Military Maps
One problem that crops up when studying anti-invasion defences is that of maps and grid reference systems.
While documents contain many map coordinates for pillboxes, roadblocks and other features, these references are virtually meaningless unless you have the same map series that was being used in 1940. As we are studying the landscape of 1940-41, it makes sense to use the same maps that those who compiled the documents were looking at.
This is important, as some references are made to non-landscape features on the map, for example:
Vickers Machine Gun (exclusive of infantry-manned m.g.s) position Serial No.6, manned by one section of E Coy. 2 P.L.K.R. at Pevensey Castle 079234. Right arc: "s" in Westham. Left arc: road 090232.
The arcs are drawn in red on the map at right; note how the description relates the right arc according to the "s" in 'Westham' printed on the map, rather than give a proper grid reference.
Without a wartime map, neither the grid references nor the location of the "s" would make any sense.
[Enlarged reproduction from 1941 Ordnance Survey 1-inch sheet 134 with the kind permission of the Ordnance Survey]
The standard maps used by the British Army were those of the Ordnance Survey at a quarter-inch to one mile (1: 253440) and one inch to one mile (1:63360). The War Office versions were based on the maps sold to civilians, with the addition of a grid reference system.
The Cassini Grid
In 1919, the British introduced a grid cooordinate system for their military maps; this British System was superceded in 1927 by the Modified British System, which is that referenced in the documents I'm consulting.
As this system was only used on military maps and not those available to the public (until later on), the Modified British System is often known as Military Grid, War Office Grid, Cassini Grid (after the method of projecting the grid onto the curvature of the earth), or simply as Purple Grid after the colour of the grid printed on some map editions.
A grid of 500km squares was superimposed on the United Kingdom; this is seen in the diagram at right outlined in red. The four squares that cover England are lettered Q, R, V and W.
These 500km squares were each subdivided into 25 squares of 100km sides, lettered A-Z (excluding the letter 'I').
The national reference for a 100km square is to give the 500km square in lower case, followed by the 100km square as a capital letter.
East Sussex therefore falls within the area of grid squares wQ and wR, shaded in blue.
[Diagram from Notes on Map Reading (The War Office, 1940)]
The 100km squares break down into 10km squares; the image at left shows grid square 0 2 highlighted in red from the quarter-inch map (left) and the one-inch map. (These extracts are not seen here in scale with each other). Note how Grid Square 0 2 on the one-inch map subdivides into 100 1km squares.
[Reproduced from the 1941 and 1943 Ordnance Survey maps with the kind permission of the Ordnance Survey]
Highlighted in blue on the one-inch map is grid square 08 23, which is the only complete square seen in the image at the top of the page; it is also seen below, shaded in blue.
The Vickers gun position was at grid reference 079 234, which implies a further subdivision of the 1km square by an imaginary grid to give a six-figure reference.
The precise reference would therefore be wQ 079 234; however, the letters can also be substituted for the number of 500km squares east and north from the zero point origin of the grid (vV as shown in the diagram above). This places square wR six squares east and one square north (vV counts as 0, not 1). Translated into a purely numerical reference, the reference becaomes 6079 1234.
As military documents normally state which map sheet number(s) they reference (and it's usually obvious which they are even if they aren't explicitly named), the letters or full numerical references are seldom encountered except with Royal Artillery Fort Record books, where seemingly very complex grids are given.
For example: "CASL Position No.1 at eastings 604 595.0 northings 116 720.0." This translates into six-figure reference wR 046 167:
- Eastings 604 595.0 (Should be rounded up to 046)
- Northings 116 720.0
The 'w' comes from the 6 prefixing the eastings, the 'R' from the initial figure of the northings.
Although this is a precise 12-figure grid reference, I round it down to six-figures to make it easier to work with and convert to the modern National Grid. Because I work primarily at the one-inch scale, an eight-figure reference is pushing the level of accuracy to its limit, but as 99% of grid references given in documents are four- or six-figure it makes sense to aim at the latter.
Converting Cassini Grid to National Grid
There's no easy way to do this, as the two systems are not related, although the grid squares on both are 1km square. The National Grid Reference (NGR) was originally introduced in 1938 to supercede the Cassini system, but the process was abruptly halted by the war, and so the British continued to use the Cassini system for the duration.
The loss of some survey data caused by the bombing of the Ordnance Survey offices in Southampton in 1940 delayed the introduction of NGR in some areas until at least 1947; in the meantime, War Office Cassini maps were made available for sale to the public until the NGR sheets were ready.
A formula to convert Cassini references to NGR has been published and a computer program compiled by John Penny - see his excellent article Straight to the point right on time [1]. The accuracy of the conversion formula is stated to be +-200m.
For my own purposes, I have modified the formula by introducing a local variation to increase the accuracy for grid squares wQ and wR.
"Accuracy" is only relative, however. While my modified formula appears to convert a six-figure Cassini reference to National Grid "accurately" (i.e. Cassini wR 046 167 converts to NGR TV 612 969 and they both mark the same location), a conversion will not necessarily lead you onto the precise spot once occupied by a pillbox or other feature, as the Cassini reference may be inaccurate itself. It should be remembered that a six-figure reference describes a 100m square in which there is a big margin of error!
Other maps
Other Ordnance Survey maps in use at the time were those of the County Series, namely in 6 inches to one mile and 25 inches to one mile scale. The latter was apparently also produced with the Cassini Grid for the War Office, although I have only come across one instance of its use so far amongst the archive documents.
References
- Penny, J. (2000) Straight to the point right on time available online at: http://humanities.uwe.ac.uk/regionhistory/rhcnew/RHCarticles/General.htm