimeter focus for an 18
× 24 centimeter plate. Film is to be preferred over plates because of
the greater number of exposures it is possible to make on a flight. The
shutter of the mapping camera must be extremely uniform in its rate of
travel so that the elements of the map may match in tone (Fig. 140). A
mount which permits the camera to be turned normal to the direction of
flight, such as the British turret mount (Fig. 87), is particularly
desirable if flying across the wind is necessary, as will often be the
case in mapping strips between towns or between flying fields. Devices
to indicate compass direction and altitude are called for in new and
poorly mapped territory, and may be expected to receive intensive study
in the future. The question of their utility is, however, bound up with
the whole question of the sphere of aerial photographic mapping. Up to
the present this has been almost entirely a matter of filling in details
on maps obtained by the regular surveying methods, or of making
pictorial maps for aviators. To what extent primary mapping can be done
by the airplane is yet to be determined.
[Illustration:
FIG. 140.—A strip map, showing effect of uneven focal plane shutter
action.]
At this point mention must be made of special cameras for securing
extremely wide angle views, thereby minimizing the number of flights.
The _Bagley camera_, devised by Major Bagley of the U. S. Engineers, is
an example. It has three lenses, a middle one pointing directly
downward, and one to either side at an angle of 35 degrees. The pictures
obtained with the side cameras are of course greatly distorted, and must
be rectified in a special rectifying camera. The resultant definition is
not good, but as the maps are made on a much smaller scale than the
original pictures, this is not a serious objection. It is a matter for
the future to decide whether the additional labor on the ground
necessary for the rectifying process is to be more expensive than the
extra flights which must be made with the ordinary types of cameras
covering a smaller angle.
=Printing and Mounting Mosaics.=—With an ordinary set of overlapping
negatives the first step toward producing a map is to _scale_ the
negatives. For this purpose one should be selected which by comparison
with a map shows no distortion, and which is on the desired scale, or is
known to have been made at the average altitude of flight. A sketch map
of the territory should then be drawn, on this scale, based on available
maps. This sketch is preferably made on a large ground glass illuminated
from behind (Fig. 141). On this all the negatives should be laid, and
their proper relative positions sought. When this is done it is evident
at once whether all the territory has been covered, and whether there
are any superfluous negatives. Each negative should then be examined as
to its scale and distortion. If it can be made to fit the scale by
simple enlargement or reduction, a line can be drawn on one edge of a
length indicating its scale. This line will later be used as a guide in
the enlarging camera. If the picture is badly distorted it must either
be replaced by another negative, or if rectifying apparatus is
available, it must be set aside for the making of a rectified print.
[Illustration:
FIG. 141.—Scaling negatives for mosaic map-making.]
The next step is to make prints from the negatives, which may be done
either by contact, or, necessarily if differences of scale must be
compensated, in the enlarging camera. If prints to an exact scale are
required the shrinkage of the paper must be determined and allowed for.
The prints must all show the same tone, and must be uniform from edge to
edge. If the focal-plane shutter is not uniform in its travel, as is
frequently the case, this means that the print must be “dodged,” or
exposed more at one edge than the other, by locally shielding the plate
and paper during exposure. A case of the step-like effect caused by
uneven shutter action is shown in Fig. 140. The effect due to uneven
shutter action is of course absent with a between-the-lens shutter,
which constitutes a strong argument in favor of that type for use in
mapping cameras.
[Illustration:
FIG. 142.—Arranging prints for a mosaic map.]
When the prints are made they must be mounted together on a large card
or cloth background. For a very small mosaic they may be juxtaposed by
simple examination, matching corresponding details in successive prints.
For a mosaic of any size an accurate outline map must be drawn on the
surface to which the prints are to be attached. The prints are then laid
out on this outline, moved to their correct positions, and held down by
pins (Fig. 142). When they are all arranged the final mounting may be
begun. The excess paper, beyond what is necessary for safe overlaps, may
be trimmed off, exercising judgment as to which print of each adjacent
pair is of the better quality, and utilizing it for the top one at the
overlapping junction. If one print shows serious distortion it may be
placed under its fellows on all four edges, thus minimizing its weight.
The edges are best made irregular by _tearing_. Straight edges are apt
to force themselves on one's attention in the final mosaic and give an
erroneous impression of the existence of straight roads or other
features. Both forms of edging are shown in Figs. 124 and 143.
An alternative method of securing the final print mosaic, where film
negatives are used, is to trim successive film negatives so that the
trimmed sections will exactly juxtapose, instead of overlap. The
sections are then mounted, by stickers at their edges, on a large sheet
of glass, and printed together. Captured German prints show that this
was the method commonly used with the German film camera (Fig. 62).
It will be noted that the procedure which has been described and
illustrated by Figs. 142 and 143 assumes the previous existence of a map
accurately placing at least the chief features of the country covered.
This draws attention at once to the limitations and true sphere of
aerial photographic mapping at the present time. With the cameras thus
far it is not possible, nor is it attempted, to do primary mapping of
unknown regions. Distortions due to lens, shutter, film warping and
paper shrinkage considerably exceed the figures permitted in precision
mapping. From the standpoint of geodetic accuracy the cumulative errors
of deviations in direction, altitude and level, peculiar to flying,
would soon become prohibitive.
[Illustration:
FIG. 143.—A partly completed map. Prints mounted over an outline
sketch map to proper scale.]
The great field for aerial photographic mapping in the near future lies
in filling in detail on maps heretofore completed as to general
outlines, or, as in the war, on maps far out of date. The war-time
procedure in country largely unknown, such as Mesopotamia, was probably
closely that which will be necessary in peace. Conspicuous points in the
landscape were first triangulated from friendly territory, and from
these the outline map was drawn, whose details were to be supplied by
aerial photographs. Much of the “mapping” of cross country aerial routes
so far done is frankly of a pictorial nature, showing conspicuous
landmarks and good landing fields—extremely valuable and useful, but not
to be confused with precision mapping. In assembling mosaics of this
kind the elaborate procedure described above is not followed. The
process is the simple one of juxtaposing adjacent prints as accurately
as possible by visual examination. Errors are of course cumulative, but
as long as exact distances are not in question this is no matter.