Contributor:
Craig Gill
In 1805, a British Naval officer called Francis Beaufort introduced a scale from 0 -12 for measuring the speed of the wind at sea. Beaufort developed the scale by matching the customs for setting a ship’s sails with the speed and strength of the wind. He used everyday terms for each level of his scale. It is now used to describe the effect of wind on the surface of the water as well as on a range of everyday objects on the land – from smoke to flags, trees and roof tiles.
- Force 0 = Calm, wind less than 1km/h, water like a mirror.
- Force 2 = Light breeze, winds 6 – 11 km/h, water rippled into wavelets.
- Force 3-4 = Gentle to moderate breeze, winds 12 – 28 km/h, flags blow about.
- Force 5 = Fresh breeze, winds 29 – 38 km/h, moderate waves, many whitecaps.
- Force 7 = Moderate gale, winds 50 – 61 km/h, sea heaps up, white foam blown about.
- Force 9 = Strong gale, winds 75 – 88 km/h, slight damage to trees and buildings.
- Force 10 = Whole gale, winds 89 – 102 km/h, severe damage to trees and buildings.
- Force 11 = Storm winds, 103 – 117 km/h, widespread damage.
- Force 12 = Hurricane, winds over 117 km/h, devastation.
Using a Compass
Apart from determining the direction of North, a compass enables you to work out a compass bearing. This is the angle measured in the number of degrees between 0 and 360, which tells you the direction from one place to another. We call North ‘0’ and therefore it follows that East is ’90’ and South West is ‘225’ and so on.
If we only use the points of a compass, we could only get eight different directions. If we keep dividing, we could get 32 different directions. However, if we use degrees, we can get 360 different directions which allows us to be much more accurate.
The Three Norths
When working on a map and compass, there are three different norths to be considered. In the UK, we only use and work with two of them, True and Grid.
- True North – Each day, the Earth rotates about its axis once. The ends of the axis are true North and South poles.
- Grid North – The grid lines, pointing to a grid north, on Ordnance Survey maps divide Great Britain into 100-kilometre sections. They are then further subdivided into one-kilometre squares east of an imaginary zero point in the Atlantic Ocean, west of Cornwall.
- Magnetic North – A compass needle points to the magnetic north pole. Unfortunately, it is not in the same position as true north. The magnetic north pole is currently located in the Baffin Island region of Canada, and from the UK, it is west of true north.
The difference between true and magnetic north is called “magnetic variation”, and its value can be found in the orientation panel or margin of an Ordnance Survey map.
