Free Women of Intelligence: Winning the Second World War with Air Photos by Christine Halsall Page B

transport planes visible among different types of aircraft, indicating that something was happening: comparative cover would show the change in use and purpose. It was then the responsibility of the PI to find the answers to the questions and report on them. Captain Dirk Bogarde summed up the fascination that PIs had for their work:

     
You needed observation, an eye for detail and memory. I loved the detail, the intense concentration, the working out of problems, the searching for clues, and above all, the memorising. 2

     
    Intervals between flying photographic sorties could range from a matter of hours in a rapidly changing situation, to a regular ‘watching brief’ of days or weeks. When an operation was being planned, one of the first orders would be for a significant increase in photographic cover of the area concerned, which would form a major part of the intelligence gathering and influence the planning and decision-making process. Even when intelligence came from other sources, for instance by electronic or human means, it invariably had to be verified with photographic cover. The plans for virtually every wartime operation included the words: ‘The photographs show …’
    Maps and charts are fundamental to every armed force activity on land, sea or air, and constitute one of the earliest forms of intelligence. This part of the course put the mathematical skills of the students to the test. Dorothy Colles passed her PI course in 1941 despite a few doubts:

     
On the last course all the WAAFs and half the RAFs failed. I have looked at all the manuals and am overwhelmed - not that one does it in a few weeks, but that one does it at all! I can see some most involved mathematical calculations ahead of me! 3

     
    By the afternoon of Diana Byron’s first day, the students were immersed in learning the standard procedures of basic scaling, recognition, identification and measurement that all PIs follow when looking at an air photograph. On each desk was set out the ‘tools of the trade’: a simple, pocket-sized stereoscope, which looked like a pair of spectacles mounted on a fold-up stand, a magnifier with graticules for measuring, photogrammetric tables and sets of trigonometry tables, a slide rule and an anglepoise lamp to provide a strong and adjustable source of light.
    The scale of an air photograph had to be determined in order to identify objects correctly by size and recognition. The process involved the accurate measurement of the size of an object relative to its size on the ground; sometimes the object was little more than a speck seen on a photograph taken at a height of 30,000ft. Before the invention of the pocket calculator, the slide rule was the most commonly used calculation tool in science and engineering. Looking like an overlarge folded-up ruler, it was used to multiply and divide and calculate functions such as roots, logarithms and trigonometry. Hazel Furney and Sarah Churchill were together on their course at Nuneham:

     
Five of us shared a room. I liked Sarah very much – she had an awful inferiority complex and was terrified of letting the family down. One day we all did a test report on an aerial photograph of which she had got the scale wrong, and then she asked me why she had got so much of her interpreting wrong. When I realised and told her what she had done, she disappeared. Later I found her sobbing her heart out on her bed, convinced she had failed. Needless to say, she wasn’t sent away. 4

     
    Sarah’s abilities with a slide rule improved and she passed the course, although she still lacked confidence in her mathematical capabilities. So she devised an ingenious solution to her problem:

     
When faced with the complexities of setting a slide-rule, I would tiptoe downstairs and ask a friend, one of the model makers, to do it for me. In fact, I got two slide rules, so that I would not have to change the setting if the photographic sorties used cameras with different focal