Author Topic: Magnetic Variation  (Read 2250 times)

Hobbo

  • Newbie
  • *
  • Posts: 40
  • Man, likes camping, hiking and running.
    • View Profile
Magnetic Variation
« on: January 24, 2014, 09:20:55 AM »
Apologies if this is in the wrong part of the forum!

Came across this website whilst reading the news today (news article here: http://www.bbc.co.uk/news/uk-scotland-highlands-islands-25841106). The site will give you the current magnetic variation for the co-ordinates you supply it with.

Rather useful, I think!

Go to http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html to have a play with it yourself.
I don't know it all and when I think I do, I tend to find karma is just around the corner...

Pete McK

  • Sr. Member
  • ****
  • Posts: 374
    • View Profile
Re: Magnetic Variation
« Reply #1 on: January 24, 2014, 01:05:27 PM »
What a simple page to use Hobbo, already added to my smartphone pages - thanks mate :)

Hugh Westacott

  • Sr. Member
  • ****
  • Posts: 306
    • View Profile
    • Walk with Westacott
Re: Magnetic Variation
« Reply #2 on: January 28, 2014, 11:33:49 AM »
Ive been considering the implications for dinosaurs like me, who still prefer to navigate by map and compass, now that grid magnetic north in the the extreme west of Cornwall is now east of grid north. No longer can we rely on the mnemonic Add for mag; get rid for grid that Captain Paranoia rightly despises. I fear that some walkers may get confused.

For some years Ive ignored the difference between magnetic north and grid north when navigating in lowland countryside both at night and in any weather conditions. I take the view that as most compasses mark degrees in 2 increments; the grid magnetic angle is rarely more than 3; and the distances over which I take bearings are usually less than 500 metres means that the inherent inaccuracy is acceptable.

According to the British Geological Survey website mentioned by Hobbo, in July 2014 the differences between magnetic north and grid north rounded to the nearest degree will be

Penzance (the most westerly town in mainland Great Britain)  1 E
Lowestoft (the most easterly town in mainland Great Britain)   3W

This simplified method has worked so well that Ive adopted the same technique when navigating in upland areas in good conditions. When the weather is foul and visibility is poor, or circumstances require maximum precision, I adopt the standard recommended procedure. Needless to say, I do not teach this method in navigation classes.

Id be interested in reactions from any fellow-dinosaurs amongst the members of this forum.

Hugh



      

Lost Soul

  • Sr. Member
  • ****
  • Posts: 265
    • View Profile
Re: Magnetic Variation
« Reply #3 on: January 28, 2014, 01:44:25 PM »
In the main Hugh, I agree with you and do likewise.  Only difference is that I do like mnemonics and find them extremely useful.

In respect of being accurate to a couple of degrees of heading on a short leg.  It is not really here nor there.  Practical realities vs theory; does it make a difference?  To illustrate - the difference between a pure mathematician and an applied mathematician.  Something you desire is 2 meters in front of you.  To reach it you step forward, each step halving the distance between you and the object - 1 metre. 0.5 metre. 0.25 metre - you get the picture.  How many steps do you have to take to reach the object.  Pure mathematician will says you will never reach it, applied mathematician says about 8 for all practical purposes.

So it is with being 2 deg out on a 500 metre leg.  Using the 1 in 60 rule which states that for every degree off heading you are then you will be l metre out of position for every 60 travelled.  So 2 deg mis-heading over 500 metres = 16.7 meters out of position.  Assuming of course you are able to walk in a dead straight line in the first place.

Any way when following a compass heading are we not taught to stop every few metres to check position on desired track etc and adjust accordingly.  The lower the visibility / the more dodgy the terrain the more frequently we should check.

Putting 16 metres into perspective, by my estimation that is about the width of one carriageway of a 3 lane motorway.