At BB we usually use Ordnance Survey Explorer maps. These maps are drawn with a blue grid criss-crossing the entire surface. We can use these grids to identify particular points on the map. Grids are made up of vertical lines (drawn from top to bottom) and horizontal lines (drawn from left to right). The vertical lines are called Northings and the horizontal lines are called Eastings. The Northings and Eastings join together to make up squares, called Grid-squares.
To identify a particular square we first look at the Northing lines. Each one of these lines is numbered in turn from left to right. We look for the line which is closest to the left of the square we want to find. Next we look at the Eastings and find the line closest to the bottom of the square. The Easting lines are numbered from bottom to top. |
Example – Red TriangleThe Northing nearest to the left of the square, which contains the triangle, is called 04. The Easting nearest to the bottom of the square, which contains the triangle, is called 28. In short hand we write: (04 28) Example – Green CircleThe four-figure grid reference is (02 27) |
Using the four-figure grid reference, we can see that in square (04 26) there is a small circle with a cross in it. The reference refers to the whole square, however the circle is very small and could be anywhere in that square. This is why we usually use six-figure grid references.
A six-figure grid reference is 100 times more precise. As before, we calculate the four-figure reference to tell us which grid square the target is in. The small circle with a cross in it is at (04 27)
Then we divide that Grid-square up into 100 smaller squares – by imagining 10 mini-Northings and 10 mini-Eastings (both numbered from 0 to 9). The picture below shows these extra divisions.
Just as with calculating a four-figure reference, we move from left to right and find the mini-Northing just to the left of the mini-Square containing the target. Then we move from bottom to top to find the mini-Easting just below the mini-Square containing the target. For the target, the mini-Nothing is 7 and the mini Easting is 2. Combining the value for the Square’s Northing (04) with the mini-Northing (7) gives the first three figures of the six-figure grid reference (047). Likewise, combining the value for the Square’s Easting with the mini-Easting gives the last three figures of the six-figure grid reference (262). The six-figure grid-reference for the target is (047 262). Normally, we don’t need to draw in the mini-gridlines – we can just approximate these in our heads. The process for calculating or decoding a six-figure grid reference is always the same. Imagine you are walking into your house: first you need to walk along the corridor before you can go up the stairs! Similarly, work out the Northings first – these are numbered left to right (i.e. along the corridor) and then work out the Eastings – these are numbered bottom to top (i.e. up the stairs). |
We work out the direction we are travelling along by looking at where North is in relation to us. There are four major directions we can travel in: North, East, South and West. From these four different directions we can give names to a total of sixteen different directions.
If we want to be even more accurate we need to use Degrees. If we start at north and turn to face East, then the South, then West and back to North again, we say we have turned through 360 Degrees.
We can use an Ordnance Survey map to find which way north is. The vertical gridlines (called Northings) travel from North to South – North is towards the top of the map. There is a slight difference between the North a compass points to (Magnetic North) and the North a map points to (Grid North). However, the difference is very small and has little effect when travelling small distances.
Picture: A typical ‘Silva’ style base plate compass |
Explorer map key images from Ordnance Survey.