Pinpoint your location with GPS

The OS grid divides the country into named squares 100km on a side...

...with numbered gridlines within each square. The first three digits are the horizontal coordinate, the final three are the vertical.

At its most basic level, a GPS receiver tells you where you are. To make any use of that information, though, you need to know a little bit about how maps work. Here in the UK, we're blessed with Ordnance Survey maps - highly detailed, legendarily accurate and covering the whole country at a range of scales. To pinpoint your location on the map, we use a grid reference.

Grids are a slightly arbitrary but very useful map feature. Because the planet is (roughly) spherical, on a global scale longitude and latitude is used, with lines a fixed number of degrees apart. Lines of longitude run north-south and tell you how many degrees east or west of the Greenwich meridian you are. Lines of latitude run east-west and tell you how many degrees north or south of the equator you are.

Because lines of longitude converge at the poles, the further from the equator you are the smaller the actual distance on the ground between any two lines a given number of degrees apart becomes. At the equator, one minute of arc (a sixtieth of a degree) is equivalent to one nautical mile. At the poles, it's zero. Lines of latitude are all parallel to the equator, so the relationship between degrees and miles stays constant no matter where you are.

Clearly this is all very complicated, with calculating distances on the ground requiring a firm grasp of spherical trigonometry. Hence the use of a grid over smaller areas like, for example, Great Britain. Sticking a bunch of parallel lines over the map lets you essentially ignore all that latitude and longitude stuff and work with a simple coordinate system.

The vertical lines on a map are aligned to what's called Grid North, parallel to a particular line (or "meridian" of longitude. True North is the direction along the actual lines of longitude, converging at the pole, so the further away from the meridian to which the grid is aligned you are, the more Grid North varies from True North. Again, you don't need to worry about this for most purposes. You also don't need to worry about Magnetic North, which is the way a compass points and a very small amount different from either of the other norths.

But to return to the grid, it's arranged on a number of scales. The lines on an OS map are 1km apart. Each one has a two-digit number. Obviously there aren't enough two-digit numbers to cover the 1,100km length of Britain, so the grid is divided into 100x100km squares and each of those given a two-letter code. The grid lines within each square are then numbered 00 to 99 both north-south and east-west.

GPSes designed primarily for navigation that are OSGB-aware will give you a number like ST125413. With any luck you'll know roughly where you are so you may be able to ignore the letters. The rest of the grid reference is made up of the actual coordinates, with three digits for the "eastings" (reading crossways) and three for the "northings" (reading upwards). The first two digits in each case relate to the numbers printed on the grid lines, with the third telling you how far between that line and the next you are. So "125" is halfway between 12 and 13, while "413" is a third of the way between 41 and 42. And there you are.

The same operation in reverse gives you a grid reference that you can punch in to your GPS so that it can tell you in which direction (and how far away) somewhere you're looking for lies, but there'll be more on that later in the series.