Map Making from the Sky
From Modern Wonder
magazine 16 July 1938.
As a retired cartographer, map maker, I was fascinated to see this story written more than a decade before I was born! It features an aircraft that my Dad had flown, and the science has not changed much./drf
EXACTLY
eighty years ago, a Frenchman named Nadar made history by taking the first
aerial photograph while up in a balloon.
And such a sensation did
his achievement create that all the newspapers printed long stories about it,
while one Paris paper published a cartoon entitled “ Elevating Photography to
the Heights of Art.”
But, both aeronautics
and photography have gone far since that day, and now aerial photography is no
longer a “hit-or-miss” affair but a very exact science.
And it is a very
important one, too, as Squadron Leader P. R. Burchall, O.B.E., former
Photographic Officer of the Royal Air Force and now in charge of survey
operations for Aerofilms Ltd., emphasized in a recent interview with Modern
Wonder's Special
Representative.
Air photographs, he
explained, are not just a means by which attractive “bird’s-eye view” pictures
can be obtained.
The vertical photograph,
taken by a camera that protrudes through a hole in the floor of the plane, does
not possess the attractive pictorial qualities of the oblique photograph which
is taken at an angle of from 35 to 60 degrees, but it is invaluable in survey
work.
It enables mapping and
revision to be carried out rapidly and accurately and provides more detailed
information than can ever be given on a map.
On the other hand, the
oblique aerial photograph with its wealth of easily understandable detail is of
enormous value to authorities who require a pictorial record of existing
conditions when planning new works, or offices, studying slum clearance in
congested areas, recording the progress of works under construction, and so on.
Lawyers, too, often find
oblique aerial photographs of great use when arguing legal matters in the Law
Courts, while as a form of pictorial advertising they are becoming more and
more popular.
Vertical photographs are
taken by a special camera that is electrically controlled and automatic in
action. It is supported on the floor of the plane in a mounting that absorbs
all vibration while a specially designed “louvre” shutter ensures that the
pictures will be free from distortion.
Before the pilot and
photographer— Aerofilms employ almost exclusively former members of the Royal
Air Force—go up on a survey flight, the exact route to be taken and the height
at which they are to fly are worked out in detail.
The scale of the
photograph and the amount of area covered is controlled by the height of the
plane and focal length of the lens used, the most usual being a height of nine
thousand feet with a twenty-inch lens, as this gives a scale of approximately
twelve and a half inches to the mile. These conditions are used when work is
being carried out on the revision of twenty-five inch ordnance sheets.
After each exposure, the
film in the camera, measuring one hundred and sixty-five feet, and capable of
taking two hundred photographs, is automatically moved forward, a special
timing apparatus ensuring that at least two-thirds of the ground shown in one
photograph is included in the picture taken immediately after it.
Inside the camera are a
small clock, an altimeter and a Veeder counter set in a line in such a way that
when each exposure is made, the light from a two-volt lamp enables them to be
photographed on the margin of the picture. By this means it is possible to
record the exact time of day at which each photograph was taken, the flying
height of the plane, the serial number of the picture, and when necessary,
crosslevels.
The “louvre” shutter,
which consists of a row of very thin metal leaves which turn on end in exactly
the same way as do the wooden bars on the old-fashioned Venetian blind, is
first wound up. Then, two seconds before it is released, a red light flashes to
warn the pilot to level up the plane.
The instant the
photograph has been taken, a green light signals the “O.K.” and the film is
automatically wound over ready for the next exposure, by means of a small
electric motor and electric control box.
The pilot engaged in
aerial photography flies his machine along a series of parallel straight lines
in such a way that about a third of the ground that appears on one side of the
photographs taken on the first “run” will appear in those taken on the second,
and so on.
He is provided with
special instruments including a gyroscopic compass and a drift sight and from
his map he picks out certain distinctive points beyond the imaginary straight
line along which he must fly, and by taking a compass bearing on it, he is
enabled to keep a straight course.
The greatest problem
that confronts the pilot is a strong side wind, for this produces “crabbing.”
That is to say, the plane is being continually forced sideways at the same time
as it is moving forward.
To counteract this
effect, the camera is swung round to allow for the angle of drift and let the
edges of the photographs run parallel. The photographer checks up the
actual direction of his flight by means of a special sight which is also used
to estimate the angle of the plane’s drift.
Within
a few hours of a flight having been completed, the films are developed and then
rough prints are made and assembled so that the area that has been photographed
can be checked accurately.
This
work is always carried out as soon as possible so that in the event of some
part of the area having been missed owing to low cloud or faulty navigation,
the gaps can be filled in by further photography on the next day.
In
studying the aerial photographs, the stereoscope is used extensively, for by
its means the ground may be studied in three dimensions. When you look at a
pair of aerial photographs through a stereoscope, trees, buildings, hills and
other objects seem positively to stand out from the picture in such a way that
you feel you are not looking at a flat picture but a perfect scale model of the
actual country.
As
accuracy is the essence of survey, the photographs are enlarged and rectified
optically until an exact fit with the ordnance map is obtained.
