If a map made for civil purposes contains inaccuracies or mistakes, inconvenience and serious financial loss may result. In a military map the consequences may be far graver—perhaps even to the extent of jeopardizing the whole operation. The responsibility of a maker of military maps is heavy. The map tells a story to those who read it, and the report it gives must be true.

In the actual making of the military map, the ground cannot be occupied either for control survey or for the amplification of photographic interpretation. The military cartographer is thus required to make a better map, and to make it from data less complete. He may be required to make it within limits of time quite unprecedented in civil practice, and he must do this with personnel who, for the most part, may be new to the work.

Many of the new and modified techniques described in this book were devised, in the first instance, to solve specific military mapping problems encountered in the North-West European Theatre. Nevertheless the text, as now presented, has primary application to civilian mapping. Some of the material was published during the war for restricted military circulation, particularly in ‘Parallax Without Tears’, ‘Field Survey Pamphlet No. 6’, ‘Heighting Without Tears’ and in ‘Air Survey Research Paper No. 18’. Other new techniques developed and used during the war have not previously been published, either because it was felt, at the time, that they had not been sufficiently tried and proved against ground checks, or because there was no time to write the methods out in form suitable for publication.

Neither was there time to follow many promising lines of investigation— in multiplex operation; in connection with templet techniques such as the spring-loaded intersection locator and the error formula. With the end of the war this situation changed; it became possible to gather up many of these loose ends, and the relaxation of security regulations now permits publication.

The Chinese put the roof on a house before they build the rest, claiming, with some reason, that they are thereby able to carry on the work without interference from the weather. Photogrammetry is here presented in much the same manner, and for much the same reason.