Welcome to the Center for Ecological Analysis of Lidar Historic Airphotos Blog

This is where I plan to display the materials I’ve been collecting as part of my investigation of strategies for using the wealth of historic aerial photography available through resource agencies, archive and museums around the world. I’ll also be posting reference material that is long out of print, maps of historic aerial data collections, and links to information I’ve found useful or interesting.

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Trimetrogon Photography

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.

 

 

 

 

South and Central American Trimetrogon Resources

Alirio de Matos  (1945) General Principles of Cartography. Brasilian Journal of Geography Vol 7:4 621:630

PDF

Sub-director Geographic and Cartographic Service, Brasilian National Council of Geography

Abstract: The author first devotes some attention to the definition of Cartography and discusses its objectives as well as the paths it must follow to achieve them. After considering various definitions of Cartography, the author does not attempt to decide among them, but suggests as a working definition it is well to include within the scope of Cartography all field work which leads to map-making.

He then shows that the composition of a map always starts from previous work, and a map 1s in fact a problem resolved by successive approximations.

He examines field operations as they are carried out in Brazil: he considers various elements about the use of photogrammetry and is of the opinion that the trimmetrogon should be used in regions where there is no cartography or where it very poor. He gives a general outline of the method to be followed.

Then he examines those methods which have greatest precision and shows how photogrammetry obtains thousands of maps in less time than other methods, and furthermore economizes very considerably on the amount of field work necessary. Next he discusses the diverse field and office operations for the production of maps, and finally examines the processes used in design and the methods of reproduction.

At the end he discusses very briefly the most commonly used projections for topographic maps.

 

Jorge Zarur (1948) Geography and Cartography for Census Purposes in Latin America. Brasilian Journal of Geography V10:4, 561:5999

Assistant Secretary, Brasilian National Council of Geography

PDF

A review of photography available for Central and South America. Indicates that there is vertical and trimetrogon photography for the following countries:

Argentina, Uruguay, Paraguay, Brazil, Mexico, Guatemala, El Salvador, Dominican Republic, Haiti, Cuba

But coverage is questionable in:

Ecuador, Peru, Bolivia, Chile, Honduras, Nicaragua, Costa Rica, Panama, Columbia, Venezuela