Allowing for true north!!

[BushTucker]

Hi all

I wonder if any experts out there can help me with a problem i have.

Ok, the prob is with a compass, to set it to true north for the southern area of dorset is about 1.6 degs east, right. Please correct me if i am wrong. I move my bezzel or the spinny bit on the compass to read 358. a bit degs. Am I doing this correctly to allow for the declination?

Been told by someone that i am not, but it is how i have always figured it done, even though i think it make no odds for my area for short distances.

Cheers

Don

 

[pheasant plucker]

easy way to remember is "from grid to mag add, from mag to grid rid" but you need to check the map you're using as older maps will have a different declination as the pole wanders around.

PP

 

[BushTucker]

I understand that bit and the map i am using is about a year old, and another map has the reduction from a certain date and both coinside.

 

[He' s left the building]

Page 9 of this pdf: http://www.ordnancesurvey.co.uk/oswebsite/mapshop/pdf/map_reading_made_easy.pdf

Map bearing to ground: add the variation.
Compass bearing to map: subtract the variation.

TBH I rarely bother unless I'm doing something that requires the utmost accuracy (hardly ever!)

 

[He' s left the building]

Am I doing this correctly to allow for the declination?

Declination is something different (which I will not even begin to describe), but is the reason why there are Northern hemisphere and Southern hemisphere instruments. Silva sell a 'globally-balanced' compass, which is the one I have packed in my kit now. How much difference this makes I haven't got a clue??!!

 

[Wayland]

Must admit I tend to orient the map and read the topography for position.

 

[pheasant plucker]

depends on the distance covered if you're given a bearing to follow imagine the difference there would be if you don't account for declination. If I'm in an area I know I don't bother with a compass most of the time as long as I know where north is and can orientate the map I'm sorted, but it's good practice to use declination and then the one time you really need it you'll have the confidence to put it into practice.

 

[Sainty]

If the magnetic declination is, as you say, 1.6 degrees East then you are spot on. As PP says, 'mag to grid get rid'.

However, if you look at this website: http://www.magnetic-declination.com/ then it says the declination for Weymouth is 2 degrees 36 minutes West, so you would be miles off if you worked on 1.6 degrees East.

Is the website accurate? Isn't everything we read on the internet 100% accurate all the time?

Anyway, worth a look all the same.

Martin

 

[pheasant plucker]

Errrrr sainty is'nt that the longitude of weymouth?? So it would'nt be the declination?

PP

 

[He' s left the building]

http://www.ordnancesurvey.co.uk/oswebsite/aboutus/reports/misc/north.html

 

[Broch]

Why do you want to set your compass to true North?

If you're using a map and Grid North for navigation you need the compass to be offset from the GN by a few degrees not True North (unless you really are walking to the NorthPole )

I started writing instructions but realised that the instructions change depending on whether your compass has a magnetic north offset adjustment (you rotate the base plate of the dial) or not and without a diagram it gets too complicated.

What model compass are you using?

You're right though, unless you are walking across a wide area of featureless terrain it makes little difference. For checking that the path you're following is the one you want (i.e. goes 18 deg not 85 deg), using a compass without offset is fine.

 

[Sainty]

Errrrr sainty is'nt that the longitude of weymouth?? So it would'nt be the declination?

PP

Apparently Weymouth is at Longitude: 2° 27' 39.4" W. I don't know what the magnetic variation is for that part of the world as it's been 25 years since I walked to coast path there. I still agree with you that it's 'mag to grid get rid' though.

Martin

 

[Joonsy]

are you getting true north mixed up with grid north - OS maps have three norths, grid/true/magnetic, you only really use grid and magnetic for normal navigation, the magnetic variation is written on OS maps with date applicable so check your OS map is an up to date one, at present in the Uk the magnetic variation is so small it can be ignored for general use when not being ultra accurate, however to allow for magnetic variation if taking bearing from map you add the variation but if taking compass bearing and transfering to map you deduct the variation. Make sure your OS map is up to date and the variation will be written on it. Grid north is those blue lines running vetically which you line up with compass and is in fact not true north.

 

[pheasant plucker]

Sorry sainty I messed up with what I meant, what I really meant is that's the difference between the greenwich meridian and weymouth and not the magnetic declination for wymouth??

PP

 

[Sainty]

Sorry sainty I messed up with what I meant, what I really meant is that's the difference between the greenwich meridian and weymouth and not the magnetic declination for wymouth??

PP

I dunno!! I prefer the answer above which says that the difference is so small these days that it can be ignored.

My advice is if you get lost go to the pub for directions. Only, don't ask them what the magnetic variation is.

Martin

 

[He' s left the building]

I was told in Oman that coffee in the South was almost green in colour, whilst in the North it was darker and brown.

If we were lost our guide said just buy a coffee and see what colour it is

 

[Joonsy]

If the magnetic declination is, as you say, 1.6 degrees East then you are spot on. As PP says, 'mag to grid get rid'.

However, if you look at this website: http://www.magnetic-declination.com/ then it says the declination for Weymouth is 2 degrees 36 minutes West, so you would be miles off if you worked on 1.6 degrees East.

Is the website accurate? Isn't everything we read on the internet 100% accurate all the time?

Anyway, worth a look all the same.

Martin

at present magnetic variation is west of grid north, as it is west you add the variation when taking bearing from map, the variation constantly changes very slowly and in years to come when it passes grid north it will eventualy be in the east, when it is in the east you will have to subtract the variation for map bearing, but that is a good many years away. grid north is completely different to true north, true north points to the north pole, grid north is for reference in map reading, magnetic is where the compass needle points, onlly grid and magnetic are used for map reading.

 

[Carbuncle]

I don't know if this is any help but you've got four options that I know of - I'll use setting a map with a 10deg west declination as an example:

1. Don't bother - in Blighty the declination seems to be in the 1-3deg west range now, and for setting your map that won't make much difference. Do resections, or go walking in the States however . . .

2. Use one of the rhymes as described above. So north on your map (the gridlines) goes to magnetic 350deg on you compass.

3. Use the manual declination scales in your compasses capsule - you line the NS lines in your capsule with the gridlines, but point the needle to the appropriate place on the manual declination scale as in this photo of my Silva Type 4 (Two things - the camera lens monkeyed around with the needle making it 5deg, and the capsule and bezel are misaligned by 2deg, one of the reasons I don't rate Silva compasses)

4. By far the best solution IMO, get a compass with declination adjustment. The two I rate most are the Suunto M-3G and MC-2G, the former a baseplate and the latter a mirror sighting compass. I prefer the baseplate compass personally. Recta do ones physically the same as them, but the capsule markings aren't IMO as good.

Heres how you set it (using the MC-2G):

Turn the compass over, and using a little key on the Lanyard, turn it so the gate/shed matches the declination on the scale.

Now the compass is set, using it is a doddle - the NS lines in the capsule line up with the map, and the red goes in the shed:

Someone mentioned global needles - I don't understand it too much, perhaps someone can explain it to me, but apparently there is a vertical component to declination that varies from north to south, meaning a needle will not balance correctly if used in the wrong zone, and will scrape against the capsule and stick. A global needle has a ring magnet in the middle (or so it appears), which stops that happening. What I do know is that it has the pleasant side effect of making the needle quicker to settle and more tolerant of being tilted, it's probably as big a plus on usability as the capsule being liquid damped, though why they don't market it as such I don't know.

Hope that helps.

 

[BushTucker]

Hi all

Sorry i could not reply sooner, we had a major blackout here and did not come back on till 23 45.

Thanks all for your tips and advice, great to know i was doing it right but thanks

 

[Tadpole]

Errrrr sainty is'nt that the longitude of weymouth?? So it would'nt be the declination?

PP

The Adjustment needed to correct Grid North and magnetic North for Weymouth is 2 deg 21 min west of grid north.