Understanding OS Maps,
Grid References and contours

• The UK is split into a large grid.

• Each grid square covers a 100km x 100km area and is represented by 2 letters.

• These 2 letters should be quoted before the 6-figure Grid Reference i.e. SK xxx yyy.

• First read along the bottom of the map to find the nearest 2-digit number to the left of your location ("Along the corridor").

• Divide the side of the square into ten equal sections. Read across then estimate in tenths how far you are into that square. 

• Next read along the side of the map to find the nearest 2-digit number below your location ("Up the stairs").

• Divide the side of the square into ten equal sections. Reading up, estimate in tenths how far you are into that square. 

• This will provide you with a 6-figure Grid Reference (xxx yyy).

• Remember to precede your Grid Reference with the 2 letters of the main OS grid. These will be shown in the corners of the map and when the numbers change from 99 to 00.

Common symbols that you will need on your DofE Expedition include:

• Roads (especially A and B roads, minor roads)

• Railway Line and Stations

• Electricity Lines

• Public Rights of Way (footpaths, bridleways, paths and byways)

• Community Landmarks - churches, pubs, public telephones, village halls.

• Camp Sites.

• Field boundaries.

• Woods and type of wood (coniferous etc).

• Hills and contours.

All maps contain a legend at one side of the map. This identifies all map symbols, as well as explaining other key information.

• The scale of a map is written as a ratio, e.g. 1:50000 or 1:25000.

• This means that, for every 1cm on the map, this represents the number of cms shown on the land - 50,000 cm or 25,000 cm respectively.

• 50,000 cm is the same as 0.5km. Therefore, 1 cm is the equivalent of 0.5km on land, or 2cm is the equivalent of 1km on the land.

• 25,000 cm is the same as 0.25km. Therefore, 1 cm is the equivalent of 0.25km on land, or 4cm is the equivalent of 1km on the land.

• Each grid square on the map represents 1km of land, therefore the grid squares on a 1:50000 map are 2cm x 2cm and on a 1:25000 map are 4cm x 4cm.

There are 3 Norths:

• Grid North is the top of the map. The grid lines on OS maps divide the UK into one kilometre squares, east of an imaginary zero point in the Atlantic Ocean, west of Cornwall. 

• Magnetic North is where the compass needle points to. As a result of the Earth spinning on a tilt on it's orbit around the sun, the magnetic north pole does change very slightly over time in it's relationship to Grid North. This Magnetic Variation and the date when it was last recorded is printed on the OS Map Legend.

• True North is the direction of a line of longitude which converges on the North Pole. Each day the Earth rotates about its axis once. The ends of the axes are the True North and South poles.

Contour lines are used to illustrate 3D land on a 2D map. On 1:25000 scale maps:

• They are brown lines drawn on the map that join points of equal height above sea level.

• The interval between contours is usually 5 metres, although in mountainous regions it may be 10 metres.

Uphill: Contour numbering reads up hill – in other words the top of the number is uphill and the bottom is downhill. 

Steepness of slope: The closer contour lines are together, the steeper the slope.

There are three types of features:

• Spot (a building such as a church)

• Linear (field boundary, stream, path or road) 

• Area (woodland)

It is often easier to use linear features when setting the map.

Look at the map and identify where you are. It’s a good idea to do this regularly from the very beginning because it will help you keep track of where you are and avoid major errors. You might even set the map in the car park and use the location of the road to help you work out which path to take.

Look around you at the various features you can see and choose a few obvious ones (the bigger the better).

Stand still and rotate the map in your hands until the features you are looking at line up with the features on the map. For example, if the path is in front of you and there’s a stream to your left, turn the map so that the stream is to the left of the path when you look down at it.

• Place your compass on the map.

• While holding the compass in a fixed position on the map, rotate them together until the needle is parallel to the grid lines and the red part of the needle is pointing to the top of the map.

• Be aware that if the red line is pointing to the bottom of the map you will be 180° out!!