Introductory material
Reading edition
All of the airplane photographs in this book, both oblique and vertical, were taken by the United States Army Air Service, except Figs. 78 and 79, which were taken by the United States Navy Air Service, and Figs. 10, 65, 69, 75, 77, and 82, which were taken by the author. To these two services the author is indebted for the permission to reproduce their photographs, and this acknowledgment is made with the same force as if made individually under each illustration.
As a guide to the evaluation of the scale of the vertical photographs, which is expressed under each photograph in the form of the natural scale, or representative fraction, the following approximate equivalents may be borne in mind:
1:10,000 = 800+ feet to the inch
1:16,000 = ¼ mile to the inch
1:21,000 = ⅓ mile to the inch