North Datum

[+Birders]

I just read this on a cache page:

 

"In light of recent events I should mention that the GPS readings were taken with the GPS set to Magnetic North with 003W variance"

 

As someone who has studied navigation I have to say I'm not sure what this means!

 

Surely, GPS receivers are not dependent on magnetic variation to locate an object unless you are comparing their readout with that of a conventional compass? In tracking towards a cache the tiny difference between magnetic and true north will not be noticeable with the kind of accuracy we're used to.

 

Maybe he put that in as a kind of "reverse spoiler"... or have I drunk too much wine tonight?

[+Eckington]

or have I drunk too much wine tonight?

......Blimey, at 5:17pm to have done that makes me green with envy and blush with shame

 

...thatt I can't match it.....I'm on shift until 10:00

Edited December 15, 2004 by Eckington

[+Team Ullium]

I just read this on a cache page:

 

"In light of recent events I should mention that the GPS readings were taken with the GPS set to Magnetic North with 003W variance"

 

As someone who has studied navigation I have to say I'm not sure what this means!

 

Surely, GPS receivers are not dependent on magnetic variation to locate an object unless you are comparing their readout with that of a conventional compass? In tracking towards a cache the tiny difference between magnetic and true north will not be noticeable with the kind of accuracy we're used to.

 

Maybe he put that in as a kind of "reverse spoiler"... or have I drunk too much wine tonight?

From what I have learned is that whatever the North setting is set to ....it only affects the compass readout on one's GPSr...and has no affect on the co-ord readout??

 

Anyone want to set me right on this belief ???

 

Bill.

[+wildlifewriter]

From what I have learned is that whatever the North setting is set to ....it only affects the compass readout on one's GPSr...and has no affect on the co-ord readout??

That is entirely correct. (Even in Ireland.)

 

It could be that the cache page mentioned (we don't know which one) had some other reference to compass bearings or directions - perhaps in the hint.

 

My eTrex Vista has a built-in magnetic compass. However, it's so close to being completely useless that I leave it switched off.

[+Flyfishermanbob]

I just read this on a cache page:

 

"In light of recent events I should mention that the GPS readings were taken with the GPS set to Magnetic North with 003W variance"

 

As someone who has studied navigation I have to say I'm not sure what this means!

 

Surely, GPS receivers are not dependent on magnetic variation to locate an object unless you are comparing their readout with that of a conventional compass?  In tracking towards a cache the tiny difference between magnetic and true north will not be noticeable with the kind of accuracy we're used to.

 

Maybe he put that in as a kind of "reverse spoiler"... or have I drunk too much wine tonight?

 

What kind of wine?

 

[+Bill D (wwh)]

It shouldn't normally be relevant, as others have said. WLW pointed out that it might relate to something else on the cache page - which cache is it?

[+Birders]

<<What kind of wine?>>

 

Waitrose best Vin Blanc - £2 a bottle.. no expense spared here y'know!

 

In the cache description there was nothing which would require the settings under discussion. Several people had experienced problems finding it and the owner had checked the co-ords and then issued the statement in my first posting. We'll just have to go and find it and check it out! As I suspected, you could set any variation and it would not have any effect on locating a postion. However, the compass readings may be slightly inaccurate... which is hardly relevant.

[+Chris n Maria]

Is it quite an old cache? I think I once saw a similar description on a cache in Hornet land. At the time there was a discussion on the forum about this and people noticed that switching north did make a difference to the position but in reality it was no more inaccurate than any GPS position.

 

Chris

[+The Forester]

Neither the GPS satellites nor a GPSr care a jot about North of any flavour. They don't use North (or South) in deriving either their or your position.

 

The output on your GPSr screen relates the relative bearing from the calculated position to a given waypoint in any selected format. That can be related to True North or Grid North or Magnetic North. They are all different and vary wildy, depending on location and selected grid.

 

Cheers, The Forester

[+The Forester]

My eTrex Vista has a built-in magnetic compass. However, it's so close to being completely useless that I leave it switched off.

I'm surprised.

 

Those fluxgate compasses are usually very good.

 

Do you recalibrate yours after changing batteries?

 

Cheers, The Forester

[+wildlifewriter]

I'm surprised.

Those fluxgate compasses are usually very good.

The theoretical principle may be very good but, as implemented in the Vista/Summit, the results are poor - certainly not as good as Magellan manage in their equivalent units.

 

The two main problems are: (1) the compass display is insufficiently damped, and (2) it is far too sensitive to the angle at which the unit is held. By comparison, any Silva compass works much better.

 

This second problem is non-trivial. In the field, one can easily produce an error of 7 or 8 degrees, with only a slight tilt. During calibration, the Vista must be absolutely level, or the resulting error becomes "fixed".

 

So... as said before, I prefer to leave it off - this having the advantages of better battery life AND not having to cart a spirit-level around as yet another item of geocaching equipment.

 

Do you recalibrate yours after changing batteries?

Why not ask the Pope if he knows anything about Catholicism?

[+The Hungry Caterpillars]

We also have big problems with the compass on our vista even when it is set up as per instructions. :mad:We normally just leave it off I have also become aware of people giving us very odd looks as I have set it up in the suggested manner after having had to change the batteries in a busy place

[+Team Ullium]

I'm surprised.

Those fluxgate compasses are usually very good.

The theoretical principle may be very good but, as implemented in the Vista/Summit, the results are poor - certainly not as good as Magellan manage in their equivalent units.

 

The two main problems are: (1) the compass display is insufficiently damped, and (2) it is far too sensitive to the angle at which the unit is held. By comparison, any Silva compass works much better.

 

This second problem is non-trivial. In the field, one can easily produce an error of 7 or 8 degrees, with only a slight tilt. During calibration, the Vista must be absolutely level, or the resulting error becomes "fixed".

 

So... as said before, I prefer to leave it off - this having the advantages of better battery life AND not having to cart a spirit-level around as yet another item of geocaching equipment.

 

Do you recalibrate yours after changing batteries?

Why not ask the Pope if he knows anything about Catholicism?

These are all the reasons I was advised on the Ramblers forum not to choose a GPSr with an electronic compass on board Wildlifewriter

 

Especially as the old hand held magnetic compass is cheaper and more reliable (OK.... outside geological anomalies) and is a lot more OS map friendly

 

Bill.

[+The Forester]

it is far too sensitive to the angle at which the unit is held. By comparison, any Silva compass works much better.

 

It is unsurprising that a single-axis fluxgate needs to be kept level when being calibrated and when in use. The dip angle of the lines of geomagnetic force in your part of the world is about 69°! If you are not keeping the thing level, you can expect that to severely distort the indicated heading.

 

Silva compasses compensate for this by having assymetric needles. One end is slightly heavier and longer than the other to balance out the dip force. They actually have slightly different models for sale in different parts of the world for this reason.

 

Cheers, The Forester

I'll ask the Pope about navigational matters when I think he's qualified to answer!

[+The Forester]

he old hand held magnetic compass is cheaper and more reliable (OK.... outside geological anomalies) and is a lot more OS map friendly

I agree, Ullium.

 

Old fashioned map and compass is hard to beat.

 

A good orienteering compass is worth its weight in gold. No batteries to run flat and no calibration required.

 

I suppose I might feel differently if I cached in the far North of Canada where compasses are tricky to use, but I don't!

 

Cheers, The Forester

[+The Hungry Caterpillars]

Iagree with Forester ,nothing better than a decent compass. We do a fair amount of Letterboxing and use a Silva sighting compass with very good results

[+Birders]

Nothing like a good compass... until it goes wrong!

 

For some years we have had a Stesco compass, purchased from one of the large camping stores - usual thing... the compass is set in a graduated ruler for use with maps, etc. No problems at all with it over years of worlwide travel... until last year in Australia. We'd got a bit lost so decided to follow the compass to take us to a main road. After driving for a mile or two Ruth suddenly exclaimed that we were going the wrong way! She had been sharp enough to realise that the shadows were all wrong for the way we were driving and the time of day. Sure enough, when we sorted ourselves out, we found our compass was 180 degrees out!

 

It had never, knowlingly, been placed near any powerful magnetic source and we can think of no reason why it should have happened.... but it's sitting in front of us now quite happily pointing south, whilst alongside it is another compas pointing north!

[+The Forester]

our compass was 180 degrees out!

 

It had never, knowlingly, been placed near any powerful magnetic source and we can think of no reason why it should have happened.... but it's sitting in front of us now quite happily pointing south, whilst alongside it is another compass pointing north!

It's extremely easy to make a compass needle or to reverse its polarity.

 

In basic survival training, military types are taught how to make one out of a pin or a needle or any sliver of suitable metal such as steel floating on a leaf in a small dish of water. It takes less than 60 seconds to magnetise it and to determine its polarity.

 

It sounds as if your compass has simply had its polarity reversed by its environment, albeit accidentally.

 

Actually, the entire Earth reverses its magnetic polarity every couple of tens of thousands of years or so. North becomes South and vice-versa. It happens very quickly. Silva salesmen must be salivating at the prospect of it happening after this Christmas!

 

Cheers, The Forester