> Notes on the Modified British System of coordinates used on the European Theatre of Operations during the WWII
> Overview of the "Modified British System" used on the European Theatre of Operations   (version française)

The historical context

The Army Forces involved in the WWII were for the most brought to define and to use specific geographical coordinate systems in order to specify the localization of points or targets located on the theater of operations. The fights of the First World War had shown all the interest of such systems which, used in conjunction with adequate cartographic data, had largely contributed to the improvement of the action of the units of artillery on the battle field. Considerations relating to artillery had precisely led the French Army to adopt in 1915 the Lambert map projection "Nord de Guerre", covering the North-East of France and Germany, and which was going to be supplemented during the inter-war period by the Lambert 1, Lambert 2 and Lambert 3 projections in order to cover the rest of the French territory.

In 1919, Great Britain adopted on its territory the "British System", based on a Cassini projection and on the use of a reference grid. This system was superseded in 1927 by the "Modified British System", more adapted to the coverage of large areas and to a military operational use by terrestrial and aerial forces.

Diagram of the grid used over the British territory ("Modified British System")

This "Modified British System" was going to be used during the World War II by the British and American Armies, while being extended to the European, North African, Middle East and South-East Asian theatres of operations.

Characteristics of the "Modified British System" used during the WWII

The "Modified British System" used during the World War II is based on the Cassini projection covering the British territory, and on a series of other projections intended to cover, while being put side to side, the whole of the theatres of operations.

Western Europe is thus cut out in ten zones, whose cartographic statements have been produced at the end of the Thirties and at the beginning of the war by the British and French Army Geographical Services (War Office, Geographical Section, General Staff for Great Britain).

In the "Modified British System", each projected area is divided into squares of 500 kilometers side, each of which designated by a letter. Each of these squares is subdivided into 25 squares of 100 kilometers side which are also lettered from "A" to "Z" omitting the letter "I".

These squares of 100 kilometers side, which play a fundamental role in the determination of the coordinates, and are thus often represented on the maps of the time, are referred in a complete way by the combination of two letters, for example 'vZ', indicating here the 100 km square Z included in the 500 km square v.

Diagram of the grid used for the "Nord de Guerre Zone"
in the "Modified British System"

The determination of the coordinates of a point in the "Modified British System" is carried out in two stages : firstly by indicating the reference of the 100 km side square inside whose this point is located, secondly by defining the exact position of the point inside this square.

A point is always located inside a square of 100 km by its co-ordinates x/y respectively measured on its x-axis (West-east) and y-axis (North-South), refered to the point of origin of this square (left lower corner). According to the scale of the map used for their determination, these coordinates are expressed in kilometers or hundreds of meters ; a network of vertical and horizontal lines separated by 10 km or 1 km is sometimes represented on the maps in order to facilitate the reading of the x and y values.

As an example, the town of Alençon (France) is located in the "vZ" square of the "French Lambert Zone 1" (LZ1). When we determine the position of the city compared to the origin of this square, using a map scaled to 1/250000°, we respectively measure x=33 km and y=83 km. Formatting this result in the "Modified British System", we finally obtain the following reference : (LZ1) vZ 331/835 or (LZ1) vZ3383, by joining the two values.

"French Lambert Zone 1" Grid

As an other example, the Strasbourg-Entzheim airfield (France) is located in the "wV" square of the "Nord de Guerre Zone" (NGZ). Its position, compared to the origin of this square using a map scaled to 1/50000°, is given by the values: x=93000 m and y=93700 m. By expressing these measurements in hundreds of meters, we obtain x=930 and y=937, which gives, in the "Modified British System" format, the following coordinates: (NGZ) wV930937.

The absolute and "canonical" reference of a point on the theatre of operations is thus defined by the combination of three data, which are:

- the name of the map projection or geographical area used for the determination of the position (example: "Scandinavian Zone 3")
- the complete reference of the 100 km side square containing the point (example: "vR")
- the numerical data indicating the position of the point inside the square (four or six digits according to the precision of the determination).

Example : Nord de Guerre Zone (NGZ) / rW705951

In practice, the soldiers brought to use this system in operational situations often used a slightly different way of writing the coordinates, including the reference to the maps used for the determination.

Several examples of references, corresponding to the same point located in the area of Boulogne (northern of France), are thus given below:

- 1/ Reference EUROPE (Air) 1/500000 Sheet "STRAIT OF DOVER" / Map Reference vG7461
- 2/ Reference GSGS 3982 (1/250000) Sheet "BOULOGNE" / Map Reference vG7461
- 3/ Reference FRANCE (Army / Air) (1/250000) Sheet 1 / Map Reference G7461
- 4/ Reference GSGS 4336 (1/100000) Sheet 5 / Map Reference G7461
- 5/ Reference FRANCE AND BELGIUM (1/50000) Sheet 38 / Map Reference 744616
- 6/ Reference FRANCE (1/250000) Sheet 38 SW / Map Reference 744616

The informations given by these references are only fragmentary if we try today to locate the point without the original maps in hand. In the case of the reference #3 for example, the only exploitable data are the followings:

- the point is located in the North of France
- the point is located into a (?)G 100 km side square
- the numerical data relating to the position of the point in the square are: 7461

It appears here fortunately that these informations, although fragmentary, are sufficient to be able to go up with the complete coordinates of that point. By consulting the grids used on the European Theatre of Operations, we note indeed that the "Nord de Guerre Zone" includes a square designated by the reference "vG". We can deduce from it that the "canonical" coordinates of the point are as follows:

Nord de Guerre Zone (NGZ) / vG7461

The researchers dealing with fragmentary coordinates will therefore always have interest to take a look to the grids used on the European Theatre of Operations, by using at their maximum the contextual information which they will have been able to collect otherwise (example: operations carried out in the north of France, in the area of Strasbourg, etc…).

The conversion towards other systems of coordinates

The "Modified British System" of coordinates is based, as we saw, on the use of a series of map projections intended to cover the totality of the Western and Central European territories.

These map projections correspond, for some, to the "national projections" used before the war by the involved countries, as Great Britain ("Cassini Grid") and France (Lambert 1, 2, 3 and "Nord de Guerre"). The coverage of the other geographical areas had to be ensured at the end of the Thirties and at the beginning of the war using completely new projections, like for example for the Italian and Iberian peninsulas.

In the case of already existing projections, the definition of the grids was very simple, as we are going to see it here through the example of the French Lambert zones, representative of the process followed for the other projections.

As we've seen it above, the Geographical Service of the French Army used five geographical projections in 1939, corresponding to five large areas cutting out its territory:

- the "Nord de Guerre" zone, covering the North-eastern part of France (as well as Belgium and Germany),
- the "Lambert 1" zone, covering the north-western part of France,
- the "Lambert 2", zone covering the central part of France,
- the "Lambert 3", zone covering the southern part of France,
- the "Lambert 4", zone covering Corsica.

(it should be noted that the Lambert 1, 2, 3 and 4 projections are always used in France nowadays, a modification of the formulas of computation having however been brought in 1948).

The coordinate systems used for these map projections locate the points using two coordinates, X and Y, expressed in meters. For example, the co-ordinates of the center of the town of Alençon, measured on a "Lambert 1" map of the time, are: X=433300 m and Y=83500 m.

"French Lambert Zone 1" Grid

" French Lambert Zone 2" Grid

" French Lambert Zone 3" Grid

The use of these grids for a coordinates conversion from "Modified British System" to "French Lambert System" is shown throuh the following example:

The "Modified British" coordinates of the Rennes-Saint-Jacques airfield are :
(LZ1) VX962494

1/ we measure the co-ordinates of the origin (left lower corner) of the "vX" square of the "French Lambert zone 1" (LZ1) grid :

These coordinates are : X0=200000 m et Y0=0 m.

2/ the Lambert 1 (X,Y) coordinates of the airfield are computed as follows :

X= X0 + (962x100) = 200000 + (962x100) = 296200 m
Y= Y0 + (494x100) = 0 + (494x100) = 49400 m

The French Lambert 1, 2 and 3 system still being used in France nowadays, it is quite easy to locate a point whose coordinates are expressed in the old "Modified British System", by reproducing the calculation above and consulting an adequate map produced by the IGN (French National Geographical Agency).

On the other hand, the "Nord de Guerre", "British Cassini", "North Italy", "South Italy"... projections not being used anymore, it is necessary to do the conversion of the coordinates toward other systems, more adapted to the localization of points on a modern map.

The "Coordinates Translator" proposed on this web site was developed for this reason : on the basis of complete coordinates expressed in the "Modified British System", it makes it possible to obtain the corresponding geographical co-ordinates expressed in terms of latitude and longitude. These co-ordinates can then be exploited quite easily using detailed geographical maps or numerical atlases like the one proposed on the MapQuest web site.

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