Cassini Grid

The Cassini Grid was a grid coordinate system used on British military maps during the first half of the twentieth century, particularly during World War II. The referencing consists of square grids drawn on a Cassini projection. For a period after the war, the maps were also used by the general public. The system has been superseded by the Ordnance Survey National Grid.

The Cassini Grid system was introduced in 1927 for maps of the United Kingdom for the British military. It is so called from the use of Cassini map projection. It modified and replaced a grid coordinate system first deployed in 1919 known as the British System. The Cassini Grid system is thus more properly called the Modified British System. It may also be referred to as Military Grid, War Office Grid, or Purple Grid (due to the grid colour on some issues of maps). It continued in use until some time after World War II.

After the war, the Cassini Grid system was replaced with the Ordnance Survey National Grid (see § Transition to National Grid). However, the Cassini Grid system continued to be used to map colonial possessions into the 1960s. Hong Kong, for instance, was surveyed in 1962 for a series of 1:10,000 and 1:25,000 maps of the territory. These were published between 1965 and 1972.

The map is divided into a grid of 500 km squares. The squares covering England are Q, R, V and W with an intersection somewhere between York and Thirsk. Ireland is covered by square I, but the Irish grid is tilted slightly to the East relative to the English grid. Each 500 km square is further divided into 25 100 km squares. The 100 km squares are also given letters (with I omitted). The full designation for a grid square is the 500 km letter in lowercase followed by the 100 km letter in uppercase. For instance, the squares covering London are wL and wQ.

Each 100 km square is subdivided into 10 km squares, and each of these is further subdivided into 1 km squares. These squares are numbered according to eastings and northings, that is, counting from the south-west corner of the 100 km square. For instance, the grid reference 25 76 refers to a 1 km square within the 10 km square 2 7. That is, it is two squares across and seven squares up (counting from zero) from the reference corner. Greater precision can be obtained by using six, eight, or even more digits, but always an even number with the same precision on both coordinates.

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