Magnetic North

[Pippa]

I read this morning (in a geophysics journal, not some tabloid) that magnetic north has been moving at over 50km/year for the last 8 years (but interestingly not magnetic south). I presume people drawing maps keep up to date with such things, and each new map takes this into account, or maybe it explains my poor skill when it comes to using a compass? And before you ask, no I haven't calculated what this means in reals terms for us in the UK, but it's different from the rate of magnetic migration during the 1990's when I was all geeky and knew about such things for DofE.

Pippa (ill, hence mindless message-board browsing...)

[Adventure Racer]

Hmm, well checking the data at http://www.geomag.bgs.ac.uk for Hartland observatory (in Devon), the declination was 3 degrees 48 minutes west the middle of 2006, 5 degrees 23 minutes west in the middle of 1996, and 6 degrees 47 minutes west in the middle of 1986. Looking at the data it doesn't actually seem to be changing significantly faster now than it was 20 years ago. I'd think there were bigger changes in most maps than that over 20 years, though I guess you should really adjust the map slightly for a 3 degree change (how easy is that in OCAD?) 3 degrees would equate to 10m out over 200m.

[graeme]

OS maps (used to?) list the rate of change of magnetic north. It's much as you say, and I see no evidence that maps keep up. People who claim to find controls on a bearing more accurate than the north lines are just bluffing. I've heard people claiming to stay within 5 degrees on maps I've made: its a joke, because I know for sure that the map isn't accurate enough.

[candyman]

Why it is very important to realise that the compass is just a tool which is used to help keep direction alongside map reading and shouldn't really be used in isolation to find controls....

[redpossum]

As a mapper the first thing I do in OCAD is to set the base map to real world co-ordinates to allow the use of GPS for mapping and planning. I then look up the British Geological Survey web site (http://www.geomag.bgs.ac.uk/gifs/gma_calc.html) and enter the grid reference of the map to obtain the latest magnetic variation. The map can then be rotated the required amount (a record of this is kept under Options > Scales for future updates). I am very aware that this is not done on a regular basis when maps are updated and whilst the odd degree here and there probably doesn't make a lot of difference when running but it can make quite a positional difference when the map is being surveyed.

The OS does indeed provide basic magnetic information on their maps but it is provided to them by BGS so it is better to obtain the information directly from the source.

[Adventure Racer]

How does setting the map to real-world coordinates work in conjunction with then lining it up with magnetic north? Surely you're throwing the (true north based) coordinates out?

[FatBoy]

Can't speak for redpossum, but I also map related to grid north. Esentially do all the mapping work with it aligned to grid north, then the last thing is to rotate to magnetic north. I always keep a legend on the map of the grid north (not true north in most cases) and magnetic north angles used, to make it easy to switch back to grid north if required, and also for info on a printed copy as the map gets older.

Before anyone asks, if I need to take a bearing while mapping when my paper is set to grid north, I use my GPS which has a magnetic compass which can be set to point to grid north.

I would agree with Graeme though, by enlarge you wouldn't be able to notice if magnetic north was out by only a few degrees, as even if you could follow the compass religously in terrain which you can't, most things you're trying to take a bearing from and to won't be in an accurate enough position on the map to show that difference.

[Gross]

What a load of sh**te... map a map... then go orienteering & see if it works:)... unless of course it's the WC...

[Gross]

OK.. throwing the WC Final 2000 into consideration.......... Dan Marston couldn't work out why his last leh split was so good......... until a training camp next year when Stanley noticed that the MN north lines were way.... out:)

[Pippa]

Why it is very important to realise that the compass is just a tool which is used to help keep direction alongside map reading and shouldn't really be used in isolation to find controls....

Yes. I can categorise my orienteering into three distinct phases:
1. Run in the direction that looks most hopeful/nicest for five minutes. Spend 20 minutes relocating.
2. Learn that the compass is always right and run on a bearing for 5 minutes. Spend 20 minutes relocating.
3. Learn that I am a bit rubbish with a compass and read the map as well. The whole time.

Graeme: I thought there was something about your sprint map at the 6 days that didn't quite make sense...must have been a minor magnetic north error.

Is no-one else as excited as me that something geological could be happening as fast as 50 km/yr?
Pip

[Gross]

Pippa.. you should've been in GB team when Goran & I were involved... World Number 1

[graeme]

Graeme: I thought there was something about your sprint map at the 6 days that didn't quite make sense...must have been a minor magnetic north error.

North was definitely correct at Kellas. At least, it was in some parts of the map. Mapping using straight North lines is overrated: we've known the planet is round for 2000 years

[Kitch]

Is no-one else as excited as me that something geological could be happening as fast as 50 km/yr?
Pip

its probably due to global warming

[Adventure Racer]

Mapping using straight North lines is overrated: we've known the planet is round for 2000 years

Good point. Even more importantly North lines shouldn't be parallel. How much should I set them to converge by in OCAD?

[King Penguin]

Good point. Even more importantly North lines shouldn't be parallel. How much should I set them to converge by in OCAD?

So little that it won't be at all discernable on an A3 1:15,000 map ?