True North vs. Magnetic North - HUH?

[rag-a-muffin]

Hey all...

 

I'm confused as to the difference between true north and magnetic north. Could someone explain what the difference is? Is one better than the other? I have a Magellan Meridian, and I'm able to choose between true and magnetic, but I don't know which one to choose. Does choosing one over the other make me more/less accurate when hunting a cache?

 

r.

[+EraSeek]

Magnetic north is where your compass points; the magnetic north pole which is currently wandering around northern Canada. True north is the actual north pole, the axis of the Earth's rotation, way up there on the very top.

 

Your compass points to magnetic north unless you have an adjustable compass.

 

Most maps are oriented to True North.

 

It is a matter of choice. Neither will make your GPS more or less accurate. If you set it to True, then it points to true north on your compass page and discribes your bearing degrees in relation to true north. If in Magnetic, it give it in relation to magnetic.

 

If you give directions in degrees, you will want to state whether it is in true or magnetic north.

 

Here is were you can find the declination (the degrees difference) between the two methods: http://www.thecompassstore.com/decvar.html

 

 

"See the wonderous works of Providence! The uncertainty of human things!" Geo.Washington

[+nincehelser]

The geographic North pole (i.e. true, or the axis of Earth's rotation) and the magnetic North pole are not in the same location due to the Earth's dynamics. The magnetic pole is actually some distance away in Canada.

 

Magnetic north is nice because it's where your compass naturally points. However, maps are usually aligned to true north. Depending on your location, you'll have to make some degree of correction (declination) to account for this and make your map and compass agree.

 

For the purposes of finding a cache, this means nothing. For the most part, you can just follow the pointer.

 

George

[+TEAM 360]

Stick with True North. I also have the feature to set the compass on Magnetic readings or GPS course readings. Walking along railroad tracks is not the time to have it set to magnetic ANYTHING.

[+Renegade Knight]

There are.

 

True North which points at the pole.

 

Magnetic North which point at the magnetic north pole which is in the 'general area' of the norht pole but really isn't there.

 

"Grid North" which is North by definition on a grid which is needed to keep things like UTM consistant in an area. North from one grid to another is different.

 

Then my personal favorite:

 

"Plan North" which is used by architects who don't like Grid North or True North, or even Magnetic North and whan their buildings all nice and square. So they will pick a North and call that Plan north. Then when they are talking north and an engineer are talking North they are not really speaking the same language.

 

==============================

Wherever you go there you are.

[+Poindexter]

Railroad tracks won't affect your readings unless your using a GPSr with an internal compass activated.

[+Cachetrotters]

quote:

---

Originally posted by TEAM 360:

Stick with True North. I also have the feature to set the compass on Magnetic readings or GPS course readings. Walking along railroad tracks is not the time to have it set to magnetic ANYTHING.

---

For the novices who won't know what you meant to say....

The GPSr will not be affected by the railroad tracks regardless of mode or orientation;

A compass will be affected regardless of declination adjustment.

 

don

[+TEAM 360]

If you are on the compass screen with the compass set to magnetic, and you are standing on railroad tracks, trying to spin around and line up the "X" on the outside edge of the compass ("X" being the correct route to the cache) you are in big trouble. Having the compass getting thrown off by the tracks while staring at THAT screen while getting to the cache will be a problem. (MeriPlat)

[+Poindexter]

Only if your using the internal compass. You can still get "magnetic" bearings when not using the internal compass as long as you are moving (not standing still) and the tracks will have no effect on the readings. The GPS simply applies the declination correction from internal tables based on your current location. It does not rely on the earths magnetic field to determine bearings from GPS movement (as opposed to using the internal compass). However, if you are standing still, you could use the internal compass if you calibrate it first. It will compensate for local deviation and give correct bearings for that location.

[+TEAM 360]

If you cache by looking at the compass screen, as I do sometimes, and the compass is being affected by the tracks, like it did, the compass will be wrong, and the route to the cache will be wrong as well, at least on the compass screen, until I change screens to where a "heading" is not displayed. If you can't see a "heading" on the MeriPlat screen you are displaying, the internal compass is automatically turned off. Anytime I am around large metallic objects, I go to "True North" (instead of Magnetic North)and "GPS Course" (instead of Magnetic) to set the compass reference and orientation by.

Regardless of whether you set the compass to True North or Magnetic North reference, if you use Magnetic Orientation instead of GPS Course Orientation, and the compass is whacky from other metallic objects, as long as you are looking at the compass screen, your bearing to the cache will be off (designated by an "X" on the outside rim of the electronic compass). Calibrate the compass all you want before you start, and when you get within metallic objects large enough, it will still be off. I have walked down railroad tracks and had the compass do the 180 degree flip flop on me, as soon as I went over to GPS Course Orientation with True North on the compass settings, it cleared up.

Something else I noticed is that when I used a Mag 315 and stood still, the track map would spin around the steady icon in the center of the screen, as it averaged. The Meridian series solves this jumping around by turning the center icon instead of the map, so all the compass feature does is stop the center icon from spinning around during averaging (I assume the Meridian non-Plats do this).

[+Poindexter]

What I'm trying to get across is that you can still get accurate magnetic bearings (as long as your moving) by selecting Magnetic North as your north reference in the setup menu and turn your magnetic compass OFF by selecting GPS Course in the compass orient menu and then these magnetic bearings WILL NOT be affected by the railroad tracks because their not relying on the internal compass sensor. If you don't get it, you don't get it, but thats the way it is.

[+Renegade Knight]

Poindexter what you say is true, if and only if.

 

1) The internal magnetic compas is not used at all.

2) The GPS north (based on movement like my GPS V which doesn't have an electronic compas) knows to show magnetic north from having declination input somewhere in the GPS.

 

My GPS can show me north because it doesn't care about the earths magnetic field or the railroad effect. It only knows north though when I'm moving.

 

==============================

Wherever you go there you are.

[+Poindexter]

>>1) The internal magnetic compas is not used at all.

 

That's what Iv'e been saying all along.

 

>>2) The GPS north (based on movement like my GPS V which doesn't have an electronic compass) knows to show magnetic north from having declination input somewhere in the GPS.

 

You don't have to input the declination unless you want to do it manually. If it's set to auto, it will do it automatically by using internal declination tables.

[+TEAM 360]

Poindexter, didn't you say?:

 

"However, if you are standing still, you could use the internal compass if you calibrate it first. It will compensate for local deviation and give correct bearings for that location."

 

No, you can't use the internal compass if you are near metallic objects, whether still or moving, unless you set it to read off GPS Course only, and not magnetic readings.

 

Other than that, I completely agree with you.

[+Waterboy]

Since the original author, rag-a-muffin, is from Richmond, VA. There is something far more common than railroad tracks that create problems for magnetic compasses here in Pennsylvania and some neighboring states. It is iron ore in the soil. Some places the definition of magnetic north is the direction to the nearest mine (usually an abandoned mine). I do not know what the conditions are in or near Richmond, but please be aware of this.

[+Poindexter]

quote:

---

Originally posted by TEAM 360:

Poindexter, didn't you say?:

 

"However, if you are standing still, you could use the internal compass if you calibrate it first. It will compensate for local deviation and give correct bearings for that location."

---

 

Yes, I did say that, and it is true as long as you don't move from the location that you calibrated it, as I said. That's the whole purpose of calibrating it. Normally, you would calibrate it away from any ferrous objects because normally you would use it away from those objects. This is the same way that you calibrate electronic compasses used in vehicles and you could also calibrate your GPS compass in your vehicle and use it there although other nearby vehicles when you are driving along will cause errors. We calibrate electronic compasses on boats to compensate for local deviations caused by ferrous metals in nearby fixed objects. As long as the relationship between those objects and the compass don't change then the accuracy is good. Put something ferrous nearby and things change. If that something is to be permanent, then you just calibrate again to compensate. Prove it to yourself, stand on the tracks and calibrate your compass and it will be accurate as long as you don't move from that location.

 

And yes, I know what it says in the Meridian manual. I don't own a Meridian so I can't test for myself. It may be that the Meridian software won't let you calibrate the compass when it's near ferrous objects. If that's the case, it's to protect the ignorant user and you would be correct for the Meridian. The Garmin MAP76S doesn't have this shortcoming and will compensate for local deviations.

 

We are getting way off track here. The purpose of my original post was to rebut your statement that you shouldn't set your GPS to "magnetic anything". I'm saying it's perfectly fine to use magnetic bearings even with the compass off when your around ferrous metals.

 

[This message was edited by Poindexter on April 10, 2003 at 11:26 AM.]

[victorjwood]

Its also worth pointing out that Magnetic North changes a bit every year, (as the earths mantle moves around?). I think I read it can change as much as 7 deg. For that reason, I prefere true north.

 

Victor

[+TEAM 360]

Good points made, though, and that's what it's all about. Thanks.

[+Poindexter]

quote:

---

Originally posted by VictorJWood:

Its also worth pointing out that Magnetic North changes a bit every year, (as the earths mantle moves around?). I think I read it can change as much as 7 deg. For that reason, I prefere true north.

 

Victor

---

 

The Magnetic North pole is constantly but very slowly moving due to changes in the magnetic field, not the movement of the Earth's mantle. In the contiguous 48 states the annual change varies from a minumum of 5' (minutes) E or W per year (meaning it will take 12 years to change the declination by one degree) to a maximum of 12' (meaning it changes one degree in five years). Obviously nothing to worry about as you will have purchased a new GPS by the time it would start to make a difference and the declination tables will have been updated. By the way, some firmware updates include updated declination tables.

[+EraSeek]

Changes in declination in 'Minutes per Year':

[+Tonsil]

Don't trust true north! Just because it's "true" doesn't mean it's true all the time. Sometimes it lies. And when it does, it leaves you with that hollow, bitter feeling inside. But it's not your fault! Oh no! You must look within and know that you're better than some two-faced compass direction. You can stand on your own two feet, and learn to be your own source of guidance. And then, only then, will you be truly free!

 

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

Have you had your house checked for Rae Dawn Chong?

[+EraSeek]

Another interesting item is that the magnetic poles flip every few hundred thousand years, so that suddenly the North magnetic pole will end up where the South is now, and the South where the North is. Evidence of this has even been found in fire rings in Australia showing that men have experienced this. As the rocks in the fire rings are heated, the molecules are realigned and set as they cool. If left alone you can trace the magnetic alignment as it was at the time of cooling.

 

I wonder what kind of ride such an event provides. It will certainly send a guy with a compass down the wrong path.

 

The magnetic field of the Earth is thought to be the result of the currents and rotation of the liquid iron magma in the interior of the Earth. It acts somewhat as a generator but is not well understood. So if you hear a large sloshing sound and your compass is acting funny, well... I'd hang on to something solid.

 

 

"See the wonderous works of Providence! The uncertainty of human things!" Geo.Washington

[+nincehelser]

Pole reversals, though fast on a geological time scale, will seem pretty slow to us. It's speculated that such events take thousands of years.

 

There are, however, some "theories" that Earth's rotational pole will shift dramtically (on the order of hours or days), or at least the crust of the planet is going to shift as quickly, causing things such as polar regions suddenly turning tropical. This is supposedly to be caused by the weight of the ice caps and/or the alignment of planets.

 

Just one more thing to worry about...

 

George

[+Don&Betty]

I recommend you set your GPS to indicate references to magetic north when it indicates in which direction the cache is, for instance. Then you merely set your orienteering compass to that heading (or otherwise look toward that compass heading) and the compass (plate or heading) points right toward the cache.

 

If you had the GPS set for true north reference, you would have to first add/subtract the local declination to obtain the magnetic heading you would look at your compass to eyeball the direction to.

 

In case you ever do need the local declination in your area, you can measure it yourself by finding on a topo map some road which runs precisely N-S (E-W would do too) then go there and see what your compass says. Any difference it shows from N-S (or E-W) is the declination.

[+EraSeek]

 

"See the wonderous works of Providence! The uncertainty of human things!" Geo.Washington

[+EraSeek]

I've been reading some on this facinating topic. The Magnetic North Pole trends on a NW path at a rate of 40 kilometers a year. If you stand on top of it your compass needle will want to point straight down at your feet. It has a daliy elliptical path which travels about 85 kilometers, as shown in this graphic:

 

 

"See the wonderous works of Providence! The uncertainty of human things!" Geo.Washington

[psyopwak]

looks like you got alot of good answers.

here is another

 

I have a book, called "Staying Found

The complete map and compass book" by June Fleming. I highly reccomend it

 

Also if you get your hands on FM 21-26 (Map Reading and Land Navigation) This is the bible of the army on all you should need to know for advanced knowledge.

 

neither one covers GPS too much

 

psyopwak

 

Posts: 6 |  Registered: April 12, 2003

[+Searching_ut]

This thread has me a bit worried now. What happens if I wake up on a backpacking trip, only to find out that North isn't north anymore. That could turn my whole world upside down.

 

As for how I set up my GPS units. I generally keep everything magnetic. This way, in those tough reception areas where you have to seek out a spot where your receiver will work, you can quickly sight your compass to the number displayed by the GPS and off you go. This is extremely handy if you use a sighting compass like the suunto vista.