The earth patterns were extensively studied with the result that definite conclusions were formulated pertaining to the best patterns to be used. Although it is out of the question to present a detailed discussion of this important phase in this résumé, attention will be called to the manner in which the earth patterns diminish with increasing altitude. The insert in [Fig. 100] shows the actual size of an image of a 50-foot airplane from 0 to 16,000 feet below the observer as compared with corresponding images (to the same scale) of objects and areas on the earth’s surface 10,000 feet below the observer.
For simplicity assume a camera lens to have a focal length equal to 10 inches, then the length x of the image of an object 100 feet long will be related to the altitude h in this manner:
| x | = | 100 | or xh = 1000 |
| 10 | h |
By substituting the values of altitude h in the equation the values of the length x of the image are found. The following values illustrate the change in size of the image with altitude:
| Altitude h in feet | Size of image x in inches | |
| 1,000 | 1.00 | |
| 2,000 | 0.50 | |
| 3,000 | 0.33 | |
| 4,000 | 0.25 | |
| 10,000 | 0.10 | |
| 20,000 | 0.05 |
It is seen that the image diminishes less rapidly in size as the altitude increases. For example, going from 1000 feet to 2000 feet the image is reduced to one-half. The same reduction takes place in ascending from 10,000 to 20,000 feet. By taking a series of photographs and knowing the reduction-factor of the lens it is a simple matter to study pattern. An airplane of known dimensions can be placed in the imagination at any altitude on a photograph taken at a known altitude and the futility of certain patterns and the advantages of others are at once evident.
Fig. 100.—Illustrating the study of pattern for airplanes. The photograph was taken from an altitude of 10,000 feet.
The insert shows the relative lengths (vertical scale) of an airplane of 50-foot spread at various distances below the observer.
It is impracticable to present colored illustrations in this résumé and values expressed in numbers are meaningless to most persons, so a few general remarks will be made in closing the discussion of low visibility as viewed from above in spring, summer and fall. A black craft is of much lower visibility than a white one. White should not be used. The paints should be very dark shades. The hues are approximately the same for the earth areas as seen at the earth’s surface. Inland waters are a dirty blue-green or bluish-green, and deep ocean water is a greenish-blue when viewed vertically, or nearly so. Mean hues of these were determined approximately.