THE INTERPRETATION OF AERIAL PHOTOGRAPHS
Oblique aerial photographs if on a large enough scale are even easier to
interpret than are ordinary photographs taken from the ground, since
they practically preserve the usual view, and add to it the essentials
of a plan. With verticals, however, this is far from the case. In them
all natural objects present an appearance quite foreign to the ordinary
mortal's previous experience of them. This may be easily demonstrated by
taking any aerial view containing a fair amount of detail and trying
systematically to identify each object. A necessary preliminary to doing
this accurately is acquaintance with and study of the ground
photographed, or of similar regions, and of objects of the same
character as those likely to be included.
The interpretation of military aerial photographs is of such importance,
and has become such an art, that it is the function of special
departments of the intelligence service. Extended courses in the subject
are now given in military schools. This instruction must cover more than
the interpretation of aerial photographs as such. General military
knowledge is essential, so that not only may photographed objects be
recognized, but the significance of their appearance be realized.
Whether attack or retreat is indicated; whether a long range bombardment
is in preparation, or a mere strengthening of local defences.
The natural difficulties of interpreting aerial views are enormously
increased by the unfamiliar nature and frequently changed character of
the military structures, and particularly by the attempts made to
conceal these from aerial observation by selection of surroundings and
by camouflage. The small scale of the photographs, in which a machine
gun shows as a mere pin point, adds to the uncertainty, with the net
result of making interpretation a task of minute study and deduction
worthy of a Sherlock Holmes.
Little detailed information on interpretation can be profitably written
in a general treatise, partly because the illustrations available are of
a highly technical military character, partly because original
photographs instead of halftone reproductions are practically imperative
for purposes of study. Nevertheless some general instructions,
applicable to any problem of interpretation, may be given, as well as a
few illustrations, drawn from military sources, which will serve to show
the detective skill necessary.
First of all it is important that the print or transparency be held in
the right position. The shadows must always fall toward the observer;
otherwise, reliefs will appear as hollows and hollows will show as
hills. The reason for this is that the body ordinarily acts as a shield,
preventing the formation of shadows except by light falling toward the
beholder. Thus in Fig. 162 the slag heap looks like a quarry when the
shadows fall away from one. The necessity for proper direction of
shadows is, it may be noted, in conflict with the ordinary convention
for the orientation of maps—at least in the northern hemisphere. A city
map, made by sunlight falling from the south, presents its shadows as
falling away from the observer, when it is mounted with its north point
at the top, as is customary. As a consequence buildings in aerial
photographic mosaics of cities occasionally look sunken instead of
standing out.
[Illustration]
Wrong way. Shadows falling away Right way. Shadows falling toward
from observer. observer.
FIG. 162.—The wrong way and the right way to hold a photograph for
interpretation.
[Illustration:
FIG. 163.—Guide to interpretation of trench details.]
The relation between the shape of the shadow and the object casting it
must be well learned. This is a part of the training of every
architectural draftsman, but the appearance of shadows from above has
not heretofore been a matter of importance. The difference between high
and low trenches, between cuttings and embankments, between shell holes,
occupied or unoccupied, and “pill boxes,” must be detected largely from
the character of the shadows. Which elevations and depressions are of
military and which of merely accessory nature, whether this black dot is
a machine gun or a signaling device, whether that dark spot is an active
gun port or an abandoned one—these are all matters of shadow and of
light and shade study. Several illustrations of these points appear in
Figs. 163, 164 and 165.
[Illustration:
FIG. 164.—Guide to interpretation of shell holes and other pits.]
[Illustration:
FIG. 165.—Illustrating the importance of distinguishing between
objects of similar appearance but different military importance.]
Shadows may be used to get exact information as to directions and
magnitudes. If we know the time of day at which a picture is taken, the
direction of the shadows will give the points of the compass. A chart
for doing this is shown in Fig. 166. The length of a shadow is a measure
of the height of the object casting it, and the exact relation between
the two dimensions is determined by the day and hour. Fig. 167 embodies
in chart form the values of this relationship for all times of the year
and day, while Fig. 168 shows the kind of picture in which shadow data
could be utilized to great profit.
[Illustration:
FIG. 166.—Location of true north from direction of shadows. Place the
dial on the photograph, the hour line corresponding to the time it
was taken being pointed in the direction of the shadows. North lies
between the two arrows, the exact direction being obtained by
joining the center of the dial to the point on the figure of eight
corresponding to the date on which the picture was taken. (Number on
figure of eight represent the 1st of the month.)]
Minute changes, both in light and shade and in position, must be watched
for with great care. Naturally growing foliage and the cut branches used
for camouflage differ in color progressively with the drying up of the
leaves. Hence a mere spot of lighter tone in a picture of a forest,
especially if the picture is taken through a deep filter, becomes
instant object for suspicion. The complete study of any position calls
for photographs of all kinds—verticals, obliques, and stereos.
Stereoscopic views are the worst foe to camouflage. A bridge painted to
look like the river beneath is labor thrown away if the stereo shows it
to be a good ten feet above the real river!
[Illustration:
FIG. 167.—Length of shadow of object one meter high, at different
times of the day and year, for latitude of Paris.]
[Illustration:
FIG. 168.—Bethune, August, 1918, illustrative of interpretation by
shadows.]
[Illustration:
FIG. 169.—Typical trench photograph showing first and second lines,
communicating trenches, listening posts, machine gun emplacements,
and barbed wire.]
A few illustrations of the more ordinary and obvious objects whose
detection is the subject of aerial photography are shown in accompanying
figures. Fig. 169 pictures a typical trench system, with barbed wire.
The trenches show as narrow castellated lines, from which run the zigzag
lines of communicating trenches, saps, and listening posts. The minute
pockmarks behind the main trench lines are shell holes and machine gun
pits. The barbed wire shows as double and triple gray bands, intricately
criss-crossed at strategic points. Another form of defence, intended for
the same purpose as the barbed wire of the western front, is that
furnished by overthrown trees in forest regions. Fig. 170 reveals a
mountain fortress surrounded by a zone of felled trees, and indicates in
striking manner the value of the information a single aerial photograph
may furnish to an attacking force. Fig. 123 shows on a comparatively
large scale opposing trench systems in which a natural obstacle—a
river—separates the adversaries. Nicks and dots indicate machine guns to
the skilled eye, and several rectangular structures are revealed as
concrete buildings which have survived unscathed the shell fire which
has obliterated, and caused to be rebuilt, nearly every other element of
the trench system.
[Illustration:
FIG. 170.—A mountain fort surrounded by felled timber.]
Isolated battery emplacements (Fig. 171) must be carefully studied to
learn if they are in use. The chief indication is given by the paths the
men make in going and coming; these show as fine light lines,
obliterated by growing vegetation if long disused. Another indication is
the blast marks in front of the gun muzzles; occasionally the sensitive
plate will catch the actual puff of smoke as the gun is discharged.
[Illustration:
FIG. 171.—Three stages in the life of a battery.]
Railways of various gauges show as thin lines, crossed by ties, and
exhibiting the characteristic curves and switches. They are particularly
important to detect because they naturally lead to guns or supplies of
importance. Abandoned railways from which the rails and ties have been
removed leave their marks on the ground and must be carefully
distinguished from lines in actual operation.
Aviation fields are easily recognized by the hangars, often with “funk
hole” trenches alongside for the men to take shelter in during air raids
(Fig. 172). Other characteristic features are the “T” which shows the
direction of the wind to the returning pilot, and of course the planes
themselves, standing on the ground. But the field may be inactive, and
the planes merely canvas dummies, so that to pierce the disguise, all
paths, ruts, and other indications of activity must be minutely studied.
Overhead telegraph and telephone lines are revealed when new by a series
of light points (Fig. 174), where the posts have been erected in the
fresh turned earth. Later, when the fields through which they pass are
cultivated, the post bases show as islands left unturned by the plow. In
winter the wires reveal their position by black lines in the snow caused
by drippings. Buried cables are indicated while building by their
trenches, and for some time afterward by the comparatively straight line
of disturbed earth.
[Illustration:
FIG. 172.—Aviation field, showing hangars, planes, landing “T” and
refuge trench.]
Just as the detective of classic story makes full use of freshly fallen
snow to identify the footprints of the criminal, so does the aerial
photographer utilize a snowfall to pierce the enemy's attempts at
deception. Tracks in the snow show which trenches or batteries are in
actual use. Melting of the snow in certain places may mean fires in
dugouts beneath. Black smudges in front of trench walls show where guns
are active. Guns, wire and other objects, however carefully painted to
match the gray-green earth, stand out in violent contrast to this new
white background (Fig. 173).
[Illustration:
FIG. 173.—Trenches and barbed wire in the snow of an Alpine ridge.
Italian Air Service photograph.]
[Illustration:
FIG. 174.—A fully interpreted aerial photograph.]
After the aerial photograph has been interpreted the results of the
interpretation must be made available to the artilleryman or the
attacking infantryman. This may be done by legends marked directly on
the photograph. Another method is to mount over the photograph a thin
tissue paper or oilskin leaf, with the interpretation marked on it. A
yet more elegant method consists in outlining all the chief features of
the photograph in ink, writing in the points of importance in
interpretation, and then bleaching out the photograph with potassium
permanganate solution. Photographic copies of the resultant line drawing
are then mounted side by side with the original photograph. Fig. 174,
which shows a fully interpreted photograph, is an example of this kind
of mounting.