There is a point somewhere off Lands End from which all points are measured.

There are two measurements:

All measurements are in metres

The UK is divided off into 100km squares starting from this point each of which is designated by two letters - for example NY is the 100km square covering the North Lakes.

Within each square a point is defined by the easting and northing measurement from the 100km square's SW corner.

For example my house is at NY317251

Which means that it is in the 100m square whose SW corner is 31700m east of and 25100m north of the SW corner of square NY. That locates my house to within 100m.

If we were not bothered about such precision we could say that my house is at NY3125 which defines the 1000m (1km) square 31 km east of and 25km north of the SW corner of NY. This is useful if we are talking about a village or some general feature.

When one is working in a small area that is entirely within the 100km square it is normal custom to omit the NY part of the reference, so my house becomes 317251.

If you need even greater acuracy then 31752514 defines a 10m square. This is only useful for small features on large scale maps.

As well as locating a site, grid references can be used to calculate how far apart two sites are.

For example the summit of Blencathra is at 324277 and my house is at 317251

From my house, the summit is (324 - 317)*100m = 700m to the east and (277 - 251)*100m = 2600m to the north of my house.

Using Pythagoras we have 700*700 + 2600*2600 = 7,250,000 = 2700*2700 (approx)

So the summit of Blencathra is only 2.7km from my house - horizontally. It happens also to be 720m away - vertically, which as you can see makes for a very steep direct ascent, particularly as the first bit is much gentler!

I hope that explains the National Grid Reference System sufficiently.