1.2 Specifying position

It is useful to be able to specify a position exactly on a map or chart. Indeed, this could be of critical importance when informing the coastguard of your whereabouts in an emergency.

Many people will be aware of the OS national grid. All OS maps are overlaid with blue grid lines which divide the map into 1 km squares. Numbers along the side of the map allow each square to be referenced.

Part of the OS grid

To reference a specific square on the grid we use the numbers to the left of and below the square. For example, to reference the square on the grid above highlighted in yellow, we’d use the number 62 along the bottom of the map (the ‘easting’) and 33 along the side (the ‘northing’). OS grid references are always given with the number along the bottom of the map (easting) first (some people use the phrase ‘bottoms up’ to remember this). So the grid reference for the square is ‘62 33’.

The reference numbers on OS maps only go from 00 to 99. This is a problem as OS maps have to cover the whole country, which is larger than 100 km x 100 km! To solve this problem the country is divided up into 100 km squares, each referenced by 2 letters. You’ll find these letters at the corners of OS maps, or at the edges of the squares, when these appear on a map. Our grid above is entirely in the ‘TL’ 100 km square, so we can provide a gird reference for our square of ‘TL 62 33’. In practice, the letters can be omitted if it’s clear roughly where the location is.

TL 62 33 is called a 4-figure grid reference for fairly obvious reasons. 4 figure grid references refer to a 1 km square, which is quite a large area. We often want to specify location more accurately than that. Because of this, sea kayakers will normally use 6-figure grid references, which specify a 100m x 100m square.

You’ll notice that there aren’t any grid lines spaced at 100 m intervals - they’d make the map look very cluttered. We need to estimate location within the grid square instead. For example, let’s create a 6-figure gird reference for the cross on the grid above. Looking first at the eastings along the bottom of the map, the cross appears about halfway across the grid square. Thinking of 10 imaginary divisions across the grid square, the cross would be on the 5th division, so we append the number ‘5’ to our original easting number to give ‘625’. Looking up the side of the map, we estimate that the cross is about three tenths of the way up the grid square, giving a northing of 333. Our complete 6-figure grid reference is ‘TL 625 333’.

Let’s look at an example on a real map.

Figure 1.2: OS map of part of the Isles of Scilly

What is the grid reference of the Peninnis Head lighthouse?

Notice that we can’t see the edge of the map here. However, you’ll notice that grid numbers are helpfully printed across the map to make giving grid references easier.

1. Two letters that identify the 100 km square – in this case ‘SV’. This would be given on the corners of the map sheet (not shown on the map extract above).
2. The ‘easting’ – the ‘horizontal’ coordinate. We use the scale on the edge of the map, which is repeated for convenience across the sheet. The first two letters identify the grid square – in this case ‘91’. For greater precision, we can add a third number to indicate how many tenths across the square the location is. In this case the lighthouse is 1/10 of the way across the square, so the third number is ‘1’ to give a complete easting of ‘911’
3. The ‘northing’ – the ‘vertical’ coordinate. Exactly the same as for the easting, but up the map. In this case, ‘093’. Note that we keep the leading zero here to make it clear that this is a 6 figure gird reference, and we aren’t referring to square 93.

The complete grid reference is thus ‘SV 911 093’.

We’d use the phonetic alphabet to say this over VHF: “Sierra Victor nine-one-one zero-nine-three”.

Giving 6-figure grid references is somewhat approximate, but for most purposes estimating the final numbers for eastings and northings by eye is good enough. A tool called a ‘romer’ - a card with graduations matched to the map scale - can be used to improve accuracy. Many hiking compasses feature romer scales at their corners, so if you’re carrying a compass, it’s likely that you can use one if you want to be super-accurate.

Romer on compass being used to determine grid reference for the beacon on Sgeir nam Maol. The romer reads about 5 for the easting and 6 for the northing, giving a grid reference of NG 395 816. Note the importance here of using the correct romer scale - this is a 1:50,000 map, so we use the 1:50k scale and ignore the 1:40k and 1:25k scales.

There’s no theoretical reason to stop at 6 figures. An 8-figure grid reference specifies position to the nearest 10 meters. It’s hard to read an 8-figure grid reference from a map (although using a romer and a 1:25,000 makes it feasible). However, 8-figure (or even 10-figure) grid references are commonly given by GPS units.

GPS unit providing 10-figure grid reference (upper right of display, marked ‘BNG’ for ‘British National Grid’)

If you end up faced with an abnormally long OS grid reference, simply ignore anything after the first 3 figures of the northing and easting to give yourself an understandable 6-figure gird reference. For example, the GPS unit in the photo above shows ‘TL 44351 60361’. I’ll read off TL 443 603, which is a 6-figure grid reference that I can reference to a map easily.