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Simon B:


For an explanation of what Magnetic Declination is & How to Correct for it, click this link to the Compass section: http://micronavigation.com/forum/index.php?topic=256.0



Hello All,

Just read Kenneth's thread but it's still sinking in for me at a slow rate. I am having brief moments of clarity that then get fogged over by my brain's default refusal to work with any kind of logic.

So, for now I thought I'd have a go at just making an adjustment on the compass for practice. I’m using the 2011 OS explorer OL13 for the Brecon Beacons.

So according to the legend on the map, as of 2011;
True North is 0 degrees and 51 minutes East of Grid North
Magnetic North is 1 degree and 53 minutes West of Grid North
Annual change is 9 minutes East.

I use Abergavenny to calculate from and I calculate I need to adjust to 1 degree 45 minutes West of Grid North. The online Magnetic Declination Calculator suggests it’s 1 degree 37 minutes West of Grid North.

For Magnetic North to True North I calculate 2 degrees and 36 minutes West. The online calculator is giving 2 degrees 26 minutes West.

1st Question.  Why or have I made wrong calculations?

I’m also trying to get my head around Mag’ Dec’ with Grid North and True North.  Say I decide to spend a day walking around the Sugar Loaf NW of Abergavenny.  2nd Question.  Do I take the Mag' North calculation against Grid North or True North? What determines this?

Thanks!
Simon

captain paranoia:
> True North is 0 degrees and 51 minutes East of Grid North

Unless you're using a map that has a True North grid representation, you can ignore True North; it's just for information.  You need to be able to relate Grid North (as per the Transverse Mercator Projected OSGB map) and Magnetic North, as per your compass.

> Magnetic North is 1 degree and 53 minutes West of Grid North
> Annual change is 9 minutes East.

> So according to the legend on the map, as of 2011;

As of what month in 2011?  If we're changing by 9' per year, then half way through a year, we'd expect to have changed by 4.5'.  So, if it's January 2011 as the start point, and we're now in September 2012, that's 12+8 months elapsed, so expect 9' * 20/12 change, or 15'.

That gives us a Grid Magnetic Angle (the difference between Grid and Magnetic Norths) of 1 degree (53-15)', i.e. 1 degree 38', which is much closer to the online calculator.  Try measuring that 1' difference with your compass.  Actually, try measuring the 1 degree difference with your compass...

The other thing to bear in mind is that, due to the Transverse Mercator Projection, the Grid Magnetic Angle varies slightly across the map.  It's not much, and I wouldn't like to say exactly how much it is, but there is a difference.  You could use the online calculator to explore this.  So, if you take a point within the map, if will have a slightly different GMA to the 'reference' specified in the map legend (which I think is specified for particular point on the map; a corner, or the centre, can't remember which).

> I’m also trying to get my head around Mag’ Dec’ with Grid North and True North.  Say I decide to spend a day walking around the Sugar Loaf NW of Abergavenny.  2nd Question.  Do I take the Mag' North calculation against Grid North or True North? What determines this?

As mentioned above, True North is of no practical use if you're using a map that does not present a True North representation.  OS maps are printed using Grid North, so you want to use Grid North and Mag North.  Assuming you want to be able to transfer bearing between map and compass, that is...

Lost Soul:
As Captain  Paranoia says forget true North work only with grid north.  Interestingly, the recently published series of The AA’s  Walker’s maps, which are nothing other than repackaging of the OS Explorer maps, only show magnetic deviation in respect of grid north.   That makes things much simpler and avoids all the confusion that can be visited on a user if too much information is given.  Or, in other words they’ve adopted the KIS approach - Keep It Simple.

Next thing to consider is the practical accuracy that you can achieve when out in the field in respect of all that high precision information you are given about a wandering magnetic pole and how it affects the little spot on the planet you are standing upon.   As Captain  Paranoia has indicated can you measure and set heading etc with a high degree of accuracy using the standard baseplate compass?   No you can’t.   With a good level of care you’d be lucky if you can get something to within half a degree.   What does this accuracy or lack of it mean in practice?

To understand that we need to go to the 1 in 60 rule.   This states that for every degree you are out in heading then for every 60 metres you travel you will be 1 metre out of position.   Example: heading you’re supposed to be on 300°; heading you’re actually following is 299°.  So after having travelled 60 metres in an absolutely straight line you will be 1 metre to the left of where you want to be.  Does this make any difference to you?  Not really, particularly if we stop every couple of hundred metres or so,  as we are supposed to, to reassess our position and adjust accordingly.

The nine minutes annual change deviation you mention.  If you could set your heading with a very high degree of precision on your compass but get deviation wrong by nine minutes and you accurately trot off an absolute straight line then after having travelled 60 metres you will be exactly 150 mm (or 2 lengths –ish of your baseplate compass) to the left or right of where you thought you wanted to be.

Given all the inherent inaccuracies in equipment etc we use then for all practical purposes just set deviation as 2°.  Which way do we make the corrections?  Remember it this way.  East is least and west is best.  So if you measure your heading off of the map as 067° and deviation is 2°west then add 2° to the map derived heading.  Thereby trotting of on a heading of 069°.  And don’t forget if you have taken a compass bearing off of a feature in the landscape then subtract 2° from if before you transfer it to the map.

Hugh Westacott:
Completely agree with the two replies.

It's worth remembering that few navigators can walk on a bearing, using a baseplate compass, to an accuracy better than two degrees, so it's worth familiarizing yourself with the useful technique of 'aiming off'. This involves building in a deliberate navigational error.

Consider the following situation. You are walking off-path towards a footbridge over a substantial stream a couple of miles away. You are in dead ground so cannot see the bridge. Your compass and navigational skills are not sufficiently accurate to be sure of reaching the exact location of the bridge. If you don't hit it on the nose you won't know whether it is located to your left or right. What to do?

Answer: build in a deliberate navigational error that will ensure that you will arrive at the stream on one side of the bridge. Then all you have to do is to turn towards the location of the bridge and follow the stream until you reach it. I find that five degrees is right for me but navigators should experiment to find the error that works best for them. In the above illustration I would add five degrees to the bearing that, in theory, would take me to the location of the bridge then, when I reached the stream, I would turn left and walk to the bridge. (If you can't find the bridge it probably means that it has been swept away in a flood!)

Aiming off is a useful technique that can be used in a number of situations.

Hugh

Simon B:
Captain Paranoia, Lost Soul and Hugh..

Thank you . Your replies have really helped out. It starts to become obvious once you know...  ;)   

I just didn't even register the month on the map for the calculation. Silly mistake but now I'll know (it was July). That said I also think I was telling myself that I had to make very accurate calculation/adjustments. After reading and re-reading various other explanations on different webpages you tend to, well, go round in circles. It's good to hear the practical realities of what is actually possible with navigating across the land, with compass in hand and of using other techniques (aiming off) to compensate for this. An Art and not just Science.

Cheers!
Simon.


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