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Magnetic north or true north?


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Magnetic bearings are the most useful when geocaching, because they will correspond with your compass! I'm not familiar with Magellan equipment, but generally, a GPS set to magnetic will automatically compensate for the local magnetic variation at it's current location, giving you accurate readings.

 

Depending on where you are in the country, magnetic north can differ from true north by as much as 25-30 degrees!

 

--

Scott Johnson (ScottJ)

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I like true north because maps and roads are generally set to true, and using my GPSr on true keeps everything consistent. My compass has a declination adjustment, so I read true on it also. Your GPSr will compensate for the area you are in if you set it to magnetic. In some areas like mine, at 13.5° E of true, it is significant.

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Interesting question, Jeff...I've not thought about it for the Plat. i tend to go with Escout's take; having the GPSr make the declination adjustment automatically keeps your map display oriented nicely when using a north orientation. If you are working with a magnetic hand compass, then Scott makes a point worth considering.

 

The Plat's electronic compass can only be oriented to GPS or Magenetic North. Since it will point to a GOTO waypoint, I leave it at magnetic, and leave my "North Orientation" to True North. In a sense, I don't care where the compass says north is...I just care in which direction my course lies. If I had to figure a bearing on a hand-held compass for that, then I'd put everything to magnetic north.

 

If that explanation is less than perfectly clear, that's because I'm not sure I understand all the issues thoroughly. But that's what I've been doing, and I haven't encountered any problems in navigating (yet).

 

Max

Often wrong but seldom in doubt

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I was using magnetic, so I'd not have to do the conversion in my head.

 

But MapSend Streets (not Streets and Destinations) does not have an option to use Magnetic. All the other Mapsend products do.

 

Now that I've stopped using Streets, I may go back to magnetic on my GPS and mapsend Topo.

 

DustyJacket

...If life was fair, a banana split would cure cancer.

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I have my GPS set to magnetic. I assume it will make adjustments for me... My regular compass (actually most that we have) have an adjustment so I can set it for the dec. in the area I'm using them. Here in Michigan it's not that much but I think it's always worth checking just to be safe. My Orienteering compass has a declination chart on the outside cover covering the entire United States so I can go anywhere and make the adjustments with fairly accurate results quickly.

 

Jon.

 

Shayl.gif

 

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The JJ&C Railroad

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While the MeriPlat has the option of setting the compass to True North or Magnetic North, it also has a section in it called "North Reference", where you can set it to

True North

Magnetic North

Mils (military) true

Mils magnetic

 

I don't use a compass in addition to the GPSr. This is very confusing, as there is no explanation in the manual or cd rom to explain these choices. I guess I will keep it on magnetic for the time being, and hope Magellan had the foresight to enable the unit to autocompensate for my location.

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In case it wasn't clear from the posts above:

 

True North - the top of the planet

 

Magnetic North - the place magnetic compasses point to (exact position moves over time, but it is not the top of the planet).

 

Maps are usually drawn referenced to true north, so if you spend a lot of time taking bearings from your GPS and plotting them on maps (or vice versa), true north is probably easier. Positions (lat/lon) should plot the same regardless of the setting (making THAT come out right requires compatible map datum settings in your GPS - a different subject).

 

Because the magnetic fields curve or 'dip' as they near magnetic north, the variation between true north and magnetic north changes based on your location. This variance or deviation is usually expressed in degrees east or west (example, 13.5 degrees East, or 13.5E for my office) Good compasses often have an adjustment you can make to correct the compass to give true readings, but this can be a pain, especially if you are traveling over a large area.

 

In an airplane, for example, everything is in magnetic, radar vectors, radio navigation aids, GPS readouts, you name it. The only real exception is wind direction in pre-flight briefings (true north), but in the air, even wind direction is given in magnetic.

 

On the ground, I keep everything in Magnetic as well. I think it is less error prone, for a variety of reasons. However, the by-the-book approach to land navigation would be to either set variance on your compass, or correct all readings mathmatically to true ("east is least...")

 

It is largely personal choice, but do know the difference and how to convert between them.

 

Good Luck,

-jjf

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quote:
Originally posted by Indiana Jeff:

While the MeriPlat has the option of setting the compass to True North or Magnetic North, it also has a section in it called "North Reference", where you can set it to

True North

Magnetic North

Mils (military) true

Mils magnetic

 

I don't use a compass in addition to the GPSr. This is very confusing, as there is no explanation in the manual or cd rom to explain these choices. I guess I will keep it on magnetic for the time being, and hope Magellan had the foresight to enable the unit to autocompensate for my location.


 

It is confusing; the manual is not entirely clear what the 4-choice option is refering to. My intepretation is this: the 4 options you list under "North Reference" are all the compass-related fields that are expressed in degrees (e.g., course, bearing, COG, etc). The compass itself has the separate setting under "Compass orientation": GPS or Magnetic North.

 

My guess here (and I suppose someone could check it out but I'm too lazy) is that if you set it to GPS, it will get its fix from changes of position derived from satellite signals, but direction will be within the context of the choice you made for North Orientation. I.e., if you told the GPS to orient to true north, and then you told the compass to go along with the GPS, it would give you a true north orientation--based, of course on satellite signals.

 

The implication (if the foregoing is accurate) is that you can tell your compass to orient to magnetic north based upon magnetic forces, or you can tell it to orient to several different things based upon GPS computations, but you *cannot* tell the compass to make a declination adjustment and orient to true north based upon a direct adjustment to a magnetic north reading.

 

I just realized a complication, though...many of the data field readouts are supposedly based upon compass readings (I think COG is an exception). My guess is that the data fields heed your North Reference setting.

 

And the other possibility is that I don't know what I'm talking about (cue the signature) I'm going to consult the yahoo meridian brainmeld.

 

Max

Often wrong but seldom in doubt

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Without getting overly technical, the 3 components of basic land navigation as we cachers/hikers may require are: GPSR, magnetic compass (adjustable for declination), and paper map. When should we use True north and when to use magnetic north.

  • GPSR only - either true or magnetic
  • Compass only - either true or magnetic
  • Map only - most maps are true north
  • GPSR plus compass - either true or magnetic but make sure both are set for same
  • GPSR plus map - true north
  • Compass plus map - true north
  • GPSR plus compass and map - true

These will eliminate the chore of remembering how to adjust for your area's magnetic declination. Of course there is always "grid north" to consider but like I said lets not get too technical.

 

Cheers, Olar

 

"You are only young once but you can stay immature forever"

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MILS and Magnetic MILS are another scale for the compass. This scale is normally used by artillery types for finer placment of ordinance. The Magnetic MILS are referenced from Magnetic North. Unless you have a need for military direction definition, I would recommend using DEGREES, paying attention to the declination in your area.

 

Personally I use True North and stay away from compasses that align with magnetic flux. Those compasses can be influenced any Iron deposits (like railroad tracks and automobile engines)

 

Thats my $0.02 (and almost worth it)

 

MtM

 

*****************************************

This Space Intentionally Left Blank

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Note to Embra,

 

Despite what others may have said, the definition of Magnetic North here in wilds of Penn’s Wood is sometimes different. The definition is “MAGNETIC NORTH IS THE DIRECTION TO THE NEAREST IRON MINE”.

 

On the serious side, there are places where the above definition is true, both in Pennsylvania and neighboring states. The magnetic compass can still be used for navigation, in most places it is good. But be prepared for surprises.

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