GRID REFERENCE

'Grid references' define locations on maps using grid values. Grid lines on maps define the coordinate system, and are numbered to provide a unique reference to features.
Grid systems vary, but the most common is a square grid with grid lines numbered sequentially from the origin at the bottom left of the map. The grid numbers on the east-west (horizontal) axis are called Eastings, and the grid numbers on the north-south (vertical) axis are called Northings.
Grids may be arbitrary, or based on specific distances, for example the United Kingdom Ordnance Survey maps use a one kilometre square grid spacing.
A grid reference locates a unique point on the map. The precision of location varies, for example a simple town plan may use a simple grid system with single letters for Eastings and single numbers for Northings. A grid reference in this system, such as 'H3', locates a particular square rather than a single point.
Points can be located by grid references on maps that use a standard system for Eastings and Northings, such as the British national grid reference system on Ordnance Survey, and located by someone else using grid references, even if using maps of a different scale.
For Ordnance Survey maps, each Easting and Northing grid line is given a two digit code, based on the British national grid reference system with origin point just off the south west coast of the United Kingdom. Since the Eastings and Northings are one kilometre apart, a combination of a Northing and an Easting will give a four digit grid reference describing a one kilometre square on the ground. In the example map below, the town Little Plumpton lies in the square 6801, even though the writing labeling the town is in 6802 and 6902 and most of the buildings (the orange boxed symbols) are in square 6901.
The more digits one adds to a grid reference, the more precise the reference becomes. To locate a specific building in Little Plumpton, a further two digits are added to the four digit reference to create a six digit reference. The extra two digits describe a position within the 1 km square. Imagine (or draw or superimpose a Romer) a further 10x10 grid within the current grid square. The grid reference for the church in Little Plumpton is estimated to be about six tenths of the way from left to right in its grid square, and about eight tenths of the way from the bottom to the top. This gives the digits 6 and 8. These are added to the four figure grid reference after the two digits describing the same coordinate axis, and thus our six figure grid reference for the church becomes 696018. This reference describes a 100 metre by 100 metre square, and not a single point, but this precision is usually sufficient for navigation purposes.
Grid references comprising larger numbers for greater precison could be determined using large scale maps and an accurate Romer. This might be used in surveying but is not generally used for land navigating for walkers or cyclists etc. The growing availability and decreasing cost of handheld GPS receivers enables determination of accurate grid references without needing a map, but it is important to know how many digits the GPS displays to avoid just reading off the first six digits. A GPS unit commonly gives a ten digit grid reference, based on 2 groups of 5 numbers for the Eastings and Northing values. Each successive increase in accuracy (from 6 digit to 8 digit to 10 digit) pinpoints the location by a factor of 10. Since, in the UK at least, a 6 figure grid reference identifies a square of 100m sides, an 8 figure reference would identify a 10m square, and a 10 digit reference a 1m square. In order to give a standard 6 figure grid reference from a 10 figure GPS readout, the 4th, 5th, 9th and 10th digits must be omitted, it is important not to read just the first 6 digits.