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battlerat and pussycat

True North vs. Magnetic North

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Hello,

 

I've often wondered whether I should configure my GPSr to true North or magnetic North? Does it matter? Some cachers would write a hint like "...hidden 20m NE from the beacon..." Depending on your GPSr setting, you could end up at the right place or not. For specific caches it matters. For most caches not (I think).

 

What's your opinion?

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Hello,

 

I've often wondered whether I should configure my GPSr to true North or magnetic North? Does it matter? Some cachers would write a hint like "...hidden 20m NE from the beacon..." Depending on your GPSr setting, you could end up at the right place or not. For specific caches it matters. For most caches not (I think).

 

What's your opinion?

 

True north is the way to go, in my opinion, especially if you are required to make projections for a cache. For small distances like 20m it hardly makes a difference, but over hundreds of meters you'll end up way off. I know some caches use magnetic north, but they normally specify this.

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For the purpose of using a GPSr, it does not matter because the coordinates are not influenced by where North is, however its been my experience that most GPSrs are set to magnetic north by default. This is probably just for historic reasons so that the electronic compass will point the same way as a magnetic compass. But as far as finding a cache goes, I think the declination between True North and Magnetic North is small enough that over a distance of a few meters it would be of little consequence when taking into account the general error in accuracy of a consumer GPS.

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The difference between True North and Magnetic North (declination) varies depending on where in the world you are - it is about 24.5 deg in Cape Town (Magnetic North is West of True North). If you project, it makes a difference of 42m+ for every 100m, so can significantly increase your search area!!!! So it is important to know which one a cache owner is using!

 

Declination changes slowly over time, usually, but every several hundred thousand years, the poles reverse :blink:

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Capeccr says it nicely - The choice of magnetic Vs true bearings can somtimes be quite important, over shorter distances that you would imagine. In Battlerat and Pussycats example however, he (she ?) uses (inter)cardinal points (N, NW, W etc). Dont forget that the difference between N and NW is 45 degrees, and typically when a user gives direcions using this method, they are seldom being precise. 45 degree steps suggests an aproximate direction, although you could the finer points (N by NNW for example). Typically if the cache owner wanted to be that precice he would have used degrees, in which case magnetic Vs True bearings would be very relevant. My guess is that in this case, 20M NW-ish of the your location would be an obvious place to hide a cache.

Edited by speedstripe

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My question would be, as I assume that not many of us would carry a separate magnetic compass with us, which north do the GPRs and smartphones use? Do they have a separate magnetic compass chip or do they work out the direction from the GPS?

Which ever they use would probably be the default for caching purposes

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My feeling is to stick to True North. GPSr's do not rely on magnetic north information at all. Furthermore all maps are based on TN, with MN offered as a declination. All users of magnetic compassess will be well aware of declination and the variance thereof and would be applying that in their calculations and observations. So the starting point is always TN, with an additional calculation for MN. Having said that, when I checked on my GPS I noticed it was set to MN, so I might be talking through my ear - but then again that is because I have not needed that facility to date. I still think TN would be my comfort zone.

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My question would be, as I assume that not many of us would carry a separate magnetic compass with us, which north do the GPRs and smartphones use? Do they have a separate magnetic compass chip or do they work out the direction from the GPS?

Which ever they use would probably be the default for caching purposes

They can do both. Some (higher end) GPS devices have a "real" electronic compass integrated, and this one will obviously align itself to magnetic north. However, as the device knows where it is, it will know how much the declination is in that area and can compensate accordingly. The same is true the other way around: if the device takes heading from the GPS data, then it can "de-compensate" and show the heading as aligned to magnetic north. It all depends on what the user has set it to.

As for smartphones, I don't know if they do (or even are capable of) the same kind of compensation. I'd assume that it's up to the software to do it.

 

My feeling is to stick to True North. GPSr's do not rely on magnetic north information at all. Furthermore all maps are based on TN, with MN offered as a declination. All users of magnetic compassess will be well aware of declination and the variance thereof and would be applying that in their calculations and observations. So the starting point is always TN, with an additional calculation for MN. Having said that, when I checked on my GPS I noticed it was set to MN, so I might be talking through my ear - but then again that is because I have not needed that facility to date. I still think TN would be my comfort zone.

Generally I agree, if something requires you to follow a bearing, do a projection or anything like that, and it doesn't say whether to use MN or TN, I'd assume TN. However, there are exceptions when you're explicitly told to use MN.

Edited by dfx

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