True North or Magnetic North

[+South Lyon Trekkers]

Does anyone pay attention to true north vs. magnetic north? Which is the default for Geocaching.com? Has anyone seen a cache based on one or the other? I know there is one in my area that specifies true north. I am just curious.

[+DavidMac]

It depends on where you are. In most areas of the eastern and central US, the difference is negligible. But if you're in Alaska or British Columbia, there can be a considerable difference (20 degrees or more). I've seen several caches which specified one or the other, but I found that you won't be off by more than a few feet if you use the wrong one.

 

Here's a map from NOAA that shows the declination between true and magnetic north:

 

[+biosearch]

Is this for multi caches? I can't think of any other time it would be used. Or am I missing something? I would think that if this were the case they should specify which they are using. Just be lucky you arn't in the military and have magnetic, true and grid, and have to convert from one to the other on the fly, oh those days, I miss them so, in the old days when you had to bring in birds and you didn't even know what a gps was.... I digress, do you know if that is the case, whether this is for multi caches or something else... DavidMac posted that map as I was typing that. That is cool. You should be able to find what the magnetic declination in your area is. And DavidMac is correct, in most cases the difference is negligable, unless (and this is a very big unless) you are traveling a large difference following that azimuth. It all depends upon what you are doing with the data, how far you are traveling, what the azimuth is leading you to, etc.

Edited October 30, 2006 by biosearch

[DogFleazJR]

Is this for multi caches? I can't think of any other time it would be used. Or am I missing something? I would think that if this were the case they should specify which they are using. Just be lucky you arn't in the military and have magnetic, true and grid, and have to convert from one to the other on the fly, oh those days, I miss them so, in the old days when you had to bring in birds and you didn't even know what a gps was.... I digress, do you know if that is the case, whether this is for multi caches or something else... DavidMac posted that map as I was typing that. That is cool. You should be able to find what the magnetic declination in your area is. And DavidMac is correct, in most cases the difference is negligable, unless (and this is a very big unless) you are traveling a large difference following that azimuth. It all depends upon what you are doing with the data, how far you are traveling, what the azimuth is leading you to, etc.

 

Use true unless specifically instructed to use magnetic, or you are using your GPS in conjuction with a compass bearing.

 

Maps are stated in True

[+South Lyon Trekkers]

Is this for multi caches? I can't think of any other time it would be used. Or am I missing something? I would think that if this were the case they should specify which they are using. Just be lucky you arn't in the military and have magnetic, true and grid, and have to convert from one to the other on the fly, oh those days, I miss them so, in the old days when you had to bring in birds and you didn't even know what a gps was.... I digress, do you know if that is the case, whether this is for multi caches or something else... DavidMac posted that map as I was typing that. That is cool. You should be able to find what the magnetic declination in your area is. And DavidMac is correct, in most cases the difference is negligable, unless (and this is a very big unless) you are traveling a large difference following that azimuth. It all depends upon what you are doing with the data, how far you are traveling, what the azimuth is leading you to, etc.

 

I'm just curious if it is an issue. I know my GPS can be set to either. There is a cache in my area that states one would need to use true north. It is a multi-cache and to find the various caches along the way one would need to use the projection feature on the GPPr.

[+South Lyon Trekkers]

It depends on where you are. In most areas of the eastern and central US, the difference is negligible. But if you're in Alaska or British Columbia, there can be a considerable difference (20 degrees or more). I've seen several caches which specified one or the other, but I found that you won't be off by more than a few feet if you use the wrong one.

 

Here's a map from NOAA that shows the declination between true and magnetic north:

 

 

Thanks for the map. I have been reading up on declination. I'd like to more proficient with the compass.

[+emurock]

Some people may not kown this but magnetic north moves about 10 miles south a year.

[QuigleyJones]

Realy? So in a few thousand years it will be at the south pole?

[+NorthWes]

The declination map is valuable - and if you're from Alaska, standing in a park in the dark in New Jersey with your magnetic compass in your hand (preset to the +22º declination in my home turf) your compass will be sending you about 30º off course. That's significant when you're searching for a benchmark set flush at ground level 60' from your starting point at dusk in a howling windstorm with dinnerplate-sized leaves blowing around ankle-deep all over the place! I was doing just that on Saturday the 28th at Fort Nonsense Nat'l Military Park (aptly named) - and remembered after fruitlessly searching for twenty minutes to reset my compass. Found it on the next pass... and swore once again it's time to get a metal detector!

[+benh57]

Realy? So in a few thousand years it will be at the south pole?

 

Possibly.. the field flips every 200,000 years or so, and we are long overdue. (last flip 780,000 yrs ago) Most scientists expect it will flip 'soon'. (in geological terms

http://en.wikipedia.org/wiki/Geomagnetic_reversal

 

http://news.nationalgeographic.com/news/20...field_flip.html

[Tahosa and Sons]

Many of my multis have some sort of compass stuff incorporated into the cache, and I usually don't say wheather its True or Mag. After a while they will finally find it.

[+edscott]

It depends on where you are. In most areas of the eastern and central US, the difference is negligible. But if you're in Alaska or British Columbia, there can be a considerable difference (20 degrees or more). I've seen several caches which specified one or the other, but I found that you won't be off by more than a few feet if you use the wrong one.

 

Here's a map from NOAA that shows the declination between true and magnetic north:

 

 

It depends a whole bunch how far you are going to go. Sure a 10-15 degree difference on a 100 meter plunge is not much, but I often use a compass for 500 to 1500 meters at a time. Break out your old geometery book and caclulate the base of a mile high triangle with a 10 degree angle at the top. Doesn't matter which north you use, but you better know which one you are using. True probably agrees with your map, magnetic north with your compass. BTW the published declinations are approximate. Better know your area if you are using a compass to find a cache.

[+Alan2]

The declination map is valuable - and if you're from Alaska, standing in a park in the dark in New Jersey with your magnetic compass in your hand (preset to the +22º declination in my home turf) your compass will be sending you about 30º off course. That's significant when you're searching for a benchmark set flush at ground level 60' from your starting point at dusk in a howling windstorm with dinnerplate-sized leaves blowing around ankle-deep all over the place! I was doing just that on Saturday the 28th at Fort Nonsense Nat'l Military Park (aptly named) - and remembered after fruitlessly searching for twenty minutes to reset my compass. Found it on the next pass... and swore once again it's time to get a metal detector!

 

Why should it matter? The bearing arrow will always aim to the waypoint even if the declination is wrong, off or mis-set. The compass rose will be off, but if you follow the bearing arrow you'll will get there anyway.

Edited October 31, 2006 by Alan2

[+EraSeek]

Some people may not kown this but magnetic north moves about 10 miles south a year.

 

Actually, no, it is moving north at a rate of about 40kms a year:

 

And it moves around its average position about 85 kms a day.

[+emurock]

Some people may not kown this but magnetic north moves about 10 miles south a year.

 

Actually, no, it is moving north at a rate of about 40kms a year:

 

And it moves around its average position about 85 kms a day.

Thank you for correcting me.

[+baloo&bd]

Realy? So in a few thousand years it will be at the south pole?

Don't sweat it, Garmin is working on a firmware upgrade to accommodate it.

[2oldfarts (the rockhounders)]

Does anyone pay attention to true north vs. magnetic north? Which is the default for Geocaching.com? Has anyone seen a cache based on one or the other? I know there is one in my area that specifies true north. I am just curious.

 

Most folks use magnetic north. Your GPSr was probably set to magnetic north right out of the box.

 

If you use a handheld compass, you will probably want to have the GPSr set to magnetic north so they will read the same.

 

Unless it specifies on the cache page to use True North just use magnetic north. When we set up a cache we use the GPSr in conjunction with the handheld compass so that means use magnetic north for our caches.

 

John

[DogFleazJR]

Does anyone pay attention to true north vs. magnetic north? Which is the default for Geocaching.com? Has anyone seen a cache based on one or the other? I know there is one in my area that specifies true north. I am just curious.

 

Most folks use magnetic north. Your GPSr was probably set to magnetic north right out of the box.

 

If you use a handheld compass, you will probably want to have the GPSr set to magnetic north so they will read the same.

 

Unless it specifies on the cache page to use True North just use magnetic north. When we set up a cache we use the GPSr in conjunction with the handheld compass so that means use magnetic north for our caches.

 

John

 

Actually, the GPS default is true and I would guess that the majority of people leave it on true. For most caching it doesn't matter, as another poster pointed out, the GPS will point a bearing to the waypoint, it doesn't care if you are set to true or magnetic.

 

If you are using your GPS with a map then set your GPS to true.

 

If you are using your GPS with a magnetic compass then set it to magnetic.

 

95% of users will never need to change this. Just leave it on true.

[+EraSeek]

Yes. The default is 'True'. The GPS and the overall system thinks in true. When you set the unit to Magnetic it offsets True by your declination value, just like time is in UTC and is offset to your time zone.

[+fizzymagic]

Yes. The default is 'True'. The GPS and the overall system thinks in true.

That's an important point. IMO, all projections that don't require the use of a compass should use true north. There's a good reason for this: the (true) North Pole is a spot defined by the coordinate system that is defined by latitude and longitude. Thus, true north is exactly defined by the coordinate system.

 

Magnetic north, on the other hand, is an approximate direction defined by the earth's global and local magnetic fields. It is defined for use with a particular instrument -- the magnetic compass. It happens to point relatively close to north in many places on the Earth, but it is low-precision and not constant. Unless you are actually using a magnetic compass, there is no reason to use it.

 

In fact, sailors in previous centuries knew enough not to count on magnetic north. They would use the compasses on their ships for a continuous bearing, but they would measure the declination using the stars as often as they could.

 

Grid north is also referenced to a coordinate system, so in certain contexts its use would make sense as well. But since UTM grids are notoriously bad for projection, you won't often see it used in that context.

[+EraSeek]

I thought this crowd my appreciate seeing just how complex the Earth's magnetic field really is:

 

Blue is inbound, yellow outbound

 

[+The Jester]

I thought this crowd my appreciate seeing just how complex the Earth's magnetic field really is:

 

Blue is inbound, yellow outbound

 

Inbound and outbound what?

[+EraSeek]

The magnetic field is directed inward in blue and outward in yellow.

[+Renegade Knight]

Does anyone pay attention to true north vs. magnetic north? ...

 

It took me 4 years to figure out my GPS was set to true.

[+EraSeek]

Actually, where the field is inbound (blue in the above graphic), which we call the north magnetic pole, is really the south magnetic pole. Out bound is north on a magnet. Thus on a magnitized compass needle the north pole of it is attracted to the "north pole" of earth because it is really the south (inbound) pole.

 

A compass needle is balenced for use by regions. The closer to the north pole you get, the more it wants to point down (to the ground).

 

In periods approaching pole reversals the field becomes unstable and multiple south and north poles occur at once.

[+michigansnorkelers]

Realy? So in a few thousand years it will be at the south pole?

 

Possibly.. the field flips every 200,000 years or so, and we are long overdue. (last flip 780,000 yrs ago) Most scientists expect it will flip 'soon'. (in geological terms

http://en.wikipedia.org/wiki/Geomagnetic_reversal

 

http://news.nationalgeographic.com/news/20...field_flip.html

 

And THAT will be the end of life on earth, as we know it.

 

Earth's magnetic field repulses cosmic rays. Without this protective magnetic field, we will get "fried".

 

Saw this on the Discovery channel recently.

[+EraSeek]

Realy? So in a few thousand years it will be at the south pole?

 

Possibly.. the field flips every 200,000 years or so, and we are long overdue. (last flip 780,000 yrs ago) Most scientists expect it will flip 'soon'. (in geological terms

http://en.wikipedia.org/wiki/Geomagnetic_reversal

 

http://news.nationalgeographic.com/news/20...field_flip.html

 

And THAT will be the end of life on earth, as we know it.

 

Earth's magnetic field repulses cosmic rays. Without this protective magnetic field, we will get "fried".

 

Saw this on the Discovery channel recently.

 

Actually, no. NASA:

"How long does an earth magnetic field reversal take and is it harmful to humans?

 

------------------------------------------------------------------------------

We don't really know for certain. Estimates from the geologic record suggest that a reversal can take less than 100 years in some instances. The effects upon the biosphere seem to be minimal since no paleontological evidence for high mutation/speciation rates correlate with the known reversals. As for humans, we have no remains from 250,000 years ago or so to look at in any detail. The major impact would be from solar activity and the apparent loss of the Earth's protective magnetosphere during this time. However, the Earth's atmosphere is so thick that it may be unlikely that earth life is even affected in the slightest!! "

 

In the case of a zero magnosphere, you would have the northern lights anywhere on earth. Not just at the poles. You have the nothern lights now at the north pole because in effect the magnetic field there lets cosmic rays into the upper atmosphere, much like what would happen with the loss of the magnosphere, and life exisits there just fine right now.

[GermanSailor]

Hi,

 

here you can check what the exact Variation for a position is. Since you just can enter whole degrees you might want to interpolate if you want to be really precise. By the way the terms magnetic declination or variation are the same, but variation is the one normally used in aviation and navigation.

 

If you want to get the magnetic bearing to follow with the compass after you figured out the true bearing on the map you add west or subtract east.

 

From Chart to Compass: West is Best, East is Least!

 

Don't forget that many other things can throw your magnitic compass off as well. Like your car, wrist-watch, power lines, iron and many other things.

 

Some multis caches state a bearing that you have to follow, so it is importat to know if it is true or mag. In the Western US you can't just ignore it. Seattle has 18° E Variation at the moment.

 

Lets say you want to go due north (true) there you have to follow a compass bearing (heading) of 342°.

 

For every degree you are off after walking for statue mile, you will end up 28 meters (92 feet) left or right of your destination. Imagine what a 19° error can to to you even if you just go for a couple of hundret feet.

 

GermanSailor

Edited November 3, 2006 by GermanSailor

[+emurock]

Realy? So in a few thousand years it will be at the south pole?

 

Possibly.. the field flips every 200,000 years or so, and we are long overdue. (last flip 780,000 yrs ago) Most scientists expect it will flip 'soon'. (in geological terms

http://en.wikipedia.org/wiki/Geomagnetic_reversal

 

http://news.nationalgeographic.com/news/20...field_flip.html

 

And THAT will be the end of life on earth, as we know it.

 

Earth's magnetic field repulses cosmic rays. Without this protective magnetic field, we will get "fried".

 

Saw this on the Discovery channel recently.

 

Actually, no. NASA:

"How long does an earth magnetic field reversal take and is it harmful to humans?

 

------------------------------------------------------------------------------

We don't really know for certain. Estimates from the geologic record suggest that a reversal can take less than 100 years in some instances. The effects upon the biosphere seem to be minimal since no paleontological evidence for high mutation/speciation rates correlate with the known reversals. As for humans, we have no remains from 250,000 years ago or so to look at in any detail. The major impact would be from solar activity and the apparent loss of the Earth's protective magnetosphere during this time. However, the Earth's atmosphere is so thick that it may be unlikely that earth life is even affected in the slightest!! "

 

In the case of a zero magnosphere, you would have the northern lights anywhere on earth. Not just at the poles. You have the nothern lights now at the north pole because in effect the magnetic field there lets cosmic rays into the upper atmosphere, much like what would happen with the loss of the magnosphere, and life exisits there just fine right now.

Thanks for the infor.

[+The Jester]

After reading this thread, I had a question pop into my head today while caching (and swearing at: lousy signals; a compass that decided to break down today; and rain). Do the GPSr's have any way of tracking the movement of the magnectic pole? In other words, will they stay accurate for magnetic bearings as the pole shifts? Or will they keep using a static position?

[+EraSeek]

The declination is programed in to the GPS. It knows from the programing what your declination is by your coordinates. Whether declination annual drift is programmed in as well, I don't know but it would not be that tough to do. You can find drift rates on the web. But the drift is minimal and I don't think the programmed declination is all that precise in the GPSr. For example, I've seen my declination at 17.5 or 18.5 or some such, and the gps displays 18. I think the offset may just be a good round figure. Another reason to use TRUE instead of magnetic.

[+EraSeek]

For example, when I have my GPS set to Mag, it displays 18 degrees east as the declination.

Here is the computed value:

Results for date: 2006.8438720703125

Declination = 17.587° changing by -0.175 °/year

Inclination = 69.562° changing by -0.026 °/year

X component = 18,247.64 changing by 15.06 nT/year

Y component = 5,783.85 changing by -56.72 nT/year

Z component = 51,366.72 changing by -79.04 nT/year

Horizontal Intensity = 19,142.34 changing by -2.69 nT/year

Total Intensity = 54,817.6 changing by -75 nT/year

[+Alan2]

I changed the date of my computer to 2099. The declination for NYC of W13.4 stayed the same in Mapsource running on my computer as it is for 2006. That seems to indicate that Garmin, at least in the PC, Mapsource program is using a fixed preset value.

 

Or the declination in NYC does not change over time

Edited November 5, 2006 by Alan2

[+Renegade Knight]

After reading this thread, I had a question pop into my head today while caching (and swearing at: lousy signals; a compass that decided to break down today; and rain). Do the GPSr's have any way of tracking the movement of the magnectic pole? In other words, will they stay accurate for magnetic bearings as the pole shifts? Or will they keep using a static position?

It would have to be a firmware update. That or it gets the info from GPS satellites. Either would work but the former seems more likely.

Edited November 5, 2006 by Renegade Knight

[GermanSailor]

After reading this thread, I had a question pop into my head today while caching (and swearing at: lousy signals; a compass that decided to break down today; and rain). Do the GPSr's have any way of tracking the movement of the magnectic pole? In other words, will they stay accurate for magnetic bearings as the pole shifts? Or will they keep using a static position?

Hi Jester,

 

no worries. The MagVar doesn't change that much every year. But as far as I know, Garmin has a Database integrated for the next decades. I imagine that they have a simple spreadsheet with data for every Lat/Long confluence and every year and the software just interpolates with the position / date.

 

But you can always enter the Variation manually on the GPS. Or just follow the TrueNorth bearings and do the computation. But if you have a map, why would you want to have mangnetic bearings? Everything you get of the chart is in True.

 

What I would do: Check what your areas variation is. If it is less than 5 degrees I would ignore it. With the distances we walk bearings on while geocaching it doesn't really matter. (See my post above). If it is more than 5° I would apply the variation to the nearest 5 degrees. (round up or down)

Don't forget you can't really follow a 1° bearing while walking - You probably can't even read it so accurate of your "whiskey compass".

 

GermanSailor

Edited November 5, 2006 by GermanSailor

[GermanSailor]

I changed the date of my computer to 2099. The declination for NYC of W13.4 stayed the same in Mapsource running on my computer as it is for 2006. That seems to indicate that Garmin, at least in the PC, Mapsource program is using a fixed preset value.

 

Or the declination in NYC does not change over time

Hi Alan,

 

I don't think there is data available for 2099, because nobody can predict the drift of the magnetic north pole for more than a couple of decades. Try to set the date to 2009 for example, it should be 13° W!

 

But I guess in 2099, most of us don't have to worry about declination any more.

 

GermanSailor

Edited November 6, 2006 by GermanSailor

[+Peconic Bay Sailors]

I changed the date of my computer to 2099. The declination for NYC of W13.4 stayed the same in Mapsource running on my computer as it is for 2006. That seems to indicate that Garmin, at least in the PC, Mapsource program is using a fixed preset value.

 

Or the declination in NYC does not change over time

Hi Alan,

 

I don't think there is data available for 2099, because nobody can predict the drift of the magnetic north pole for more than a couple of decades. Try to set the datum to 2009 for example, it should be 13° W!

 

But I guess in 2099, most of us don't have to worry about declination any more.

 

GermanSailor

 

I'm sure that by 2099 we will not be using MapSource any more...

and the GPS that has been implanted in your brian will have the proper declination...

Edited November 5, 2006 by Peconic Bay Sailors

[+Foothills Drifter]

Howdy......

When it come right down to it...... it really don't matter !

We still find what we are lookin for... don't we.............

 

Vern...

Edited November 6, 2006 by Foothills Drifter

[GermanSailor]

error

Edited November 6, 2006 by GermanSailor

[+Alan2]

I changed the date of my computer to 2099. The declination for NYC of W13.4 stayed the same in Mapsource running on my computer as it is for 2006. That seems to indicate that Garmin, at least in the PC, Mapsource program is using a fixed preset value.

 

Or the declination in NYC does not change over time

Hi Alan,

 

I don't think there is data available for 2099, because nobody can predict the drift of the magnetic north pole for more than a couple of decades. Try to set the date to 2009 for example, it should be 13° W!

 

But I guess in 2099, most of us don't have to worry about declination any more.

 

GermanSailor

 

Actually it went up to W13.8 from 13.4. Nice to know it's a least moving

[+trainlove]

Actually by 2099 there WILL be something better than GPS, or GLONOSS or Galileo.

Wouldn't it be great if a handheld receiver could use all three systems today? I have used Loran-C once while geocaching, fun to have a 15 foot long whip antenna out of your backpack hitting every tree in the woods I walked through.

 

I have seen Aeronautical charts where in addition to the lines of constant declination (every half a degree) there are lines of constant rate of change of declination (minutes I think), i.e. at a particular spot it might be 15.5 degrees west, and changing at -7.5 minutes per year, so 15.5W today, 15.37W next year, 15.24W one more year...

 

I was unaware that Garmins have more than a static table of declinations, I use Magellan and know the declination is rounded to whole degrees (yuck, but at least you can project with tenths or hundreths). But how does one change the date? I know one can change the 'display time' but I thought all GPSr's actually know the time from the satellites and therefore one can't fake them out.

[+Alan2]

I changed the date in Mapsource in my computer not in the GPS.