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French 4 figure map references

On some French trench maps, points are marked with 4 digits. See example below with those numbers in red.

Extract from Rémonville 1/20.000 25 October 1918

These numbers are simplified map coordinates known as abbreviated hectometric coordinates, below is an explanation using an
example. Suppose someone needs to mark a point "p" on the map with coordinates longitude (x) 69,560 km and latitude (y) 92,410 km.
This point should be in the square with coordinates x 69 km and y 92 km in the lower left corner as shown below:

The procedure is as follows: 1. Coordinates must be rounded to the nearest hectometre: x is now 69,6 instead of 69,560 and y is now 92,4 instead of 92,410. 2. Only the last digit from the kilometre values is required, this results in: x= 9,6 and y 2,4. 3. A further simplification is done by dropping the comma's and joining the 2 values to: 9624

Point p will be marked on the map at the intersection of the lines 96 and 24:
Other points can be marked in the same square using the same system, the format is always 9x2y in this square.

Please note that there could be several 1km squares with the same base number on large maps.
Example: Square 69,82 would be 10km to the south of square 69,92 on the same map. Large French maps have a letter for each 10 by 10 km area in the following format:

The letters A and F can be seen in the example map extract. To avoid confusion when referencing to a point the 10km square letter
should be added: Examples from that map extract should read A8208 or F7895.

Souces:
Topographie de Campagne by P.Phillipot
The U.S. Air Service In World War I Volume III by Maurer, Maurer
The story of the 27th Division by O'Ryan, John F

 

 

 

 

 

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