Showing how true north, grid north and magnetic north are different using an orange.
An orange or round piece of fruit
Four drawing pins
Pen or marker
An OS map, especially the legend
Demonstrate true north and magnetic north as per this video https://youtu.be/PPJ4MsflJ8E. i.e. put pins in the top and bottom/ on the axis to show the north and south pole. Demonstrate spinning on the axis to give a greater visual. Then add in pins for magnetic north and magnetic south. This is where the magnetic field is in the earth and it is not the same as true north. This magnetic field varies over time.
Draw a line around the central meridian and draw the UK roughly over the line. This helps to demonstrate where the meridian is and how grid north and true north are so similar in the UK that we do not need to worry about them.
Ask them to imagine the orange being much bigger and trying to put our 2 dimensional map onto the orange. Can you see how it will not fit? This is why in countries other than UK or those on the central meridian, grid north is not the same as true north.
To show how to adjust the bearing to point to take into account magnetic variation see https://youtu.be/oPwL0lrOdFA?list=PLJp4yCtYcXprknSY_FAUpWG5ZbDwHmfY7.
In the legend of the OS it will say the exact magnetic variation for that map.
To adjust your bearing remember "Grid to mag = add".
Now ask the scouts to put their learning into practice by adjusting bearings. Both from the map and from landscape.