Map references crop up all the time in unit war diaries. If you are trying to work out the precise location of a unit during the war then you need to be able to understand how the reference system worked.
The genesis of trench maps
War of movement on the Western Front in France and Flanders came to an end in October 1914. Between then and July 1918, the opposing armies occupied a complex system of trenches that ran from the North Sea coast to the Swiss border. The trench systems and the artillery and other positions behind them became the focus of intensive observation; they were examined, analysed and sometimes modelled in detail before each raid or general attack. It was vital to pinpoint the enemy’s artillery, machine gun posts, headquarters, dumps and other locations. Details of the enemy’s positions came from observation from the air (by balloon or aeroplane) and from the front-line troops who had raided or taken prisoners. A sophisticated and thorough mapping of the enemy’s trench systems grew up; hundreds of thousands of prints of maps were issued. However, like most military things they had a special jargon all of their own and to the uninformed a Great War map is not an easy thing to intepret.
The mapping system
The maps of the Western Front compiled by the British Army during the Great War adopted the following grid system:
1. The whole area shown on each map was first divided into a series of large rectangles, each identified by a capital letter of the alphabet. These rectangles were in turn subdivided into smaller squares numbered 1,2,3 . and in the larger ones up to 36. These squares covered a ground area of 1000 by 1000 yards. So for example, one of these 1000 by 1000 squares would be identified as S6 or T1 or B14.
2. Each of these numbered squares was then subdivided into a two by two matrix of four squares, each measuring 500 by 500 yards. These were lettered in lower case a,b,c and d. a is top-left, b is top-right, c lower-left and d lower-right. So the reference for one of these 500 yard squares would be for example S6a, or T1b or B14d.
3. Usually, especially for artillery purposes, much greater accuracy was called for. To achieve this, each of the 500 yard squares was then subdivided into a ten by ten matrix of 50 by 50 yard squares. A specific point could then be identified by counting along the x-axis (West to East) in 50 yard segments, and then up the y-axis (South to North). So our example references could give S6a.4.3, or T1b.6.9 or B14d.1.2.
4. To give an even more precise reference, imagine the 50 yard square was divided not into a ten by ten grid, but a hundred by hundred one. Each square in this grid would measure 5 yards square – about the size of a machine gun pit or a small blockhouse. Our example references could be S6a.42.38 or T1b.60.99 or B14d. 11.25. In other words, orders could be given to an artillery battery to aim at location given by a map grid reference just 5 yards square. Whether they would hit it is another matter.
5. Bear in mind that each single map has say 40 or 50 of the numbered 1000 yard squares on it, so the numbers 1,2.36 may appear more than once. If there is any confusion, look out for the capital letter of the larger squares to which they belong.
This example is an extract from a map of the Vimy Ridge area in mid 1917. Trenches are shown in red. Note the square “6”. Try to work out the map references for the two ends of the trench named Severn Alley.
I make them 5.d.8.1 and 6.c.8.0. Did you?
Producing the maps
The drawing of maps was the job of the Topographical Sections of the Field Survey units of the Royal Engineers. Production of the millions of maps was the job of the Army Printing Section.
Trench maps today
Original maps are a sought-after collectors item, often attracting high prices.
Free and online collections of maps
Collections of maps you can buy
Some maps, notably those in the collections of the National Archives and the Imperial War Museum, have now been electronically scanned and can be bought on DVD. There are currently three systems available that I know of:
 Maps produced by the Western Front Association. Excellent value for money in that you get lots of maps for not much money but no frills in that you must use your own image-viewing software and there is no search facility;
 Maps produced by the Naval & Military Press. Rather expensive at over £100 but well worth it if you are likely to use maps a lot. Excellent search facility, a great collection of maps and some good information too;
 Maps produced by Linesman. The most sophisticated product, allowing overlays of modern-day maps and integration with GPS.
Single maps you can buy
 The National Archives and the Imperial War Museum sell reprints of selected maps in their in-house shops,
as do publishers G H Smith in Yorkshire. You can also sometimes find them in the bookshops of the visitor centres and museums on the battlefields in France and Flanders.