The Basics of Navigation |  |  |
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Background to navigation
For centuries the traditional method of navigation has been to find your position on a map and adjust which way you’re travelling, depending on your destination.
Over the years map quality and accuracy improved and larger areas were covered with a choice of scales. In the UK, the Ordnance Survey was set up to provide detailed mapping to the armed forces, and later expanded to supervise mapping for use by the general public.
The national grid system
To make navigation simpler the country was divided into equally sized sectors in a grid system. Grid squares are aligned along north-south and east-west axes and cover the whole country. Large areas covering hundreds of square miles are assigned letters such as SK or NY. Within each of these lettered sectors grid lines are allocated individual numbers ranging from 00 to 99. By combining the area letters and the grid line numbers any position can be accurately plotted and used for navigation.
Longitude and latitude
On a global scale the Earth’s surface has been similarly divided into a grid system, but one that has to allow for the curvature of the Earth’s surface. The grid axes run north-south, starting from a line drawn from pole to pole and running through Greenwich in London, and east-west starting at the equator. The axes of this grid are called longitude and latitude, and are sub divided into degrees and minutes for precise locations.
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Adjusting for a round earth
One of the biggest problems with the mapping system is that to keep the grid squares equal size, the axes can’t follow the true lines of longitude and latitude, but instead have to follow a notional north-south line. The notional grid axes remain constant, but for navigational purposes adjustments have to be made to relate these axes to the magnetic North which is neither the same as the grid axes or constant.
Traditional navigation using a map and compass takes the difference between grid north and magnetic north into account by adding or subtracting the relevant number of degrees to a compass bearing. To make the calculation easier, traditional paper maps have the difference between grid and magnetic north shown at the bottom of the map, along with the date of the data as magnetic north moves at approximately half a degree a year.
When using a GPS, a lot of the calculations involved are done automatically for you, once the GPS knows which system you want it to use. Almost all GPS receivers come with a choice of UTM (Universal Transverse Mercator), Lat/Long and Grid settings, and once selected the receiver’s software will convert the generated position into a reference applicable for the map system where you are.
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