There has to be a way of knowing where you’ve seen the butterflies you record, and this is done by using grid references (also called map references). These may seem a bit daunting at first, but they are easy when you know how, especially with the help of the internet.

## Useful websites

Two websites we would recommend to help you:

This allows you to put in the name of a nearby place, and then navigate your way to where you saw your butterflies on aerial photographs. When you have pinpointed the right place, you click on it and it will give you a grid reference.

The grid reference will come up in the form of two letters followed by ten digits. You need to treat the ten digits as two sets of five, and give us the first three of each – so SY8065084590 should be read as SY **806**50 **845**90 and you tell us SY806 845.

If you saw quite a lot of butterflies in an area and can’t pinpoint each individually, you can just give us the first two digits of each set – so the example will give SY80 84, though check the whole of the area is covered by these numbers by clicking on a few spots within it.

This enables you to convert a postcode to a grid reference. As with the site above, the grid reference will come up as ten digits, so please follow the instructions above.

## Using an Ordnance Survey map

You will need a map of the area you have sighted your butterflies – there is more information about this at the bottom of this page.

- On Ordnance Survey maps the UK is divided into squares, each of which is 100 kilometre (km) in size.
- Each of these is given a pair of letters to identify it, and Dorset covers part of four of these: ST, SU, SY and SZ. You can see this on the White Holes map.
- There is a panel to the side of all OS maps which gives the key and also shows the squares this map covers. On some maps the letters are also shown on the four corners of the map itself.
- These 100km-sized squares are then divided into squares of one kilometre, which are described with numbers.
- The numbers are shown at the bottom of the map by a row of numbers (called Eastings) and up the sides by another row of numbers, called Northings.
- You always read the Easting first and the Northings second – you can remember this by thinking about an aeroplane, which has to go along the runway before it goes up into the sky.

These numbers allow you to identify a kilometre square: Portland Bill, for example, is in square 67/68, 67 being the Easting and 68 the Northing.

To give even more accuracy, you imagine each of these squares divided into another ten squares, so if your butterfly was seen halfway between 68 and 69 you describe it as 685. If it was very near 69 but not quite there it can be described as 689. There is inevitably a degree of estimation in this, but it is possible to be quite accurate. You use the same technique for the Northings, so Portland Bill is more accurately described as being at 677683.

## Ordnance Survey maps

Ordnance Survey publish two sets of maps for the UK:

- The Landranger series which is 1:50,000 – so it covers more area but in less detail
- The Explorer series which is 1:25,000 – more detail but less area

### Landranger maps covering Dorset:

- 183 Yeovil and Frome
- 184 Salisbury and the Plain
- 193 Taunton and Lyme Regis
- 194 Dorchester and Weymouth
- 195 Bournemouth and Purbeck

### Explorer maps for Dorset:

- 15 Purbeck and South Dorset
- 22 New Forest
- 117 Cerne Abbas and Bere Regis
- 118 Shaftesbury and Cranborne Chase
- 129 Yeovil and Sherborne