Convert an OS map point to longitude and latitude.
Britain is covered by a series of maps produced by an organization called Ordnance Survey (OS). OS grid references are based on 100-kilometer grid squares, and each is identified by letter pairs. To reference a point inside this area, digits are used to identify a sub-square within the grid square. For example: TG 5140 1317 represents a 10-meter box with its (south-west) origin 51.40 kilometers across and 13.17 kilometers up within the TG square. Converting an OS grid reference to longitude/latitude is easy using an online utility.
Instructions
1. Visit Movable Type's National Grid Reference conversion page (see Resources).
2. Enter the OS grid reference into the textbox titled "OS Grid Ref." Remember that a grid reference identifies a square (with the size of the square determined by the number of digits). A latitude/longitude coordinate identifies a point. Its precision is also implied by the number of digits.
3. Click outside the "OS Grid Ref." textbox and the conversion will be made in the "WGS84 Lat/Lon" and "OSGB36 Lat/Lon" textboxes.
4. Decide which conversion suits your purposes. GPS systems worldwide generally use the "WGS84 Lat/Lon" conversion. This is based on an elliptical model that is an approximation to the entire earth. The Ordnance Survey itself usually uses "OSGB-36," which is based on an elliptical model of the earth's surface.
