Grid reference
From Wikipedia, the free encyclopedia
A grid reference is a standard method for the location of 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 United Kingdom Ordnance Survey maps use a one kilometre 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 absolute accuracy is not required. On a map that uses a standard system for its Eastings and Northings, such as an Ordnance Survey map which uses the British national grid reference system, one can identify a point on the map and describe this point to someone else, even if that person uses another map of a different scale. With a standard system, the more digits one adds to a grid reference, the more accurate the reference becomes.
For Ordnance Survey maps, each Easting and Northing is given a two digit code, based on the British national grid reference system and the line's position relative to a 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.
In order to locate a specific building within Little Plumpton, it is possible to add a further two digits to the four digit reference to create a six digit reference. The extra two digits describe a position within the 1 km square. Imagining (or drawing, or superimposing using a Romer) a further 10x10 grid within the current square can label the bottom and left hand sides of this square with labels similar to the Eastings and Northings of 0-9. If the position of the church in Little Plumpton was being described, it would be guessed that it is 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. We are left with the digits 6 and 8. The convention is to add these 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 actually describes a 100 metre by 100 metre square, and not a single point, but is usually sufficient for navigation purposes.
Using large scale maps and an accurate Romer it would be possible to create digit grid references of larger numbers. This might be used in surveying but is not generally used for land navigating for walkers or cyclists etc. The growing availability and cost of GPS systems allow for accurate grid references without the need for a map, but it is important that one know how many digits the GPS is reading to, so that one does not merely read off the first six digits. A GPS unit commonly gives a ten digit grid reference, so in order to give a six figure reference from that the fourth and fifth digits can be ignored, while the ninth and tenth digits can be used for rounding.
