Grid reference
From Free net 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 1 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 you will usually spot what you are looking for, so 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, you can identify a point on your map and describe this point to someone else, even if they are using another map of a different scale. with a standard system, the more digits you add to a grid reference, the more accurate the reference becomes.
For Ordnance Survey maps, each Easting and Northing is given a 2 digit code, based on the British national grid reference system and the line's position relative to a point just of the south west coast of the United Kingdom. Since the Eastings and Northings are 1 kilometre apart, a combination of a Northing and an Easting will give a 4 digit grid reference descibing a 1 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, we can add a further 2 digits to our 4 digit reference to create a 6 digit reference. The extra 2 digits describe a position within the 1 km square. We need to imagine (or draw, or superimpose using a Romer) a further 10x10 grid within the current square. we can label the bottom and left hand sides of this square with labels similar to the Eastings and Northings of 0-9. If we were describing the position of the church in Little Plumpton, we could guess that it is about 6 tenths of the way from left to right in its grid square, and about 8 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 4 figure grid reference after the 2 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 an 8, 10, 12 etc. digit grid reference. 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 you know how many digits the GPS is reading to, so that you do not just read off the first 6 digits. A GPS unit will commonly give a 10 digit grid reference, so in order to give a 6 figure reference from that you will have to ignore the 4th, 5th digits while using the 9th and 10th digits for rounding.
