Projected coordinate system
A Grid Reference is a standard method for location a point on a map. The grid itself is a coordinate system imposed on the map, and is numbered in such a way as to provide a unique way to identify any given feature.
Grid systems vary, but the most common is a square grid originating at the bottom left of the map. Each grid line is numbered sequentially. The grid numbers on the east-west or horizontal axis are called Eastings,and the grid numbers on the north-south or vertical axis are called Northings.
Grids may be arbitrary, or based on specific distances - for example the 1:50000 Ordnance Survey maps use a 1km square grid spacing.
A Grid Reference locates a unique point on the map. The resolution of the reference provides for some given margin of error - for example a simple town plan may only use a simple grid system such as letters for the Eastings and a single number for the Northings. A grid reference in this system, such as 'H3' only locates a particular square rather than a single point, but you'll usually spot what you're looking for, so absolute accuracy isn't required. For Ordnance Survey maps, a six-figure grid reference is usually used, providing a resolution down to 100 metres on the ground.
To give a grid reference, quote the Eastings first, then the Northings. For example, a reference of 687089 specifies square (68,08), and the point (7,9) within it. For casual use, estimating the tenths by eye is usually sufficient, but for complete accuracy, a romer is used to mark the point exactly.
The simple map below shows the principle - the grid reference above locates the church in the village of Little Plumpton.
