In reply to Katy44:
As swiped from my webpage! :
GRID NORTH.
The vertical gridlines on an O.S. map are a consequence of trying to represent the spherical shape of the earth on a flat sheet of paper. In order for these gridlines to meet at the poles, in the case of the northern hemisphere, the lines at the bottom of the map would be further apart than those at the top. Adjoining maps would be difficult to produce. For this reason, the gridlines are parallel and therefore the land as represented on the map is actually a distorted view, which in the case of larger scale maps is negligible.
MAGNETIC NORTH.
Magnetic north changes over time due to the earth’s molten core which in basic terms "sloshes" about due to forces exerted upon it by the climatic conditions, the gravitational effects of the moon and magnetic influences of solar flares, local variations have to be taken into account, for example the Cuillin Hills in the Isle of Skye have a high iron ore content and due to its volcanic formation can have an effect on magnetic variation. Magnetic variation is predicted by several international organisations, International Geomagnetic Reference Field, being one such organisation.
When using a compass we have to know how to calculate the difference between GRID NORTH and MAGNETIC NORTH, and when to add or subtract this variation. This depends on two things:-
Whether magnetic north is west or east of grid north & whether you are transferring bearings from map to terrain or terrain to map.
1. When using a map, the saying "West is best, east is least" meaning if magnetic north is west of grid north then ADD the variation and if magnetic north is east of grid north then SUBTRACT.
2. When transferring a bearing taken on a landmark the reverse of the above is true.
An O.S. map tells you what the difference will be at a given year and in what direction magnetic north is moving, e.g. :-
"Magnetic north is estimated at five and a half degrees west of grid north for 1993 decreasing by about a half degree in three years" (refer to No. 1. Above)
This means that in 1996 magnetic north would be five degrees west of grid north and in 1999 four and a half degrees west of grid north and so on. These half degrees are not critical unless you were travelling great distances on a bearing only. As the markings on most compass bezels are at two degree intervals it is easier to round down to the nearest whole degree.
If you are going to use an old map, say more than twenty years old then it would be better to calculate the magnetic variation yourself, as magnetic variation doesn’t change at a constant rate and its movement can speed up or slow down, the original estimated movement maybe well out. This can be done if you know your exact position and can identify a landmark in the distance which you can locate on your map. Take a bearing on the object and note the reading. Using your compass as a protractor, do the same on your map, the difference between the two readings is the magnetic variation. The further away your chosen landmark is, the more accurate the variation figure will be.
http://www.astrecks.co.uk/navigation.htm
Jeff
