Contours are imaginary lines on the map that join points of equal height above mean sea level. The height between the contour lines is the vertical height and this will be defined in the key of the map you are using. On the Ordance Survey 1;50,000 scale maps this can be found at the top left of the map in the border an in the map key section on the right hand side under heights.
Contour interpretation has two aspects: the spacing of the contours indicating the steepness and the curves they make as they follow the hillside around indicating the shape of the land, known as relief.
So the two distinct elements in the representation:
To summarise the rules for contours on Ordnance Survey Landranger and Explorer map;
- Contours and spot heights show heights in metres above mean sea level
- The vertical interval on Landranger maps is 10 metres. On Explorer maps it is 5 metres in lowland areas but 10 metres in upland areas; you need to check the key of the map you are using.
- Every fifth contour ( The index contour) is printed more thickly, to help with counting between contours and the following of the lines around the hills.
- Where space permits, contour lines are numbered with their height, always printed so that the top of the number points uphill.
- On landranger only, if the slope is to steep for the four intermediate thinner lines to be drawn seperatly, some or all of them are missed out. The thicker contour lines are always present. As a rule of thumb when just one thin contour line is missing the slope steepness is about 1 in 3.
- Contours continue under rock features, the black markings being simply overprinted.