Many of the early aerial photo collections used a “Trimetrogon” camera. As the Third version of the Manual of Photogrammetry puts it:
“In the United States the tri-lens system of map compilation is now usually designated as the “Trimetrogon”method. The word “Metrogon” is the commercial name of the kind of lens generally used, but the method of compilation is not affected by the kind of lens nor its focal length. This type of photography is accomplished by an assembly of three cameras, one pointing vertically downward in the conventional manner and two cameras mounted obliquely, pointed in directions perpendicular to the line of flight. The two obliquely mounted cameras are so placed that they will photograph both the horizon and a small part of the area covered by the vertical camera. The three cameras are exposed simultaneously and thus photograph a strip of ground extending from horizon to horizon in a direction perpendicular to the flight line. (See Fig. 1 for an illustration of a trimetrogon camera installation.)
For purposes of small scale reconnaissance mapping, trimetrogon photography,as compared to standard vertical photography, has the following advantages
a. A single flight covers a wider strip of terrain, thus permitting the distance between flight lines to be approximately six times as great.
b. The highly precise flying required for vertical photography is not essential, permitting photography to be obtained under adverse conditions.
c. Variations in altitudes, directions, and spacing of flights do not form serious compilation hazards.
d. The need for geographic control is greatly reduced.
e. Per unit area, the requirements for personnel, planes, cameras, and film are much lower.
f. Much less time and money are required for compilation
g. Excessive tilt of the photographs causes no serious problem or inaccuracy, in this method of compilation.
2. CAMERAS USED. Either the K-17 or K-3B type cameras are used in the trimetrogon assembly. In either case they employ the six-inch cone equipped with the Metrogon wide-angle lens and the A-S vacuum back magazine exposing a 9X9 inch nega6ve. These cameras have an angular coverage of approximately 74° as measured across the axis of the focal plane. Thus three of them provide more than enough coverage for the required 180° from horizon to horizon. While the lenses used in these cameras are called six-inch lenses, their focal lengths actually may vary from 149.2 mm to 156.6 mm.”
Figure 1. Trimetrogon camera installed in a B-17
The trimetrogon system solved a major problem for early aerial photography- the limited flight altitudes of commonly available airplanes. For instance, photography for the National High Altitude Photography program was flown at 40,000 feet, whereas the ceiling for the B17 shown above would have been 36,000 feet and would have been flown closer to 20,000 feet . Without the ability to fly at high altitude, the area covered on the ground by a conventional nadir photo was limited- the oblique photos could cover much larger areas. At an altitude of 20,000′ (6097 m) the usable swath of the three photos could be as high as 20 miles (32 km) wide.
A scanned version of the Third Manual’s chapters on processing of oblique images and trimetrogon imagery can be downloaded here.