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The Great War (1914-1918) Forum

Map Security


Old Tom

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May I create another thread form Mike,s excellent post in the Trench communication thread. One again I have copied a part of it.

'However, they also had to overcome the 'security' issue of the enemy listening to messages and therefore also knowing the location of your troops which would mean enemy artillery fire coming down. This was achieved by the use of the 'Photo Protractor' that was attached to the observers map, this was a celluloid protractor (like school ones) with a movable arm on, the location of the troops being giving by reading the distace along the 'arm' and the angle it was at. Only the observer and the receiving HQ knew where the 'protractor' was fitted on the map, therefore the information sent was secure.'

It has always struck me that the map reference system adopted by the BEF was cumbersome. I also understand that the French used a decimal system not unlike the modern Ordnance Survey. However, the photo protractor gets round the problems of complex references. I wonder if it was used more widely than is implied. Within a formation a reference point could be published daily. While it would not be helpful to an officer on the ground with a small portion of map, but at company level and above there would appear to be advantages. Comments please!

Old Tom

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Of course they changed to a map grid system in the 1920s, probably because it enabled calculations to find polar coords from a pair of map references. The squared system didn't enable this, but had the advantage of being more efficient in that it needed less characters for a given degree of precision, something not entirely to be sneered at considering that morse code was widely use and fewer characters had some benefits.

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You have a point! I think the comparison is that the BEF maps needed 2 letters and 4 digits to give a location to 50 yds in a local arrea whereas a decimal system would need 6 digits to get to 100 yds. I never learnt morse (although my semaphore was not too bad) and do not appreciate if a mix of letters and digits causes any difficulty.

Old Tom

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Thanks again! Interesting that the protractor is refered to as fixed to the map, rather than hold on the map and that the protractor is set at a angle to the grid lines. I suppose that setting at an angle could have been a security measure as, perhaps, the angle could be changed daily.

Old Tom

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