True Or Magnetic North

[rtjs]

A newbie here (and yes I have searched the forums).

Please could you tell me whether I should set my Explorist 210 to true or magnetic north when geocaching? ( I gather for general use magnetic north is preferable). I ask because my first trial with the unit did not get me anywhere near where I think the cache was hidden!

 

Thanks

 

Richard S

[+Nellies Knackers]

Compasses obviously point to magnetic north but coordinate systems use true north(I think)

My etrex is set to true north and has always done me fine but I admit I did have to look and see as I didn't remember ever setting it.

 

Added: Of course the critical bit is the datum! set to wgs84 and use DD MM.mmm format.

Edited November 30, 2005 by Nellies Knackers

[+Cardinal Red]

Hi rtjs

 

A newbie here (and yes I have searched the forums).

 

Actually this is one of the more popular questions that is asked VERY frequently.

 

The short answer is, don't worry about it. It doesn't matter if you don't use a paper map.

 

For the long answer type these three words in as search keywords:

 

magnetic and north

 

(The trick is the AND modifier. OR can also be quite usefull at times).

 

That will keep you busy reading for quite some time.

[rtjs]

Thanks, I gather then I must set the coord system to Lat?/long rather than OSGB?

 

Bw

 

Richard

[rynd]

If you are using a map you need to be sure that your GPSr is set the same as the map. That being said a usually use magnetic so I don't have to do any converting to use a regular compass (don't forget declination). By the way a magnetic compass is a handy little tool to have while geocaching.

 

Hope that helps.

[+Eckington]

Thanks, I gather then I must set the coord system to Lat?/long rather than OSGB?

 

Bw

 

Richard

For geocaching, whether finding or placing, the GPSr MUST be set to WGS84 or errors occur

[Garmin8888]

How far out with WGS84 against OSGB will the same Geocache be please?

[Moss Trooper]

Helloooooo rtgs..

 

Only time yer need to worry about Magnetic North or True North is when yer playing with bearings.

 

A little verse for you.. an all other cachers listening in.

 

Mag to Grid get rid

Grid to To Mag Add

 

Meaning that if you take a bearing from your map, then you need to add the magnetic variation.. like wise if you take a bearing and and transfer it to a map you need to deduct the magnetic variation.

 

The magnetic variation can be found at the top of any ordinance survey map. This does change over time.. be it ever so slow, but not that fast as to upset yer navigation.

 

But be careful I was once accused of missing out the magnetic variation on a cache.. which didnt involve a map. but no worries.. they found it.

 

Hope this helps ya..

Edited November 30, 2005 by Moss Trooper

[barryhunter]

If you have the Position format left as a Lat/Long, but still have OSGB36 set as the datum, or the other way (i.e. changed one but not the other), then the difference is typically between 70-200m depending on your location in GB.

 

As I've said before you can use either (lat/long with WGS84 _or_ British National Grid with OSGB36) but must change both the display format and datum together. Lat/Long (so WGS84 is required) is required for (many) puzzles or interacting with the geocaching.com website etc, but Grid References (so OSGB36 is required) is much easier for working with maps. (see also this very early version of a page explaining the differences)

Edited December 1, 2005 by barryhunter

[+wildlifewriter]

How far out with WGS84 against OSGB will the same Geocache be please?

There's no need to shout.

 

Here is a list of some typical variances over the British Isles...

 

Site                 Position     Err(Mtrs)  Bearing ºT	
   
Wick           N58 26.923 W3 05.191	95   253  (to OSGB)
Fort William   N56 49.430 W5 06.418	78   260  (to OSGB)
Bonnyrigg      N55 52.506 W3 05.918	90   269  (to OSGB)
Dumfries       N55 04.484 W3 36.859	88   275  (to OSGB)
Isle of Man    N54 15.861 W4 27.658	84   281  (to OSGB)
York           N53 57.484 W1 03.707	107  283  (to OSGB)
Derby          N52 55.206 W1 28.400	107  288  (to OSGB)
Pwllheli       N52 53.098 W4 24.556	87   290  (to OSGB)
Norwich        N52 37.660 E1 18.163	126  289  (to OSGB)
Reading        N51 27.332 W0 57.537	116  296  (to OSGB)
Plymouth       N50 21.824 W4 08.138	102  305  (to OSGB)
   
   
Ballycastle    N55 12.353 W6 14.797	67   269  (to Ir Grid)
Armagh         N54 21.356 W6 39.020	63   279  (to Ir Grid)
Sligo          N54 16.409 W8 28.582	47   283  (to Ir Grid)
Dublin         N53 21.575 W6 19.693	71   291  (to Ir Grid)
Killarney      N52 03.317 W9 29.933	57   319  (to Ir Grid)

 

This table is for interest and guidance only. Distances and bearings have been rounded for convenience.

 

-Wlw.

[+The Forester]

The difference between true and magnetic North has got nothing whatsoever to do with the red herring of grid versus Lat/Long co-ordinates or with the difference between Ordnance Survey's Airy spheroid and GPS's WGS84.

 

True North refers to the axis of rotation of the Earth, regardless of which spheroid is used. Magnetic North refers to the point on Earth where the geomagnetic flux lines enter the Earth. That magnetic pole is constantly on the move and is seldom if ever located at the geographic pole. In fact the magnetic North pole is not always located in the Northern hemisphere! There is plenty of geological evidence that the geomagnetic poles reverse, sometimes with astonishing rapidity (weeks, not centuries or millennia).

 

To answer the question: the user should set his/her GPSr to magnetic bearings if s/he is using a compass to measure direction.

[Lactodorum]

There is plenty of geological evidence that the geomagnetic poles reverse, sometimes with astonishing rapidity (weeks, not centuries or millennia).

Oh my gawd!!

 

I hope this doesn't happen while I'm a reviewer or I'll have to edit nearly 10,000 caches from North to South !!

[+Teasel]

To answer the question: the user should set his/her GPSr to magnetic bearings if s/he is using a compass to measure direction.

True enough in theory. But in the UK, magnetic variation seldom matters as it's so small. For geocaching, the best use for a compass (other than the often embarassing task of finding the right way out of the car park) is for the last 20m under tree cover. The arrow is jumping around like mad, and you can't zoom in to the map far enough to get a feel for which way the cache is, so you end up slowly wandering around at random, hoping you can get the distance to go down. But with a compass you have a much better idea of which way to wander! No need for magnetic variation though -- nearest 45deg is good enough!

 

IMHO, when walking in good weather, if you're relying on compensating for magnetic declination to keep your errors low, then you'd probably be better off shortening each leg of the journey and finding some intermediate features to look for. In bad weather, if you think you can hold a bearing to within 2deg, even with a friend to send ahead, you're certainly better than I'll ever be!

[+wildlifewriter]

True enough in theory.  But in the UK, magnetic variation seldom matters as it's so small.

Not small enough, for some geocaching circumstances...

 

This morning, Wuthered and I will be making an attempt to find this new cache in Co. Down. It seems to be an offset letterbox hybrid (??) but the description does not specify True or Mag for the bearing.

 

At an offset of 695 metres, this is going to give a choice of two locations for the cache - more than 60 metres apart. Not trivial, by any means.

 

Will we get the FTF? Will it stop raining long enough for us to try?

 

Watch this space...

 

-Wlw.

[+The Blorenges]

In fact the magnetic North pole is not always located in the Northern hemisphere!  There is plenty of geological evidence that the geomagnetic poles reverse, sometimes with astonishing rapidity (weeks, not centuries or millennia).

Really? Then that explains a lot: Sometimes we're unable to find a cache...or I can't find our car in a car park...or my house keys are in a completely different place from where I'm searching...or the cat disappears and then turns up again in a totally unexpected place.

 

It's all due to our internal magnetic compasses being askew.

 

Mrs B (Much relieved that she's not suffering from "senior moments"...yet. )

[+The Forester]

in the UK, magnetic variation seldom matters as it's so small. 

...

if you think you can hold a bearing to within 2deg

In the NorthWestern highlands of Scotland the magnetic variation is 6 or 7 degrees and in some parts of the world it's well over 20°.

 

 

A 5° error over a 6 mile walk results in a lateral error of half a mile (1 in 60 rule), which is not only avoidable but non-trivial if that puts you on the wrong side of a half-mile wide sealoch!

 

It's best to minimise avoidable errors by taking into account such things as magnetic variation and grid convergence. It can make a huge and very easy improvement in the accuracy of your navigation. Remember that the 7° (or whatever) is in addition to the 2 or 3 degrees resolution to which you can read a good compass. Sod's Law decrees that errors tend to be cumulative rather than mutually self-cancelling.

 

Cheers, The Forester

[markandlynn]

Avoiding all the other posts above if you are using an OS map the grid lines are set to True north therefore using true north with an OS map makes more sense than calculating for magnetic variance which if you get it the wrong way round can double your navigational error (in degrees not distance).

For goecaching as stated above use WGS 84.

This is covered a little bit on the Forum FAQ on our website.

[+The Forester]

There is plenty of geological evidence that the geomagnetic poles reverse, sometimes with astonishing rapidity (weeks, not centuries or millennia).

Oh my gawd!!

 

I hope this doesn't happen while I'm a reviewer or I'll have to edit nearly 10,000 caches from North to South !!

Fortunately Lat/Long refers to True North, not magnetic North, so you'll be safe.

 

Silva will clean up though. Just think of all the extra compasses they'll be able to sell with the 'South' end of the needle painted red!

[+Moote]

As the Canadians have the yearly task of mapping the exact position of magnetic north it might be worth reading their website, which can be found Here The poor chap who undertakes this task is called Larry Newitt, and there was a really good TV program about his job and the 100 year old equipment that is still used every year to measure and find the exact spot.

Edited December 1, 2005 by Moote

[NickPick]

Avoiding all the other posts above if you are using an OS map the grid lines are set to True north therefore using true north with an OS map makes more sense than calculating for

I hate to be picky, but OS map gridlines use a third north - Grid North, which is an arbitrary grid laid out by the OS. This also has a variance from True North, which depends on where you are in the country (New Forest east sheet it's 0.43 degrees West of True North, in Pitlochrie, Scotland, it's 1.5 degrees East of True North)

 

Fortunately, if you're using a paper OS map it'll have this information, and an estimate of magnetic declination, printed in the 'Technical Information' section of the map key.

 

As for using a GPSr for Geocaching, make sure it's set to show units of degrees and decimal minutes (Hddd mm.mmm) and datum of WGS84, and it should point you directly at the cache (Depending on tree cover, satellite geometry, the cache setter's coords being correct, etc)

 

If you're using a silva type compass to make a bearing for the final approach (when you've stopped and the GPS doesn't know which way you're facing), setting the GPSr to magnetic north means you can read the bearing from the GPSr, dial it into your magnetic compass and point in the right direction.

[+The Forester]

if you are using an OS map the grid lines are set to True north

Wrong!

 

The grid lines of an OS map refer to Grid North, not true North.

 

Grid North is equal to True [typo eventually corrected!] North only on the Central Meridian of the chosen grid system (4°W, in the case of the OS grid.)

 

The difference between Grid North and True North is called Convergence and increases not only with Longitudinal distance from the CM but also with Latitude. This is because while the Eastings Grid lines are parallel with eachother, a line pointing to True North converges on the North Pole. All Grid Eastings lines which are not located on the CM point to somewhere other than the True North Pole.

 

To transfer from a mapped Grid bearing to a Magnetic bearing, you must take into account both Convergence and Variation, which may be of same or opposite sign, depending whether you are East or West of the CM.

 

If you look at the bottom right-hand corner of an OS 1:50,000 LandRanger sheet you can see a diagram which shows the relationship as it relates to that map.

 

Cheers, The Forester

[edited to correct 'Grid' to 'True'}

Edited December 1, 2005 by The Forester

[markandlynn]

Avoiding all the other posts above if you are using an OS map the grid lines are set to True north therefore using true north with an OS map makes more sense than calculating for

I hate to be picky, but OS map gridlines use a third north - Grid North, which is an arbitrary grid laid out by the OS. This also has a variance from True North, which depends on where you are in the country (New Forest east sheet it's 0.43 degrees West of True North, in Pitlochrie, Scotland, it's 1.5 degrees East of True North)

 

Fortunately, if you're using a paper OS map it'll have this information, and an estimate of magnetic declination, printed in the 'Technical Information' section of the map key.

 

As for using a GPSr for Geocaching, make sure it's set to show units of degrees and decimal minutes (Hddd mm.mmm) and datum of WGS84, and it should point you directly at the cache (Depending on tree cover, satellite geometry, the cache setter's coords being correct, etc)

 

If you're using a silva type compass to make a bearing for the final approach (when you've stopped and the GPS doesn't know which way you're facing), setting the GPSr to magnetic north means you can read the bearing from the GPSr, dial it into your magnetic compass and point in the right direction.

OOPS! forgot that

[+Teasel]

Grid North is equal to Grid North only on the Central Meridian of the chosen grid system (4°W, in the case of the OS grid.)

Wrong!

 

Grid North is equal to Grid North everywhere, except at the poles, where it's undefined!

 

Sorry, couldn't resist!

 

ps even with a 7° variation, I stand by my belief that if you're walking 6miles on a single bearing then you should expect trouble, regardless of whether you tweaked your dial before setting off. You'd be much better off stopping every mile or so to look at the map, work out where you are, and take a new bearing.

[NickPick]

Grid North is equal to Grid North everywhere, except at the poles, where it's undefined!

Of course, if you're at the North pole, there isn't any North - every direction is South!

[+Teasel]

Grid North is equal to Grid North everywhere, except at the poles, where it's undefined!

Of course, if you're at the North pole, there isn't any North - every direction is South!

Well, to be really picky, at the North pole no direction is South. Whichever direction you move in, your distance to the South pole will stay the same. So, if you're getting no closer to the South pole, whatever direction it is you're going, it certainly ain't South! Only once you've moved a little way will there be a direction which takes you closer to the South pole.

 

I'll get me coat!

[+Snosrap]

So which way will your compass point??

[+Tharagleb]

It looks like the absolute best thing to do is to move to an area with magnetic variation of zero, and only cache withing 5 miles of your house. You may want to live in a mobile home, because the magnetic field varies a bit from year to year.

 

/My gps is set on magnetic so I can use it with an external compass.

[+purple_pineapple]

If you're using a silva type compass to make a bearing for the final approach (when you've stopped and the GPS doesn't know which way you're facing), setting the GPSr to magnetic north means you can read the bearing from the GPSr, dial it into your magnetic compass and point in the right direction.

OK, stupid question alert!!

 

If the GPSr has no reception due to tree cover, then how can the bearing it suggests for the cache be trustworthy? And if it does have reception, would it not just be easier to follow the GPSr?

[+Brenin Tegeingl]

OK, stupid question alert!!

 

If the GPSr has no reception due to tree cover, then how can the bearing it suggests for the cache be trustworthy? And if it does have reception, would it not just be easier to follow the GPSr?

 

Not a stupid question .

 

Before you hit the bad reception area, try and find a position giving you a clear view of where the GPSr is indicating. Get the bearing off your GPSr and transfer it to your compass. Sight a easily identifiable object along the imaginary line your compass is pointing along. Now move location and repeat, where the 2 imaginary lines cross, should be the general location of the cache. This works for me, as long as I remember to bring my compass with me .

 

Dave

Edited December 1, 2005 by Mancunian Pyrocacher

[+The Forester]

Grid North is equal to Grid North only on the Central Meridian of the chosen grid system (4°W, in the case of the OS grid.)

Wrong!

 

Grid North is equal to Grid North everywhere, except at the poles, where it's undefined!

Whoops!

 

Silly typo now corrected.

 

BTW, never use Transverse Mercator near the poles, only polar stereographic or somesuch. At Latitudes higher than about 82°, Mercator turns to a pile of pants. (so do magnetic bearings!!)

[+The Forester]

Not a stupid question .

 

Before you hit the bad reception area, try and find a position giving you a clear view of where the GPSr is indicating. Get the bearing off your GPSr and transfer it to your compass. Sight a easily identifiable object along the imaginary line your compass is pointing along. Now move location and repeat, where the 2 imaginary lines cross, should be the general location of the cache.

Not a stupid answer to not a stupid question!

 

The intersection technique can also be used when recording the co-ords of a new cache placed next to a rockface or other location which has lousy GPS reception locally. The method is most accurate if the included angle of the two intersecting angles is within about 30° of a right angle.

[+Archer4]

... Ordnance Survey's Airy spheroid and GPS's WGS84.

 

To join in the really really picky competition - the OS don't use the Airy meridian - they use the Bradley meridian which is 5.79m west

[+kevenh]

Thanks, I gather then I must set the coord system to Lat?/long rather than OSGB?

 

Bw

 

Richard

It will be easier.

 

The OS map coordinates are given on the cache web page, but everywhere else lat/long is used.

 

I think the gpx and loc downloads are lat/long only.

 

It's handy to be familiar with how to quickly change between OSGB and lat/long on your GPSr

It will impress your fellow walkers who only speak Landranger!

[+The Forester]

To join in the really really picky competition - the OS don't use the Airy meridian - they use the Bradley meridian which is 5.79m west

OK, you joined, so you can take a joke.

 

The joke's on you.

 

Airy spheroid has got nothing whatsoever to do with a datum.

 

It's simply the two dimensions of a spheroid, hence the name "spheroid".

 

OSGB36 does indeed use Bradley's transit meridian, but that's not a spheroid, only a single point on Airy's spheroid.

 

Cheers, The Forester

[+Nellies Knackers]

Theres a lot of old spheroids being talked here!

So whats the difference between a sphereoid and a geoid (other than spelling)?

[+purple_pineapple]

Theres a lot of old spheroids being talked here!

So whats the difference between a sphereoid and a geoid (other than spelling)?

quite, and where does an altoid come into it???

[+The Forester]

So whats the difference between a sphereoid and a geoid (other than spelling)?

A spheroid is quite simply an ellipsoid of rotation.

 

Take an ellipse and spin it to make the 3-D shape of an ellipsoid or spheroid (same thing).

 

The geoid is the lumpy bumpy 3-D thing that results from the fact that the Earth's gravity and shape is lumpy and bumpy and uneven.

 

Calculating co-ordinates on a spheroid is simple, which is why we geodesists do so.

 

Plotting such geodetic co-ords onto the geoid is reasonably simple and massively less difficult than doing the same calcs on the mathematically very complex geoid.

[+The Forester]

where does an altoid come into it???

An altoid converts the geoid into a Haemorrhoid?

[+Archer4]

OSGB36 does indeed use Bradley's transit meridian, but that's not a spheroid, only a single point on Airy's spheroid

Aha. Thanks

 

An altoid converts the geoid into a Haemorrhoid?

Is that for navigating by the seat of your pants?

[+Haggis Hunter]

where does an altoid come into it???

An altoid converts the geoid into a Haemorrhoid?

Except when it's an Android

[+Happy Humphrey]

I'm careful to leave my compass behind when geocaching, for the above reasons.

 

 

HH

[+The Forester]

I'm careful to leave my compass behind when geocaching, for the above reasons.

 

 

HH

Is that called "Preparation H", HH?

[+Pharisee]

I've made reference to a bearing on one or two of my caches and as these bearings were taken using a magnetic compass, I always quote them as magnetic. For example, the hint for one of them says:

 

"Under the fence at the end of a tree that has fallen on a bearing of 120 degrees magnetic."

 

As the only other fallen tree in the area had fallen at about 90 degrees to the one in the hint, it didn't really matter.... I was just being a 'smart-arse'

[+rutson]

As the only other fallen tree in the area had fallen at about 90 degrees to the one in the hint, it didn't really matter.... I was just being a 'smart-arse'

Well if the (silly?) hat fits......

[+Happy Humphrey]

Is that called "Preparation H", HH?

Ouch!