How to find a point on a trench map

The trench maps used in the Great War resulted from the exigencies of war. The mapping system used, the scale and accuracy of the surveying and quality of the drawing would leave a lot to be desired if designed from scratch during peacetime but they were not, they were made in the heat of battle. Seen in this light, they should be considered a monumental achievement.

Although the Army had planned what to do in the event of a European war, they were found wanting when it came to equipment. The British Army had until then always fought small scale wars of movement, they were not prepared for large scale static warfare. As part of this equipment, the maps available in 1914 were either the woefully inadequate French maps, some little better than sketches, or the better Belgian maps. Some good French maps were available, the Plans Directeur, but these only covered a small part of the front.

The importance of producing good maps in a very short time led to the decision to use an Imperial System (i.e.yards) to be super-imposed on the Belgian maps that used the Metric System (i.e.metres). This grid system was then extended over the available French maps. The resulting grids give rise to some confusion in those modern minds more familiar with British Ordnance Survey maps. A fascinating description of this and other aspects of trench maps can be found in the Topography of Armageddon by Peter Chasseaud. For a comprehensive history of the mapping of the Western Front, see Peter Chasseaud's book Artillery's Astrologers: A History of British Survey and Mapping on the Western Front, 1914-18, available at Amazon.co.uk or elsewhere.

Click image for a larger version.

Each numbered rectangle is 36,000 yards wide. The rather odd numbering sequence is due to the extension of the grid from Belgium to France. To draw one rectangle on a single map of manageable size, a scale of 1:40,000 was used. Each 1:40,000 rectangle was referred to by the number in the centre of each one. This scale is suitable for strategic or even tactical purposes but lacks sufficient detail for field use. NB. Late in the Great War, square 36c was changed to 44a, and square 36b changed to 44b. To provide a map reference, each 1:40,000 rectangle was divided into 24 rectangles, lettered A to X, six in a row. Each lettered square was then further divided into numbered "squares", some numbered 1-30 and some numbered 1-36, 6 in a row.

Click here for a complete map of the 1:40,000 rectangles dated 31st January 1918.

Although trench maps used for Great War research are generally those drawn at 1:10,000, the map reference is derived from the 1:40,000 sheets. As can be seen below, the 1:10,000 maps do not fit comfortably on the lettered and numbered grid.

Each 1:40,000 map was then subdivided into four to a scale of 1:20,000, referred to by compass points, NE, NW, SE and SW.

Every 1:20,000 rectangle was further subdivided into four at 1:10,000 for use in the field. The resulting 1:10,000 maps were named after the compass point of the quadrant they are in followed by a number as in the figure below. This means that areas mapped at 1:40,000 required sixteen 1:10,000 maps for the same area.

The grid used on the 1:40,000 was retained for the 1:20,000 and 1:10,000 maps so a single map reference was valid on all of them. Unfortunately, the requirement to base maps that used yards on a metric system results in an odd layout. The next figure shows the actual fit of the 1:10,000 maps on the 1:40,000 grid reference system. The map edges often cut part way across a grid square. The area covered by a 1:10,000 trench map is about 8000 by 5000 metres, divided into 1000 and 500 yard squares.

The main grid system used for trench maps is shown below. Early maps used a slightly different system, click here for details. The system shown used below was sometimes marked as the "Revised System of Squaring"

It can be seen that a 1:10,000 sheet such as NW4 covers all or part of squares B, C, H and I and that the squares in the top and bottom lettered rows were numbered 1-30, the two centre rows each numbered 1-36.

On a 1:10,000 trench map, the lettering of the squares were given as large letters in the corners of each square, the numbers being presented in the centre as the sample below. Each numbered square 1000 yards wide was further subdivided into four 500 yard squares, lettered a, b, c and d row by row (although the letters are not generally printed on the map)

Section of 1:10,000 sheet 51bNW2, Oppy. This in turn is part of the 1:40,000 sheet 51b.

A point such as point 1 (the tip of the arrow) above is expressed as B30a 3.5, i.e.:-

• B, the main 1:40,000 square it is in,
• 30, the numbered square within B,
• a to indicate which of the four 500 yards squares
• 3 to indicate the distance east in the square. This is called the Easting
• 5 to indicate the distance north in the square. This is called the Northing

Just B30a 3.5 would give a reference to a number of points in France and Belgium as it refers only inside one 1:40,000 rectangle, the square B is unique only within one rectangle, 51b in this example. To fully refer to a point, it is convenient to include the sheet reference such as 51bNW2, however, most Great War references were written without the sheet reference, so one has to refer to the 1:40,000 sheet to determine which 1:10,000 sheet to use. This often leads to confusion. Point 1 is uniquely identified as 51b B30a 3.5.

The point 51b B30a 3.5 gives a position to a resolution of 50 yards as each small square is 50x50yards. To achieve better resolution, each 50 yard square can be thought of as being divided into a further ten rows and ten columns. For point 1, 34.55 would indicate 3 squares from the left and 4/10th of the way into that square and 5 squares to the north and 5/10ths of the way into that square. Looking at another way, the point is 3.4 squares from the left and 5.5 squares up, written as 34.55. If this system is used, leading zeros must also be used as in point 4 in the 5 yard resolution below.

Square 30 has a dotted grid to show the idea of the squares, real trench maps do not have these. Perhaps the easiest way to get a precise measurement is to make a scale such as the one shown below positioned over square 30. It is made from card with the scale carefully marked off from the side of a grid square. In the example here, the card is positioned to measure point 3, the tip of the arrow is placed in the corner of the cut-out card. The distance from the left (the Easting) is taken from where the vertical line crosses the scale, similarly, the distance up (the Northing) is taken from the horizontal scale.

The more accurate values for refences in the form B30d 24.83 can be estimated, in this case 2.4 and 8.3 squares.

		The figures shown here cover the British sector of the Western Front and shows many of the 1:10,000 trench maps superimposed on the main grid. Click images for larger versions The maps are quite large and best printed A3 in colour. The town outlines on the modern maps were produced from geographical data from the Digital Chart of the World, 1993, so the urban areas are shown larger than they were during the Great War.
A comtemporary map showing Army Administrative and L of C Areas of the British Armies in France. NB. Late in the Great War, square 36c was changed to 44a, and square 36b changed to 44b.	A comtemporary map from the Supplement to Catalogue of the Maps of the Theatre of War issued by the GSGS 31/1/1917