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Map Reading Techniques (Tutorial)

Posted: Wed Sep 13, 2006 11:12 am
by Dizzy
Hi all, as promised I've been busy with the camera and my worn out map to hopefully give newcomers to Letterboxing some idea on how to use a compass with your map. This tutorial will help you plot most of your clues onto your map before going out on your search.

Firstly when using a map get yourself a good detailed one. The one I use that shows just about every detail is Ordanance Surey 1:25 000 Outdoor Leisure Map

As for compasses, see the other thread on this Forum or search this site.

How to read the grid references on the map.
Ok, Along the sides, top and bottom you will find grid number. Say with the grid reference of 73 77, the first number 73 you will read along the top or bottom of the map. The number 77 would be read along the left or right hand sides of the map. So think vertical first, then horizontal.
Try this grid reference out and you should end up at Bonehill Down.

You can also use a six grid reference number say 731 775 this will give you a much more accurate bearing. The 1 & 5 added would mean that grid 73 77 is split into 10 blocks up and 10 blocks across, so the 1 would be 1 block across and the 5 would be 5 blocks up in the grid.If this sounds confusing you will get some idea from my photo below.

Some people use even more accurate grid references and can be up to 10 numbers!


Adding the box to your map.
Firstly you have to remember to deduct 3 degrees from you clues. For this example we will used my Dizzy Box, Dizzy the 100th Member (still out there).
Ok, Widecombe Church bears 259 1/2° so we will reduce that to 256 1/2°, do this with every bearing you plot onto your map.

Set your compass to the appropiate bearing, in this case we will use 256 1/2°. Align your compass with your map so that the north red arrow within your compass bezel points north and the black lines within the bezel line up with the light blue vertical lines on your map. Make sure you don't chance your compass bearing of 256 1/2°. See the photo below.


Plot a line on your map with a soft pencil, then do the same thing with this bearing: Tunhill Rocks High Point 188 1/2° (so on the map 185 1/2°)
Where the lines cross over on your map should be where the box is!
If you have a laminated map only!
I then use a fine tip permanent marker to plot the cross, this will wipe off with a little cream cleaner.

If anyone wishes to add to this tutoral please feel free as we all use our own methods and I'm sure they all work just as well in their own way.

Happy hunting!


Posted: Wed Sep 13, 2006 11:41 am
by Dizzy
Reading your compass on the moors.

All you have to remember when using your compass out on the moors is to set your compass with the normal bearings, not deducting the 3 degrees you subtracted when plotting onto your map.

Set your compass to the bearing i.e. 259 1/2° (example only), keep the north red point of your compass at all times within the red arrow inside the bezel of your compass, see photo below:


If the subject is not in line with the blue arrow on your compass, walk left or right until you have it in line with your settings on your compass. You will then have to walk on this line trying to not veer off until you have the second bearing in line i.e Tunhill Rocks High Point 188 1/2°. When you have this bearing on line whilst walking on the same line to Widecombe Church, you should have the box at your feet!

All the best,


Posted: Wed Sep 13, 2006 4:42 pm
by Devon Dumplings
Thanks for the tutorial :D
Out of interest, why do you deduct the 3 degrees when plotting on a map?

Posted: Wed Sep 13, 2006 5:50 pm
by NUMBER 70
Thanks for the tutorial.

Posted: Wed Sep 13, 2006 6:45 pm
by Dizzy
Devon Dumplings wrote:Thanks for the tutorial :D
Out of interest, why do you deduct the 3 degrees when plotting on a map?
Hi guys, From what I understand there is a magnetic variation between true and magnetic north.
The needle in the compass points towards magnetic north, which apparently lies to the north of Canada. The lines on the map point north at the top of the world, the axis about which we all rotate.
For extreme accuracy, it may be necessary to subtract approx. 3 degrees to allow for the difference between the magnetic and grid North poles when plotting onto our maps.

Now I'm confused! :D


Posted: Thu Sep 14, 2006 8:49 am
by The Sly Fox
You're confused?

Me too ... I have always used a variation of 5 degrees. Something I was told years ago.
Hmm? It seems to work though. I'll try 4 next time ... lol

Great piece of work though Dizzy :)

Posted: Thu Sep 14, 2006 11:46 am
by NUMBER 70
Interesting one but the answer is nearer to hand than you think and is with you every time you step onto the moor...what am I talking about well read on.

According to a Scouting web site (keep reading this looks correct) magnetic deviation depends on where you are in the world, it then goes on to say... CHECK your map. yes folks it on your Dartmoor map in the big white box sat over Plymouth.

it tells you....

At the centre of the North and South sheets true north is 1 Degree 29' (minutes) East of grid North.
Magnetic north is estimated at 2 deg 13' and 2 deg 11' west of grid north respectively for july 2004. annual change is approx 12' east

Magnetic data supplied by the british Geographical survey
Copyright: Crown copyright 2002
Out of interest let me work it out if it changes 12' I am assuming you add

so that would mean adding 2 x 12' to get July 06 figures to each above and would be

1 degree 53' and 2 deg 37 and 2 deg 35 respectively

which are converted to get degrees...

1 degrees 53 minutes = 1.883 degeees
2 degrees 37 minutes = 2.616 degeees(think this should have been subtracted
2 degrees 35 minutes = 2.583 degrees(think this should have been subtracted

I think that makes it roughly 3 degrees
with a bit of hindsight I think you must subtract the latter two not add moving it towards 1.8 degrees

Now that was interesting stuff

Posted: Thu Sep 14, 2006 5:51 pm
by Dizzy
WOW number 70, now that's what I call well informed! Excellent research by the way! :D


Posted: Thu Sep 14, 2006 9:27 pm
by The Sly Fox
Well **** me!!

Now I am even more confused. I have splinters from scratching my head.
I guess three degrees is about right then.

Well solved Number 70. Seems you were correct Paul.

Stupid Boys Brigade ... mutter ... mutter ... grumbe :cry:

Posted: Fri Sep 15, 2006 7:29 pm
by Nik - KOTM
If you purchase a NEW map this information will be written on the map magnetic variation and all that .

I think it is 2012 when True North and Magnetic North will be the same which will make all things easier for us on Dartmoor.

but I am well impressed with the detailed description for the "how to navigate" 10/10 Well done!

Posted: Fri Sep 15, 2006 11:17 pm
by Dizzy
Nik - KOTM wrote:
I am well impressed with the detailed description for the "how to navigate" 10/10 Well done!
Thanks Nik, it's was all down to a joint effort by everyone on this thread. I hope newcomers to this fantastic website leave a post to let us know how they get on with our advice.


Posted: Sat Sep 16, 2006 8:11 am
by StannyD
Excellent tutorial. This is an area I have always found a bit tricky and I had no idea you had to vary the bearings when plotting points. Maybe I can find a few more boxes from the clues now!
Cheers Dizzy!

Posted: Sat Sep 16, 2006 1:01 pm
by Nik - KOTM
Another tip I was given was allow 15 minutes for every kilometre and add 2 minutes for every contour you have to climb up, going down is a bit easier than the ups of life.

Posted: Fri Oct 26, 2007 10:52 pm
by NanoRuler
A late reply, I know, but the variance is created because a compass points to magnetic north and not to the north pole. The problem is magnetic north is not static at all, but instead "wanders" about.

To compound matters, the earth's magnetic field doesn't run straight north-south, either. In some parts of the world the deviance can be 30 degrees! Any decent topographical map will tell you the magnetic deviation, the year the map was printed and the annual change in deviation.

Using that information you should be able to accurately calculate the correct deviation for the year that you're in.
won't be all that many years before we in the UK may need to


Posted: Sat Oct 27, 2007 6:11 pm
by jjpage
thanks for the tutorial,keep up the good work