The war of 1939-45 provides an interesting landmark in the progress of air surveys. In 1939 the science was young, but yet sufficiently advanced to leave no doubt that it had come to stay. Excellent maps had been made in Switzerland, Germany, Holland, Canada, the U.S.A. and, to a lesser extent, in other countries. While it was admitted that ‘photogrammetry’, as the science is now called, had possibilities, there was no general agreement as to its place in the survey world as a whole.

On the continent the instrument manufacturers saw new scope for their genius. The instruments of Zeiss in Germany, Wild in Switzerland, and Santoni in Italy, masterpieces of design and precision, were beginning to be used by government and by contracting surveyors, though not in this country. Here, interest was of a different kind. A few plotting instruments were designed in England, and experimental models were built, but the official solution of the air-survey problem was geometrical, not optical or mechanical. The treatment of tilted photographs was effected by mathematical analysis rather than by optical projection. This difference of treatment in 1939 is a most noticeable feature to the student of the history of photogrammetric development.

The literature of the subject up to this time tells the same story. Continental authors, dealing with the various problems in solid geometry, seek their solutions by instrumental methods, and their results call for the greatest admiration. The British literature, on the other hand, advocates graphical and mathematical solution, and, as far as I am aware, every method advocated requires an arduous treatment of the effects of tilt. The only instruments required are simple co-ordinate measuring devices which measure in the planes of the photographs, and not in what may be called ‘terrestrial’ coordinates. Both are theoretically sound, though they had not then been so widely used as to convince the critics that they were serious survey methods. Many of the critics were hostile, or at least not progressive, and they would not admit the simple truth that in full-scale tests air surveys had yielded results which compared favourably with ground surveys in respect of accuracy.