Using Compasses with Maps?

Question

I have a Compass, the sort you use for Land Trekking. As you know a compass works on a Magnetic North basis, whereas say an Ordnance Survey map shows True North.
My Compass has two black lines on it; one is longer than the other and they are 17mm apart at the outside edge, although they both aim at the centre but without touching, as segments.
Now when the compass is placed on the map and has settled it will be aiming at MAGNETIC NORTH. One of the lines marked on the compass glass has to be lined up with it so that I will know what other parts of the compass (North, East, South, West) will be in the correct positions.
I need to know therefore which line to use to achieve this.
One of you must be a dab hand with a compass, so can you help me, please.

Answer 1

It's a bit tricky to go into here. BUT never fail, the internet is at hand:-

http://www.learn-orienteering.org/old/

If you scroll down that page you will find some handy lessons on compass usage.

Answer 2

There are three norths commonly in use in Great Britain:

Grid North: the direction of a grid line which is parallel to the central meridian on the National Grid.

True North: the direction of a meridian of longitude which converges on the North Pole.

Magnetic North: the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain.

Differences
The horizontal angular difference between True North and Magnetic North is called MAGNETIC VARIATION or DECLINATION. The horizontal angular difference between Grid North and Magnetic North is called GRID MAGNETIC ANGLE. It is this angle which needs to be applied when converting between magnetic and grid bearings.

Differences between the three norths are given on our 1:50000 scale OS Landranger® and OS Explorer 1:25000 maps.

On OS Landranger Maps, the difference between True North and Grid North is given for each corner of the map and is shown in the legend as is the difference between True North and Magnetic North. The information is shown diagrammatically in the middle of the North margin. In both cases the year for which the value was compiled is given and once its value has been corrected for the current year, its value can be accepted for the whole sheet.

The second link provided will give you a precise magnetic degrees variation for any location, based on a specific date. Shouldn't get lost with this. The lines on your compass will be accurate, but only to the date of manufacture!

You can compute the true bearing from a magnetic bearing by adding the magnetic declination to the magnetic bearing. This works so long as you follow the convention of degrees west are negative (i.e. a magnetic declination of 10-degrees west is -10 and bearing of 45-degrees west is -45).

Answer 3

The longer line is the one you want to use. If you are using a standard OS map, on the key there will be a an angle that will give the relative direction from Magnetic North to true North alignment.

Answer 4

there should be something, on the chart identifying the magnetic variation. it s long time since I last looked at a OS map, I tend to look more at marine charts.

where there is usually a compass rose identfying the date of the survey, the variation on that date and an annual correction factor to be applied.