How to read a WW1 Trench Map

Tom Tulloch-Marshall
Military Genealogy & Operational Records Of The Great War

British Army
World War One Trench Maps
~ an example ~

Ok - it's a bit heavy going as far as downloading is concerned, but this is about as simple as I can make it, - but note that the map below is reproduced at a reduced scale and resolution and is not of the quality which you should expect with a normal report.
This part map shows a small section of the 1:10,000 scale Trench Map reference "36C NW3" updated to 25th August 1915 - one of a small series of maps which were current issue when the Battle of Loos began on the 25th of September that year. The lower area, below The Hohenzollern Redoubt and down as far as the Vermelles - Hulloch road was attacked by the British 7th Infantry Division and the The Hohenzollern itself and the area to the north was attacked by the 9th (Scottish) Division.

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Many casualties from both of these Divisions passed through No 23 Field Ambulance which had set up at a position known as "Railway Crossing", - the precise location being recorded in the war diaries using the standard map code format "G.9.a.2.6.". That location is replicated on the map section below and you can see that it coincides with the intersection of two farm tracks and the railway line running north-eastwards out of Vermelles. That is very typical of the degree of precision which can be achieved using the map references which are commonly found in unit and higher echelon war diaries.

Not only does this put in context the part played by No 23 Field Ambulance during the early stages of the battle but it also allows the precise location of the unit to be pinpointed today, - the railway track is long-gone, but it's path is now used as a cycle-way and with some fairly simple measurements it is possible to find the site occupied by No 23 FA with some exactitude. The "Hohenzollern" position is clearly visible across the fields to the east.

So how does the diary map reference "G.9.a.2.6" get us to the location illustrated above ? - Very easily, and whilst the precise detail may vary slightly from map to map the essential principles are always the same. The following is a slightly simplified explanation, but in principle it works like this >

G.
The complete trench map is divided into 6,000 x 6,000 yard squares which are designated by a capital letter, in the example above the area shown falls within "G" (you cant see the G on the section above, but it is quite obvious on the full size map).

9.
The main lettered square (in this case G) is sub divided into 36 (sometimes 30) sub-squares each 1,000 x 1,000 yards. These squares are numbered 1-36 (or 1-30) starting top left of the 6,000 yard square and ending bottom right. The number of the 1,000 yard square generally appears dead centre of the square, as in the example above.

a.
Each 1,000 yard numbered sub-square is divided into four 500 x 500 yard squares which are letter coded - a - b - c - d - from top left to bottom right as shown above. Be aware though that it is quite normal for the a, b, c, and d notations not to be given on the maps, though their sequence within the 1,000 yard squares is always the same >

ab
cd

2.
You will see that the edges of the 500 yard squares are divided into ten equal lengths (though again, these are not always marked on the maps). The first number in the final two digit code (in this case "2") is always the distance from the bottom left of the 500 yard square, moving to the right - ie two tenths of the 500 yard square - 100 yards east of the western side of the square.

6.
The second digit is your south to north instruction - 6/10ths of the 500 yard square vertically upwards from the point established by "2" above. You have now reached "G.9.a.2.6" - No 23 Field Ambulance.

What if the map reference had shown something like "G.9.a.25.69." ? - That is simply a more precise reference but it isnt unusual to see that form in diaries, especially for Artillery targets. Again its quite simple, - after you have plotted the "2" as above, simply imagine the space between the 1/10th markings in the 500 yard square to be sub-divided again into ten more divisions (then ditto for the "6" component), thus bringing your "accuracy" down from a 50 x 50 yard sub-square to a 5 x 5 yard square.

And that's how it works, every time, - if you are using the right map and you have the right coordinate code then you cant go wrong (well that's the theory !). If you have any problems then get back to me.

G.	The complete trench map is divided into 6,000 x 6,000 yard squares which are designated by a capital letter, in the example above the area shown falls within "G" (you cant see the G on the section above, but it is quite obvious on the full size map).
9.	The main lettered square (in this case G) is sub divided into 36 (sometimes 30) sub-squares each 1,000 x 1,000 yards. These squares are numbered 1-36 (or 1-30) starting top left of the 6,000 yard square and ending bottom right. The number of the 1,000 yard square generally appears dead centre of the square, as in the example above.
a.	Each 1,000 yard numbered sub-square is divided into four 500 x 500 yard squares which are letter coded - a - b - c - d - from top left to bottom right as shown above. Be aware though that it is quite normal for the a, b, c, and d notations not to be given on the maps, though their sequence within the 1,000 yard squares is always the same >
	ab cd
2.	You will see that the edges of the 500 yard squares are divided into ten equal lengths (though again, these are not always marked on the maps). The first number in the final two digit code (in this case "2") is always the distance from the bottom left of the 500 yard square, moving to the right - ie two tenths of the 500 yard square - 100 yards east of the western side of the square.
6.	The second digit is your south to north instruction - 6/10ths of the 500 yard square vertically upwards from the point established by "2" above. You have now reached "G.9.a.2.6" - No 23 Field Ambulance.
	What if the map reference had shown something like "G.9.a.25.69." ? - That is simply a more precise reference but it isnt unusual to see that form in diaries, especially for Artillery targets. Again its quite simple, - after you have plotted the "2" as above, simply imagine the space between the 1/10th markings in the 500 yard square to be sub-divided again into ten more divisions (then ditto for the "6" component), thus bringing your "accuracy" down from a 50 x 50 yard sub-square to a 5 x 5 yard square.