Declassified

Interpretation of aerial photos

CIA database

The CIA CREST Database contains , among other things, information about slide rules or calculating disks for interpreting aerial photographs. This is declassified correspondence, in which much information has been blacked out, such as the name of the manufacturer of the calculator.

In one of these letters, from 1964, a (blacked out) manufacturer proposes an aluminum slide rule.[1]   A standard slide rule from this manufacturer cost $28.50; the photo interpretation version would cost $30 to $35.

Pickett?

When you hear about an American aluminum slide rule, you immediately think of Pickett. A second letter[2]  from 1964 confirms this assumption: it states that the manufacturer moved to Santa Barbara, California, five months ago. Pickett did indeed move from Chicago to Santa Barbara in 1964.[3] 

The letter refers to the type N4ES, which again points to Pickett. The letter also reveals something about the production process and contains instructions for the slide rule, in typescript with handwritten notes.

Unfortunately, only 5 of the 29 pages have been released.

The general side of the proposed slide rule should have the scales LL0, LL1, A, DF = CF, CIF, T, S, L, C = D, DI, K, LL2, LL3.

Most striking is that the altitude scales range from 1.6 nautical miles to 203,000 or even 250,000 nautical miles. The average distance from the Earth to the Moon is 207,600 nautical miles. The instructions also explicitly mention the Moon. What was the CIA doing there?

Other Pickett models

Pickett had already made slide rules for interpreting aerial photographs around 1951:

Model 520 Type A-1 and Model 700 Type A-2 for the US Air Force.[4,5]  These slide rules had the general scales L, A, DF = CF, CI, T, S, C = D, DI, K. The altitude scale ranged from 100 to 250,000 feet, so a factor of 6000 smaller than the ruler intended for the CIA.

It is not clear whether the slide rule from the correspondence was ever put into production.

Calculating disk

As early as 1961 it had been suggested to replace the slide rule that was then in use, but was no longer suitable for state-of-the-art photography, with a calculating disk. The idea was not taken up at the time because "slide rules had no future in photo interpretation".[6]

When the suggestion was resubmitted in 1965, with a revised design, action was taken.
In 1966, the NPIC issued a draft Request for Proposals[7]  to supply six of either a prototype 8-inch plastic calculating disk or a combination of an 8-inch calculating disk and a slide rule no longer than one foot in length. I do not know if this resulted in an actual calculating disk.

There does exist an American calculator for aerial photography: the "US Navy Calculator For Vertical Aerial Photography, MIL-C-19729" which was made by Felsenthal sometime between 1957 and 1964.[4,8]   This calculating disk has a diameter of 5¼ inches, so significantly smaller than what the NPIC required.

References

1. Photo Interpreter's Slide Rule, June 29, 1964. CIA CREST Database CIARDP78B04747A001000060004-8 

2. May 25,1964. CIA CREST Database CIA-RDP78B04747A001000060015-6. 

3. "Pickett & Eckel, Inc.", Collection of Historical Scientific Instruments, Harvard University,  People/Pickett & Eckel, Inc

4. https://sliderulemuseum.com/Military.shtml

5. "How to use the U.S. Air Force Aerial Photo Slide Rule / Model 520-type-A1 (10-inch) / Model 700-type-A2 (6 inch)", USAF Technical Manual M 520-700; Eric Rinehart, "Pickett United States Air Force Aerial Photography Slide Rules", Journal of the Oughtred Society Vol. 32, No. 1, 2023 page 55; 2 USAF Aerial Photo Model A-1 Pickett slide rules in "Transfer of Property", NPIC, November 17, 1970,  CIA CREST Database CIARDP78B05171A000200020025-9

6. Procurement of circular slide rules, June 8, 1965; suggestion from June 1961, re-submitted in1965. CIA CREST Database CIA-RDP78B04770A002600120002-0 and CIARDP78B04770A002600120001-1.

7. Design Objectives for Circular Slide Rule, January 20, 1966. CIA CREST Database CIARDP78B04770A002600120004-8. The National Photographic Interpretation Center (NPIC) was a partnership between the CIA and the Defense Intelligence Agency since 1961.

8. MIL-C-19729 "Computer, Aerial Photographic Data" , 1964-1999

A Dutch version of this paper is published in MIR92, July 2024