Map Making From a Balloon

by Roth Lammert

At the turn of the century map making was still pretty crude. Photographs used for surveying and charting were usually made with a photographic theodolite, the axis of which could be set at a known inclination to the verticals by means of levels. Two photographs were needed for the reconstruction of an object or landscape. Attempts were made from balloons, but the usual methods just did not work. The spherical level was so placed that the upper and lower edges of the plate were horizontal when the bubble occupied the marked circle. Linear levels were attached to the camera to correct the setting of the spherical level if necessary. The plate was also set at a known inclination to the horizon, about 45 degrees, at the moment of exposure.

The photogrammetric gun was originally devised for the purpose of making approximately oriented photographs of a landscape, from which horizontal distances could be obtained by simple means with sufficient accuracy for military purposes. An expert marksman could set the level very accurately, despite the swaying of the balloon, and the setting of the moving bubble was accurate to the limit imposed by the small scale (1 to 25,000) of the maps, or to about 0.2 degree.

From two balloon photographs taken from different viewpoints a landscape could be reconstructed quite accurately. If the vertical lines deduced from the indications of the level of the instrument at the two exposures were drawn on such a reproduction, the two lines should be parallel if the bubble occupied its true zero position at each exposure. Tests showed the indications of the level could br relied on to within 0.1 degree.

More severe tests were furnished by the comparison of details of the reconstruction with the corresponding parts of good maps and of the real object or landscape. For example, a plain should show an appreciable difference in elevation and a stream should never be represented as flowing uphill. These comparisons ascertained that the moving bubble of the level gave the position of the vertical line to within about 0.05 degree which was about equal to the degree of precision with which the position of the bubble could be observed.

In an age where civilization was just beginning to reach for aids to progress, this was a milestone. Here a camera (in all essentials a copy of the eye) was used to supplement crude natural estimates of distance and orientation. Aerial photography had begun and busy inventors would continue to make improvements until aerial photography became perfected in our modern planes of today.