Ok, we've looked at the basics of what a map is and what a compass does and how to set a map. Couple of things before we move on. How many 'norths' are there? Not as daft a question as it sounds. There are 3 norths; True, Magnetic and Grid. And luckily they are all fairly self explanatory. True north is the north pole, where all the explorers go. Magnetic north is where the compass needle points to (that is, the red needle does NOT point to the north pole). It actually points to somewhere a little to the west of the north pole (something to do with the magnetic field of the earth), I guess if you wanted to research this there must be plenty of internet stuff that explains it. Anyway, suffice to say, if you followed the red needle for a long way you would not end up at the north pole. Lastly, grid north is where the grid lines point on the map. And if you think about it, as the grid lines on the map that run north-south are parallel, they cant ALL run to the north pole! The 2 that we're really interested in are grid north (as we have a map) and magnetic north (as that's where the compass points to). All we need to know for now is that the difference between grid north and magnetic north can be found on all ordnance survey maps and it's important to have an up to date figure for it - typically it is about 2 degrees at the moment, but changes over the years. So think back to what we did when setting the map. We plonked the compass on the map, held both together and turned around until the red needle pointed to the top of the map. The sharp eyed will have realised that because there is a difference between grid and magnetic north, that this can't quite be right. Correct, it isn't but the difference is so small that we can ignore ot for now.
Next, a really useful thing to be able to do is guestimate, or know, how far a bit of a route is. For a really rough guess you can count the number of grid squares that the route covers. Each grid square is 1km, and for the record the diagonal across a grid square is 1.4km (for the trig fiends it equals the square root of 2). For a more accurate measure we need to be a bit more precise. Look at the compass. It has a number of things that look like rulers on the sides. Indeed, one is a mm ruler so we could measure the distance in mm and then convert to metres or kms knowing the scale. WRONG. Who wants to work out how far 17mm represents on a 1:40000 scale map at night in a storm. So the clever compass people have put romers on the clear baseplate of the compass. The romer is a little ruler that is already to the same scale as the map being used and the numbers on the romer already represent, 100 metres, 200 metres etc. Clever, hey. But, and it's a big but. The compass baseplate often has 2 or 3 romers on for different scale maps. Have a look. Typically there might be a romer for 1:50000, 1:25000 and 1:63360. What?! What's 1:63360 all about. If you're old like me you'll know (?) that 63360 is the number of inches in a mile. And loads of compasses still have this romer on. Ignore it (unless of course you still have, and use, inch to the mile maps). More recent compasses have 1:50000, 1:25000 and 1:40000 romers on - all most useful. So, get your map, identify the scale and have a go at measuring the distance between 2 fairly close points. Use the wrong romer and you'll either be halving or doubling the distance you're measuring.
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