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True Vs Magnetic


N1AYS
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I'm new to geocaching so this may be a dumb question. I'm very familiar with calculating true and magnetic bearings/headings but would like to know which are commonly used by geocachers. I.E. what can be assumed if headings/bearings are not stated as true or magnetic?

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Good question. I've seen some people give barring, then say "hint, my compass is magnetic" I've also seen some that specify "true" north. Most of the time it's just follow the arrow to the coord’s you entered. No north barring needed.

 

I'm going to go out on a limb here, and say that it's probably true north if not specified because magnetic north moves, and also because true north is more common on maps that have GPS designators like a UTM grid or a Lat Long grid. Of course if it is a UTM grid, then it could be grid north, but that's just a couple degrees at most from true most places.

Edited by geckoee
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I have a rule of thumb that any geocacher who's sophisticated enough to use a compass properly, and understand declination, will specify true or magnetic when giving a compass bearing in a cache description. Conversely, if a cache page says "proceed on a bearing of 223° for 200 feet", I assume magnetic north, since most likely the person is just looking at their compass without adjusting for declination. This theory hasn't proven me wrong yet!

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Criminal, Leprechauns, I'll buy that.

 

I was thinking that bearings and distances were most likely computed from a map, and not from sighting an object and counting paces. There is still much to learn for this gecko.

Edited by geckoee
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Actually, bearing and distance can be relative to True North as well.

I did a 2 stage multi recently. The checkpoint gave a bearing and a distance TN (true north). I just used my GPS to project a waypoint for the given distance and bearing and found the cache easily.

 

When providing someone a bearing and distance, you should always mention whether its TN or MN.

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...I assume magnetic north, since most likely the person is just looking at their compass without adjusting for declination.  This theory hasn't proven me wrong yet!

Interestingly, I assume the opposite. When a cache page specifies a bearing without specifying magnetic or true north, I use true north, as that is the standard. This has always worked for me, even though we have a relatively large (15 degree) declination here.

 

To make your life even more complicated, there is actually a third north that some people use: grid north, or the apparent north from a UTM projection. Close to the edges of UTM zones, that north can differ from true north by quite a bit!

 

So I have another policy: whenever a bearing is specified without a reference, I notify the cache hider that there is a problem and ask them to specify which north they mean. I do it very nicely, and hiders have been very cooperative. I'd like it if the approvers would do this during the approval process!

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Conversely, if a cache page says "proceed on a bearing of 223° for 200 feet", I assume magnetic north, since most likely the person is just looking at their compass without adjusting for declination.  This theory hasn't proven me wrong yet!

I just did a cache where the second leg was due North but magnetic or true wasn't specified. I followed magnetic as the cache description mentioned use of a compass but my theory turned out to be wrong! Spent an hour hunting in the wrong place, reset my GPS to true and walked straight up to the cache. My lesson was if it's not specified try both. At the distance of the leg it made a difference of over 100 feet. BTW for any caches I've placed requiring a bearing I always specify and usually it's magnetic although I did place one where I specified true. I can't remember what the default setup was on my Magellan Meridian but I believe it was magnetic. It's easy to change it in the set up.

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I have my "Eclipse" set to TRUE. I use the compass fairly extensively in conjunction with the GPSr. When I set the GPSr for the reading in degrees, I can then "shoot" the bearing with my mirror compass and see exactly where that heading is. This is also great for tri-angulating into a poor reception area.

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IMO, the cache page should say True or Magnetic, because the difference can result in a lot of extra walking...

 

Check my DNF Log here. Now this one is entirely MY fault. The cacher hider wrote exactly what he should have, but me being the newbie cacher, with my brand new GPSr didn't read the dadgum manual first. LOL.

 

That little error ended up in me walking an extra 8 - 9 KM!

 

Oh well live and learn I guess, but be sure I will be verifing TN or MN in the future.

 

Scott :)

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I am in agreement with many here. My three multis involve bearings or projections and they are all in true. The declination here in So Cal is significant and I also specify in my description that the finder use true. I believe this is proper form.

 

With many compasses, you can set the declination or use a marker, and then read true. Its so much easier to keep everything consistent.

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I've done a lot of my caches using bearings and I just give a bearing. I won't say if its mag or true. They will have to figure out the east is least if they want to put it on a map, because in the field its magnetic, and true on the map.

 

To correct for declination you want the map bearing (True) and field bearing (Magnetic) to be equivalent.

Map bearing to magnetic bearing:

Map bearing - declination = Magnetic bearing.

298° - 10° = 288° |

 

Magnetic bearing to map bearing:

Magnetic bearing + declination = Map bearing

103° + 10° = 113°

 

 

Land Navigation Links and More.

http://www.princeton.edu/~oa/manual/mapcompass.shtml

http://members.impulse.net/~mlynch/land_nav.html

http://www.learn-orienteering.org/old/

http://www.edu-observatory.org/gps/gps.html

http://www.trailsillustrated.com/skills/glossary.cfm

http://topomaps.usgs.gov/

http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl (used for determining current declination)

http://education.qld.gov.au/curriculum/are...ings/coequ.html

http://www.pubplan.nau.edu/courses/alew/pl...urvey/index.htm

http://www.colorado.edu/geography/gcraft/n...coordsys_f.html

http://www.lostoutdoors.com/newmap.html

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:rolleyes:

I am disappointed in seeing the following response.

 

"I've done a lot of my caches using bearings and I just give a bearing. I won't say if its mag or true. They will have to figure out the east is least if they want to put it on a map, because in the field its magnetic, and true on the map."

 

I graduated with a degree in forestry along with working as a charter pilot and flight instructor. I understand navigation and woodsmanship but I didn't realize that "in the field its magnetic". I wonder why better quality compasses are made so they can have declination set into them. I suppose if someone was trying to make finding a cache more challenging, they might put in an instruction like "go in a northeasterly direction for a bunch of feet and you will find the next clue".

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I suppose if someone was trying to make finding a cache more challenging, they might put in an instruction like "go in a northeasterly direction for a bunch of feet and you will find the next clue".

And thats been done with this Cache.

One should learn how to use an non-adjustable compass and make it functional with a map, all it takes is a little rhyme and practice..

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I'm a newbie here. Can someone explain the difference between true and magnetic north and where you see each?

There are four norths I've come across.

 

True north is always pointing at the north pole.

 

Magnetic North always points towards the magenitic north pole which is close to the north pole but not at the north pole. Thus it is normally different from true north. Pluse if a line of magentic influece is locally tweaked for whatever reason your compas may not even point at magnetic north. However for all intents and purpses what your compas says is north is magentic north so it really dosn't matter.

 

Grid north is a survey solution to the problem of a curved earth. It should not be used for geocaching.

 

Plan north is something an architect will invent and has no relation to anything other than what makes a plan look the best. It should not be used for geocaching and quite frankly shouldn't be used by architects either but as they say "If wishes were horses..."

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I agree with N1AYS before you had a fancy GPS which holds your hand in the woods there was navigation with map and compass. Ninety nine percent of the time maps are printed using true north I think only once I have come accross a map that was adjusted to magnetic. They are very rare.

 

One of the very first things you should learn when using a compass is how to adjust for declination. Sure if you are sighting in a feature and you want to walk to it the bearing would be magnetic but I really don't get why it would be useful to do that just walk to the ridge or feature you are looking at. If you are sighting in features so that you can figure out where you are on a map using triangulation you would need to adjust your bearing to true north so that it could be plotted on the map.

 

Sorry Lep I think I have to disagree with you I don't think the cache reviewer should dictate that the owner needs to specify true or magnetic. Why not just hold the cachers hand all the way to the cache? I thought the point of this game was the challenge of finding the cache and figuring out where it is hidden. It should be left up to the cache owner.

 

Magnetic Declination

Edited by IMLost
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If you use one of the inexpensive clear compasses, its real simple. If it has the declination adjustment, hold the outer ring, and move the inner part until the inner red alignment arrow is pointing at the declination on the outer ring. (here it is 13.5 E of north).

 

For the other compasses that do not have the adjustable, they usually have a declination scale printed in red. Turn the compass over and draw a black line with a marker at the declination. When you line up the red needle over this mark, you can read bearings in true. If you travel far from your home area, maybe use something other than a permanent marker.

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I didn’t realize that I would start such an opinionated discussion when I originally asked my question. Thanks to everyone who have posted such thoughtful answers. I have concluded that the answer to my question is, there isn’t any standard use of true or magnetic directions in geocaching. For those on the east or west coast who are not going to specify T or M and where the variation is 15°, remember that the difference between two points, one on a true heading and one on a magnetic heading, each one mile from the starting point or apex of the angle is 1382 feet or a little over a quarter of a mile.

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I agree with N1AYS before you had a fancy GPS which holds your hand in the woods there was navigation with map and compass. Ninety nine percent of the time maps are printed using true north I think only once I have come accross a map that was adjusted to magnetic. They are very rare.

 

One of the very first things you should learn when using a compass is how to adjust for declination. Sure if you are sighting in a feature and you want to walk to it the bearing would be magnetic but I really don't get why it would be useful to do that just walk to the ridge or feature you are looking at. If you are sighting in features so that you can figure out where you are on a map using triangulation you would need to adjust your bearing to true north so that it could be plotted on the map.

 

Sorry Lep I think I have to disagree with you I don't think the cache reviewer should dictate that the owner needs to specify true or magnetic. Why not just hold the cachers hand all the way to the cache? I thought the point of this game was the challenge of finding the cache and figuring out where it is hidden. It should be left up to the cache owner.

 

Magnetic Declination

If the finding of the cache involves the use of map and compass, they true north would be appropriate. I know of many caches that involve the use of a compass, and NONE that involve the use of a map. It is my preference to use magnetic north in my caches, and I annotate the cache page accordingly.

 

That's what it really boils down to, telling the finder which to use.

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...For those on the east or west coast who are not going to specify T or M and where the variation is 15°, remember that the difference between two points, one on a true heading and one on a magnetic heading, each one mile from the starting point or apex of the angle is 1382 feet or a little over a quarter of a mile.

More important than projecting waypoints a mile is that even a projection of 200 feet, a more likely one for caching, will take you 50 feet off at 15 degrees error in declination. Add 50 to the EPE and they'll have to send to searchers in there to recover you before you find the cache. :P

 

Alan

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Just did a simple cache with a projected location. Trial and error determined the cache was at a magnetic north projection. I wish this was specified on the cache page. I was running late to pick-up my son and trying to squeeze in a cache :lol: When playing games, please make the rules available to all players. Specify true or magnetic and I'll find the dang thing.

It sounds trite, but we were taught to navigate according to true north, as most maps are laid out that way. And navigation means getting where you want to go.

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