# Travel >> Travel Preparation >> Maps & Navigation

How to Convert Grid References

Derived from the latitudinal and longitudinal map references, Ordinance Survey, or OS Grid grid coordinates, help travelers in the United Kingdom find their way around. Great Britain is bisected by the two-degree longitudinal line. In traditional terms, this would give half the country positive cartological value while the other half is negative. In order to clear up confusion, an imaginary line was drawn 400 km to the east of the natural line. This gives a new Y axis, or "Northing" for the OS grid. The natural latitude was also moved 100 km north to provide a more convenient X axis, or "Easting."

Instructions

- 1
  Find Great Britain on the OS map. The largest boxes on the OS map are 500 km by 500 km and split into a five-by-five grid, or 25 smaller boxes. Each of these boxes is given an alphabetical designation. Further designation is made by splitting each of the smaller 100 km boxes into 100-meter, 10-meter and 1-meter grids. The map coordinates can be given in either alphabetical or numeric values from the OS starting point.
- 2
  Convert the alphabetical designation of the grid box to the numeric distance of that point from the starting point. Do this for both the Northing and Easting coordinates.
- 3
  Subtract 400 km from the Easting distance. This will move the vertical axis back to the two-degree longitude. If the sum is negative, the point is to the east of the two-degree longitude. If the sum is positive, the point lies to the west. Use this point as a reference to find outlying longitudinal points.
- 4
  Add 100 km from the Northing axis. This replaces the latitude at the 49-degree point. Every 95 km, the latitudinal degree advances one digit.