Map Making from the Sky

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.