Magnetic vs. True North

[+oshf]

I have found myself looking for caches that require projecting a waypoint. I've figured out how to do that with my gps (a delorme pn-40) but I now understand that there are two choices - magnetic north and true north. Understanding that sort of thing doesn't come easy to me so I need some help.

 

1. What is the difference and how do I know which I should look for when searching for a cache?

 

2. What is the default for my gps and is there a way to switch it to the other?

 

Thank you for your help.

[+Isonzo Karst]

Unless the cache owner tells you differently, I'd assume true north. Your gps is set for true north. I'm not aware of any way to change it over to magnetic north.

 

True north is the place at the top of the globe where the lines of longitude converge. Magnetic north is the place where the lines of magnetic force come out of the earth. They are NOT the same spot. And magnetic north shifts, albeit slowly. The further north you live, the greater the potential difference between a true north bearing and a magnetic north bearing.

[+Chrysalides]

More info than you probably want to know:

 

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

 

To figure out your magnetic declination:

 

http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp

 

(edit : URL copy / paste error)

Edited November 11, 2009 by Chrysalides

[Mr.Yuck]

Unless the cache owner tells you differently, I'd assume true north. Your gps is set for true north. I'm not aware of any way to change it over to magnetic north.

 

True north is the place at the top of the globe where the lines of longitude converge. Magnetic north is the place where the lines of magnetic force come out of the earth. They are NOT the same spot. And magnetic north shifts, albeit slowly. The further north you live, the greater the potential difference between a true north bearing and a magnetic north bearing.

 

Not to be a nitpicker, but Magnetic Declination varies from east to west, not north to south.

 

2004 US Magnetic Declination Map

 

P.S. Definitely varies over time. I remember being taught in the Army in the late 80's that it was zero degrees in Ft. Benning Ga.; looks like it would be 4 degrees right now. I've never been to Ft. Benning, but used their maps for training purposes.

Edited November 11, 2009 by TheWhiteUrkel

[+Markwell]

While not really on topic specifically this should help: FAQ on Mag v True

 

Depending on where you live in New York, your variance could be anywhere from 10-15°. Since a lot of your caches are found in the Albany area, you're probably about 14° off. Then it's a matter of distance. If you're 14° mag declination, that means that it could be wrong by 14°. But only if they mean magnetic and you do true north (or vice versa).

 

If you ARE wrong in your assumption of mag vs true, in the Albany area, if a cache has you go project 60 feet and you're off by 14°, you'll only be 10.83 feet away. If they have you go 300 feet, you'll be 69.75 feet off. I checked and in the Albany area you could be off 30 feet if someone had you project a waypoint of around 120 feet. Since 30 feet is about as good as a GPS usually gets, I would say...

 

If someone asked me to project a waypoint more than 120 feet in the Albany area and didn't give me clear direction on true vs mag, I'd ask for clarification, but probably guess at true.

Edited November 11, 2009 by Markwell

[+steve p]

Not to be a nitpicker, but Magnetic Declination varies from east to west, not north to south.

 

2004 US Magnetic Declination Map

Actually, it can vary in any direction. Look at the west coast on the map you posted the link to. North to south can have different variances, as can east to west. It all depends on where you are.

[+Coyote's Girl]

Magnetic Declination World-Wide And it just gets crazier from there.

[Mr.Yuck]

Not to be a nitpicker, but Magnetic Declination varies from east to west, not north to south.

 

2004 US Magnetic Declination Map

Actually, it can vary in any direction. Look at the west coast on the map you posted the link to. North to south can have different variances, as can east to west. It all depends on where you are.

 

Well, that's true, with the "curving" of the lines in many cases. I stand corrected. []

 

I also remember it being 7 degrees in my area (Extreme Western NY) in the 1980's, now it's 10 Degrees.

[+Team Cotati]

I tried this and it worked.

 

Went to: google.com

 

In the search window found there, I typed: "magnetic vs true north", without the quotes and pressed the enter key.

[+WRASTRO]

Unless the cache owner tells you differently, I'd assume true north. Your gps is set for true north. I'm not aware of any way to change it over to magnetic north.

 

True north is the place at the top of the globe where the lines of longitude converge. Magnetic north is the place where the lines of magnetic force come out of the earth. They are NOT the same spot. And magnetic north shifts, albeit slowly. The further north you live, the greater the potential difference between a true north bearing and a magnetic north bearing.

I couldn't believe this unit does not have a way to switch between true and magnetic so I went to the website and scanned the manual. I did not find any mention of a way to change the setting but I did notice a screen shot that showed "T", which indicates true north in most cases. This leads me to believe there is a way to switch between true and magnetic.

 

I would suggest going to the Delorme forums and asking there.

[+edscott]

Unless the cache owner tells you differently, I'd assume true north. Your gps is set for true north. I'm not aware of any way to change it over to magnetic north.

 

True north is the place at the top of the globe where the lines of longitude converge. Magnetic north is the place where the lines of magnetic force come out of the earth. They are NOT the same spot. And magnetic north shifts, albeit slowly. The further north you live, the greater the potential difference between a true north bearing and a magnetic north bearing.

 

Not to be a nitpicker, but Magnetic Declination varies from east to west, not north to south.

 

2004 US Magnetic Declination Map

 

P.S. Definitely varies over time. I remember being taught in the Army in the late 80's that it was zero degrees in Ft. Benning Ga.; looks like it would be 4 degrees right now. I've never been to Ft. Benning, but used their maps for training purposes.

 

As others have pointed out, it varies either way, but as the OP said the potential is greater from north to south. Think of it this way. You're somewhere around Chicago with a declination of about zero. If you move east or west at the same latitude the greatest declination you experience may approach 20 degrees as you circle the globe. If instead you go north along the line of longitude you are standing on the declination will eventually reach 180 Degrees.

 

Might also mention that the generalized published maps are just that .. generalized. There are loads of local anomalies that have an effect on magnetic north.

Edited November 12, 2009 by edscott

[+delphic]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Edited November 12, 2009 by delphic

[+WRASTRO]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

[+mapper66]

 

2. What is the default for my gps and is there a way to switch it to the other?

 

Thank you for your help.

 

On the PN-40 you can go to Device Setup - Units and scroll down to Bearing to change between magnetic and true north.

[+delphic]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

 

I grew up in seattle and it yes it works in the northwest. Again, all you need to rememeber (from seattle) is that your declination needs to be set east...your thumb pointing in that direction will help you rememeber.

[+WRASTRO]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

 

I grew up in seattle and it yes it works in the northwest. Again, all you need to rememeber (from seattle) is that your declination needs to be set east...your thumb pointing in that direction will help you rememeber.

Can you explain to my muddled brain how I will know the approximate declination if my thumb is pointing to the southeast? Our declination is roughly 18 degrees so my thumb should be pointing north-northeast.

[+Team Trailgoon]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

 

I grew up in seattle and it yes it works in the northwest. Again, all you need to rememeber (from seattle) is that your declination needs to be set east...your thumb pointing in that direction will help you rememeber.

Can you explain to my muddled brain how I will know the approximate declination if my thumb is pointing to the southeast? Our declination is roughly 18 degrees so my thumb should be pointing north-northeast.

 

I teach Map & Compass, and the way I remember it is the magnetic needle always points toward the Mississippi River (sort of / mostly) because magnetic north is in Canada, roughly in the middle, way up at the Arctic Ocean with the Polar Bears. If you extend the Mississippi all the way up to the Arctic Ocean, that's about where Magnetic North is. Also, your magnetic needle always points more toward the Mississippi the farther north you are. The finger/thumb thing confuses me, but is pretty much the same idea.

[Skippermark]

I also remember it being 7 degrees in my area (Extreme Western NY) in the 1980's, now it's 10 Degrees.

What will happen in the future. I know it moves, but what will happen years from now if it keeps moving? Will we end up upside down with somewhere in the south actually being north?

 

I'm asking a real question here, not being silly. What would happen if your area ends up being 85 degrees someday? That would mean mine would be 90 and north would be somewhere out west towards the west coast.

[+fizzymagic]

What will happen in the future. I know it moves, but what will happen years from now if it keeps moving? Will we end up upside down with somewhere in the south actually being north?

That's actually a very good question. There is abundant evidence for many magnetic field reversals in the past, so I would expect that it will happen again in the future. Moreover, there is also a lot of evidence that these magnetic field reversals happened very quickly (on a geologic time scale). There is a reasonably good wikipedia entry on it.

 

The original and details of the Earth's magnetic field are not well-enough understood for any prediction for the future. At present, the Earth's magnetic field is getting weaker. But a reversal is not likely to happen in our lifetime.

 

Even so, though, the magnetic poles can and do wander around on human timescales, which (for me) is a good enough reason not to use magnetic north as the reference for a projection is true north is available. Since wih a GPS true north is easily obtained from motion, there is no need to use magnetic north.

[+Stunod]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

 

Don't work where I live...just west of Chicago. I'd point east, but the declination is almost 3 degrees west.

 

For declination in the US by Lat/Lon (or even ZIP) go HERE

Edited November 12, 2009 by Stunod

[+edscott]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

 

I think he means the one in Illinois....

[+edscott]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

 

Don't work where I live...just west of Chicago. I'd point east, but the declination is almost 3 degrees west.

 

For declination in the US by Lat/Lon (or even ZIP) go HERE

 

Unless you're shooting a long line to the cache 3 degrees isn't that significant. It's best to use just the map or satellite image to get within a few hundred feet of the cache then get out the compass and take the bearing from a point that is easily identified on the map.

[+WRASTRO]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

 

I think he means the one in Illinois....

Yep, the one at about N 42 degrees. Seattle is at about N 47 degrees, which would make the Chicago in Illinois south of the Seattle in Washington. Thanks for trying to help clear things up for me.

[+fizzymagic]

I think he means the one in Illinois....

Yep, the one at about N 42 degrees. Seattle is at about N 47 degrees, which would make the Chicago in Illinois south of the Seattle in Washington. Thanks for trying to help clear things up for me.

Heh.

 

Actually, the direction from Seattle to Chicago is just a hair north of due East. From SEATAC to ORD the azimuth is 89.9 degrees.

 

But your point is completely valid. The declination in Seattle is nowhere near 90 degrees! I think the "Chicago" trick is only intended to give the sign of the declination.

[+edscott]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

I think he means the one in Illinois....

Yep, the one at about N 42 degrees. Seattle is at about N 47 degrees, which would make the Chicago in Illinois south of the Seattle in Washington. Thanks for trying to help clear things up for me.

 

Wrong.. it's further south, but not south. If you don't understand that's OK... it's not on the final.

Edited November 13, 2009 by edscott

[+WRASTRO]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

Hmmm, Chicago is south of Seattle. Might not work for me.

I think he means the one in Illinois....

Yep, the one at about N 42 degrees. Seattle is at about N 47 degrees, which would make the Chicago in Illinois south of the Seattle in Washington. Thanks for trying to help clear things up for me.

 

Wrong.. it's further south, but not south. If you don't understand that's OK... it's not on the final.

Fine. You win that nit. Would you care to try to actually answer my question about how to use the finger and thumb method to determine declination? If not then you have scored your point with your nit. I do understand. And I also understand that your reply is needless attempt to belittle for some reason. Next?

[+WRASTRO]

I think he means the one in Illinois....

Yep, the one at about N 42 degrees. Seattle is at about N 47 degrees, which would make the Chicago in Illinois south of the Seattle in Washington. Thanks for trying to help clear things up for me.

Heh.

 

Actually, the direction from Seattle to Chicago is just a hair north of due East. From SEATAC to ORD the azimuth is 89.9 degrees.

 

But your point is completely valid. The declination in Seattle is nowhere near 90 degrees! I think the "Chicago" trick is only intended to give the sign of the declination.

Yep. I just went back and read the original "thumb" post again. It is only a reference for figuring out if you are positive declination or negative. I think this is of limited value and focused more on the concept of determining the proper declination. I couldn't for the life of me figure out what use there could be for me to point my thumb at Chicago when I was trying to determine the proper declination.

 

I admit I don't understand the azimuth concept even though I get the same result as you. Chicago is at a more southerly lattitude than is Seattle. In my mind this means that Chicago is south of or further south than Seattle. How is it that one would fly slightly north to go south? Perhaps this is one of those great circle things.

[+BigFurryMonster]

What will happen in the future. I know it moves, but what will happen years from now if it keeps moving? Will we end up upside down with somewhere in the south actually being north?

 

There's two (potential) effects here. One is the gradual movement of the magnetic north- and southpoles. The magnetic northpole is somewhere in northern Canada right now, and will slowly move but still stay in the arctic region, somewhere "near" the true north pole.

 

The other effect is the possibility of a complete reversal of the Earth's magnetic field. The magnetic north would become the magnetic south and vice versa. There will be a large effect on all sorts of equipment making use of the magnetic field (and on nature). Geocaching may then not be the first thing to worry about.

If the reversed magnetic poles will stay at their positions, I do not know.

[John E Cache]

A quick trick that I learned long ago and is amazing in it's simplicity. This applies mostly to compass navigation but could be used to set declination on your GPSr.

 

To remember which way to set your declination, point to true north with your index finger (or north star...no need to be exact here). With your finger pointing, extend your thumb to Chicago. Obviously, you are using your left hand in the west and your right hand to the east. Your thumb is pointing in the direction of magnetic north (as is your compass arrow) and you can easily set your declination in the correct direction. Check your map or other source for how many degrees to correct.

 

This works anywhere in the continental United States. You could use other landmarks or major cities for other regions on the planet.

The 0 degree declination line moves and doesn't always pass through Chicago.

 

[+edscott]

The 0 degree declination line moves and doesn't always pass through Chicago.

 

But it has been relatively steady for a hundred years or so. The original post was just a way to get a quick take on whether Magnetic North was east or west of true north if you are in the continental US. I saw nothing more than that said or implied in the original post.