Mag or True?

December 19, 2001

ry for what must be a very repetitve question...(I'm brand new 'round here)

Do most Geocacher's have their GPS unit set to Mag or True? Does it matter? I didn't see this mentioned on the FAQ.

Also, do any caches ever have you use a GPS to get to a certain location, then require use of a compass?

Thanks, Tim

December 19, 2001

here is the most recent discussion on this topic. I believe there were others before it, try searching "true north" in the message titles.

December 19, 2001

I set my GPS for magnetic bearings, so I can use a compass to shoot the bearings.

FWIW,

CharlieP

December 20, 2001

Move to central Illinois where declination is negligible. I keep the GPS in true just because I am annoyed when I am driving on a North-South or East-West road and the GPS shows 88 or 182 degrees when it is actually 90 or 180. Just makes me crazy. Same thing happens when the speed on the GPS doesn't match the speedometer. Drives me crazy.

I happy to see I'm not the only one who "shoots" bearings. rdw

[This message has been edited by rdwatson78 (edited 20 December 2001).]

December 20, 2001

Move to central Illinois where declination is negligible. I keep the GPS in true just because I am annoyed when I am driving on a North-South or East-West road and the GPS shows 88 or 182 degrees when it is actually 90 or 180. Just makes me crazy. Same thing happens when the speed on the GPS doesn't match the speedometer. Drives me crazy.

I happy to see I'm not the only one who "shoots" bearings. rdw

[This message has been edited by rdwatson78 (edited 20 December 2001).]

December 20, 2001

Well ... other than moving to Illinois ... why complicate your life ... your compass shoots magnetic ... set the GPS to magnetic and never have to worry about wheither or not you've compensated correctly. Wheither hiking or Geocaching ... you really need the compass ... even if it's a back up for any compass you may have built into your GPS.

Just a suggestion

December 20, 2001

Well ... other than moving to Illinois ... why complicate your life ... your compass shoots magnetic ... set the GPS to magnetic and never have to worry about wheither or not you've compensated correctly. Wheither hiking or Geocaching ... you really need the compass ... even if it's a back up for any compass you may have built into your GPS.

Just a suggestion

December 21, 2001

Set it TRUE !

The GPS TRK/DTK when set to TRUE will match up with most charts and maps as wells as highways, streets.

If you have to use a compass as a back up (yuck) you can use Magnetic then and not worry about the other 99.9% of the time.

If you want/need to use a compass often, use a model that has a Mag Var correction scale and use True Compass Headings so the compass will match up to Maps and roads too.

December 21, 2001

In regards to navigating to a geo stache, True or Magnetic makes no difference at all.

It is only important when comparing the compass and GPS to one another or to a map.

MERRY CHRISTMAS !

December 21, 2001

quote:
Originally posted by garminpilot:
If you have to use a compass as a back up (yuck) you can use Magnetic then and not worry about the other 99.9% of the time.

Incidentally, I use a compass with my GPS receiver 100% of the time, and I'd hardly say I like yukky things.

quote:
If you want/need to use a compass often, use a model that has a Mag Var correction scale and use True Compass Headings so the compass will match up to Maps and roads too.

Most geocachers don't use maps. While the elite, the receiverless, and the orienteering cachers often use maps, a simple survey in these forums will show plainly that most geocachers don't use paper maps for navigation. Since they aren't plotting bearings on paper maps, there is no reason to add the extra complexity of declination adjustments.

Most people who use a compass with their receiver (from the anecdotal evidence gathered in these forums) use a cheap plastic compass like the kind you can get anywhere for $10. They "follow the arrow" on their GPS receivers. They don't use paper maps. There is no reason for them to use true north.

Now, when they get proficient in real-world navigation, and once they are at home navigating with paper maps, then they can make the step up to using true north, plotting on paper, and using declinations. In the meantime, though, they'll get along perfectly fine in magnetic.

[This message has been edited by ClayJar (edited 21 December 2001).]

December 21, 2001

I know that this question will probably start an intellectual war between some of us. So if you don't mind, keep it simple for my sake. here's the problem. as i am traveling around, there are a minimal amount of roads on my 3+. my course doesn't completely follow the roads on the unit. does it being set to mag or tru affect this?

December 21, 2001

quote:
Originally posted by madphatboy2:
as i am traveling around, there are a minimal amount of roads on my 3+. my course doesn't completely follow the roads on the unit. does it being set to mag or tru affect this?

Not one bit.

(Slightly longer answer: setting your receiver to magnetic, true, or other north reference will affect which way is "up" and the actual bearing/heading numbers, but the reason your course doesn't completely follow the roads is because of the bits of error in the position of the roads. When you're connecting dots, it doesn't matter which way you hold the paper.)

December 21, 2001

that's what i thought, but i wasn't sure,

thanks.

December 21, 2001

I'm with ClayJar on this. (Gee, what a surprise!) This is kind of a Ford vs. Chevy, Garmin vs. Magellan issue. There are folks that will prefer each method, but for the majority of cachers, magnetic is the way to go. In about 70 cache hunts, I've only used paper maps once. Even then, I probably didn't really need them, but it was nice to have the backup as this was a hike through some rough country on unmarked trails. It was simple enough to adjust for declination that one time. By the way garminpilot, if you have not tried using a compass while geocaching, you should really give it a try. Most of the time, I'd just as soon leave home without my GPS as my compass.

On another note, I highly recommend a sighting compass instead of a mapping compass. It makes picking a spot on your line to the cache SOOO much easier.

Thanks,

brokenwing

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http://www.cordianet.com/geocaching

December 21, 2001

I too would agree with Clayjar. I use a compass a lot, I also use a printed map a lot.

Most navigational tutorials encourage you to adjust for declination at the compass. Buying a higher end compass with adjustment, putting a tape arrow on the bottom, etc.

I disagree for three reasons, first, adjusting for declination is a waste of time for compass only navigation. Since it is just an extra chance to screw-up readings, I say "don't". And, since having different ways of taking readings also creates more opportunities for error - just always use Magnetic.

Second, book adjustments for declination don't mean squat, it varies widely from area to area. You really need to find the local declination by taking bearings from a clearly identifiable location. So, when you use a map, take the bearings and just draw a new northing line on the map.

Third, when it gets cold, dark, and dangerous everyone, and I mean everyone, gets stupid. You don't all want to die because Earl is holding his compass too close to the big belt buckle (with his name on it) so you hedge your bets. You seperate a bit and take individual readings, even if one is with a tiny dime store compass built into a whistle that plays 'On the way to Mand-o-lay...'

Since some compasses won't have a declination adjustment, you'd have to do math to compare readings unless everyone is using straight magnetic. Believe me, when it *really* counts, even simple addition can be very difficult and stressful. And, of course, when weather and or environment has driven you to compass dependant navigation, chances are you can no longer take bearings of geological features to plot with the map, which takes us back to point 1.

-jjf

December 23, 2001

Here is a link about Magnetic Declination changing from year to year:

http://www.thecompassstore.com/decvar.html

Magnetic Declination Chart:

Be glad you dont live in the northeast United States, because the magnetic declination is from 15° to 20° off from true north.

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

Geoff's GPS Information Page

Geoff's Information on GPS units, and price guide for several units

December 23, 2001

Geocachers,

I lurk around here a bit and feel compelled to comment on some of the things I've read here.

I make a living using Map, Compass and on occasion GPS so I will give you a little insight into how the people that do this for a living go about it.

All bearings or Azimuths are shot using True North. It is not any harder to use, if you have a decent compass (Silva Ranger is the industry standard).

I would never go anywhere into any even semi remote or anyplace you are not familiar with, without a paper copy of the map.

Do not worry about local attraction when you are using your compass. Yes it does exist, but in nearly twenty years of beating the the brush, every day, in many different states, it has not ever been a problem as far general navigation goes.

Foresters are now just starting to use the consumer GPS units for some limited uses. Namely double checking where we think we are when we have mapping errors to contend with (ie... why cant we find the road that is suppose to be here).

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December 23, 2001

Forester,

Thanks for the comments. I see your point of view and can certainly see that in your line of work you have a need to rely on paper maps. We do however, have to be honest about a few things. In the sport of geocaching, the large majority of caches are not in even vaguely remote areas. Most are within a few yards of a defined trail and the overwhelming majority are in heavily traveled areas. (City and county parks, for example.) Due to this, the need for paper maps when geocaching is pretty limited. As a geocacher, what I do use on just about every hunt is my GPS and my compass. From my point of view, it would be silly to set the GPS to true and have to adjust for declination every time I hunt a cache on the off chance that some day I might want to orient myself to a paper topo map. Remember too, that topo maps can be displayed on a mapping GPSr. With this, the need for a paper map, and the subsequent need to orient myself to it, is virtually eliminated. Overall, setting my GPSr to magnetic just makes my life simpler. Simpler is good.

Thanks,

brokenwing

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http://www.cordianet.com/geocaching

December 23, 2001

r is good.

HOW TRUE!

Not to mention, it's not like the true/magnetic choice is set in stone (or burned in ROM). If you DO someday have the need to follow true north off a map, it takes all of about 10 seconds to switch the gps from magnetic to true.

December 24, 2001

I'm a fairly new geocacher, but an experienced hiker / navigator (airplanes worldwide). So why don't geocachers use maps?? It seemed obvious to me you would need / want one. I just downloaded the waypoints from GeoCache.com to EasyGPS, then to my GPS, then back to my TopoQuad software. Print maps of where you want to go, courses, etc. showing caches, ALL IN TRUE!

OK, if the GPS breaks, then you need to use the compass, and compensate. That's the only reason. Unless you WANT to make it harder!! LOL!!

Klemmer

December 24, 2001

quote:
Originally posted by Klemmer:
...I just downloaded the waypoints from GeoCache.com to EasyGPS, then to my GPS, then back to my TopoQuad software. Print maps of where you want to go, courses, etc. showing caches, ALL IN TRUE!

Klemmer

That sounds great! What is Topo Quad? I did a brief search at Google, but couldn't find anything significant. How can I get this? Thanks, Tim

December 24, 2001

quote:
Originally posted by Klemmer:
So why don't geocachers use maps?? It seemed obvious to me you would need / want one.

Rather than just say "because", let me give you a narrative description of the average cache hunt around my area. (Incidentally, Louisiana's caches average a 1.7/1.8 on geocaching's star scale.)

quote:
Okay, I park and look at my receiver. It says the cache is 0.15 miles bearing 221 degrees. I look that way. Oh, my! It's a trail. I get out and start walking down the trail.

0.11 miles... 513 feet... 307 feet... 254 feet... 129 feet... 85 feet... 73 feet (and by my compass, off the right side of the trail)... 82 feet...

Okay, so I hop off the right side of the trail and walk about 50 feet along the bearing my receiver indicated. I take another bearing... 23 feet over around 10 o' clock. I walk about that far, check to be sure my receiver is content with the spot, and start looking.

Is it under that log? No. How about by that big rock? Aw, nope. Maybe they hid it up in that tree? Not a chance. Aw, man! It's got to be between those rocks! YES! Okay, now let me trade and log. There, done.

Hmm... I just came in on this bearing, so let me backtrack back to the trail (which I can see right over there, but I marked a waypoint when I left the trail anyway). Oh, good, I'm back at the trail. Now I guess I'll walk back to the car.

Walking... walking... walking... Ah, yes, the parking lot. Wow! Isn't my car just that cool! [Ed. Note: ] Okay, let me put my backpack in the back. Okay, now I'll hook my receiver back to its umbilical. Seat belt, key, and on the road again.

Now, if you can tell me what part of that I need a map for, I'll give you a Nobel Prize (well, I guess I could e-mail you a picture of one from the encyclopedia or something).

While I may bring a map for the really long hikes, and I'd definitely print something out for cross-country hiking, for the vast majority of geocaching around my area, using maps is cumbersome and pointless. (Not to mention, if the hike only takes up an inch or less of a 1:24000 USGS topo quad, how useful is that map going to be to you anyway?)

GPS@(mynick).com

[This message has been edited by ClayJar (edited 24 December 2001).]

December 24, 2001

quote:
Originally posted by Klemmer:
So why don't geocachers use maps?? It seemed obvious to me you would need / want one.

Rather than just say "because", let me give you a narrative description of the average cache hunt around my area. (Incidentally, Louisiana's caches average a 1.7/1.8 on geocaching's star scale.)

quote:
Okay, I park and look at my receiver. It says the cache is 0.15 miles bearing 221 degrees. I look that way. Oh, my! It's a trail. I get out and start walking down the trail.

0.11 miles... 513 feet... 307 feet... 254 feet... 129 feet... 85 feet... 73 feet (and by my compass, off the right side of the trail)... 82 feet...

Okay, so I hop off the right side of the trail and walk about 50 feet along the bearing my receiver indicated. I take another bearing... 23 feet over around 10 o' clock. I walk about that far, check to be sure my receiver is content with the spot, and start looking.

Is it under that log? No. How about by that big rock? Aw, nope. Maybe they hid it up in that tree? Not a chance. Aw, man! It's got to be between those rocks! YES! Okay, now let me trade and log. There, done.

Hmm... I just came in on this bearing, so let me backtrack back to the trail (which I can see right over there, but I marked a waypoint when I left the trail anyway). Oh, good, I'm back at the trail. Now I guess I'll walk back to the car.

Walking... walking... walking... Ah, yes, the parking lot. Wow! Isn't my car just that cool! [Ed. Note: ] Okay, let me put my backpack in the back. Okay, now I'll hook my receiver back to its umbilical. Seat belt, key, and on the road again.

Now, if you can tell me what part of that I need a map for, I'll give you a Nobel Prize (well, I guess I could e-mail you a picture of one from the encyclopedia or something).

While I may bring a map for the really long hikes, and I'd definitely print something out for cross-country hiking, for the vast majority of geocaching around my area, using maps is cumbersome and pointless. (Not to mention, if the hike only takes up an inch or less of a 1:24000 USGS topo quad, how useful is that map going to be to you anyway?)

GPS@(mynick).com

[This message has been edited by ClayJar (edited 24 December 2001).]

December 24, 2001

Forester, I'm glad you wrote. I thought I made it clear, conventional wisdom is to address declination at each reading. I want state for the record, a person could follow the old Prince book, Freedom of the Hills, or whatever, and do well.

I certainly navigated that way for a long time. What started me thinking was a case of 'inverse correction', IE, a compass adjusted to declination in the wrong direction. I caught it from visual bearings (first looking for unexpected metal messing up my compass). I mentioned the incident to an AMGA alpine guide and he gave me a copy of an interesting article. Apparently, the NFS did an internal review after some snafus during a collaberative operation and found that field personnel were not all taking accurate bearings.

'Inverse Correction' was mentioned as a common error. Easy to see why, you loosen the little screw in a Silva Ranger and look through the back, inconsitancy in E/W terminology used in declination charts, simple human error...

Well, anyway, the NFS concluded that it was an issue of training. But I, statistical quality control enthusiast that I am, wondered if the problems cited were not somewhat inherent in the procedures. I started by experimenting with mag. north general aviation flight plans (there are two camps in that area as well). Then I tried day trips, and eventually a full on expedition. I like it, for the reasons I stated earlier.

However, I only forcefully disagree with you on one point. One need look no further than the Accidents in North American Mountaineering (published annually by the American Alpine Club) to find examples of the perils of ignoring local variation.

-jjf

December 27, 2001

Forester has it right. People that make their living navigating will use TRUE courses.

And Coyote tagged it with the TRUE orientation of digital maps that many of us use on our lap tops before we hit the trail as well as download onto our GPS receivers and PDA's

WE are eliminating the need for Mag Var corrections. That is a good thing

Clay Jar should put his wobbly compass in the pack with those cumbersome maps and use them all for back up only. If his arguement that maps are not necessary (and they are usually not)then the same goes for that error prone compass. Use the Track compass on the GPS with it's handy digital display combined with conventional compass card.. Take it from professional navigators..it's the best way to cruise to a cache.

December 27, 2001

quote:
Originally posted by garminpilot:
ClayJar should put his wobbly compass in the pack with those cumbersome maps and use them all for back up only. If his arguement that maps are not necessary (and they are usually not) then the same goes for that error prone compass. Use the Track compass on the GPS with it's handy digital display combined with conventional compass card.. Take it from professional navigators..it's the best way to cruise to a cache. [emphasis by ClayJar]

You have apparently not been listening to the discussions that have been going on in these forums for as long as they've been here. Allow me to enlighten your somewhat rude and elitist self. (Congratulations, you've ticked off ClayJar, but no worries, he won't hold it against you.)

When you stop moving, the pseudo-compass on your GPS receiver stops functioning. Only GPS receivers with a built-in electronic compass can continue to give accurate bearings when you stop moving. That rules out using the compass on the Magellan GPS 310/315/320, MAP 330/330M, and Meridian GPS/Gold/Marine; the Garmin GPS 12/II/II+/III/III+/V, eTrex Yellow/Legend/Venture/Camo, and GPSMAP 76; and many, many other receivers. While you can trust the built-in electronic compass on the Magellan Meridian Platinum and Garmin eTrex Summit/Vista, this is obviously the exception not the rule.

So, while you can readily do without maps on 500-foot cache hunts, doing without a real, live magnetic compass is indeed a real problem, and can end up wreaking having on the non-professional cachers out there.

(Okay, I'm back to my normal self now. Still, there really wasn't a need to resort to condescension. It would have been perfectly fine to ask why we even bother with a compass, and I would have gladly pointed out these very same reasons, just without having to feel a bit miffed. )

December 27, 2001

quote:
Originally posted by garminpilot:
ClayJar should put his wobbly compass in the pack with those cumbersome maps and use them all for back up only. If his arguement that maps are not necessary (and they are usually not) then the same goes for that error prone compass. Use the Track compass on the GPS with it's handy digital display combined with conventional compass card.. Take it from professional navigators..it's the best way to cruise to a cache. [emphasis by ClayJar]

You have apparently not been listening to the discussions that have been going on in these forums for as long as they've been here. Allow me to enlighten your somewhat rude and elitist self. (Congratulations, you've ticked off ClayJar, but no worries, he won't hold it against you.)

When you stop moving, the pseudo-compass on your GPS receiver stops functioning. Only GPS receivers with a built-in electronic compass can continue to give accurate bearings when you stop moving. That rules out using the compass on the Magellan GPS 310/315/320, MAP 330/330M, and Meridian GPS/Gold/Marine; the Garmin GPS 12/II/II+/III/III+/V, eTrex Yellow/Legend/Venture/Camo, and GPSMAP 76; and many, many other receivers. While you can trust the built-in electronic compass on the Magellan Meridian Platinum and Garmin eTrex Summit/Vista, this is obviously the exception not the rule.

So, while you can readily do without maps on 500-foot cache hunts, doing without a real, live magnetic compass is indeed a real problem, and can end up wreaking having on the non-professional cachers out there.

(Okay, I'm back to my normal self now. Still, there really wasn't a need to resort to condescension. It would have been perfectly fine to ask why we even bother with a compass, and I would have gladly pointed out these very same reasons, just without having to feel a bit miffed. )

December 27, 2001

Brokenwing

Your right, like most subjects there are always two camps

It seems so clear to me though!

Always choose a FORD (truck) over a Chevy

GARMIN is the only GPS

TRUE is the best way to navigate

As to your suggestion. I teach land navigation and instruct pilots in regards to air navigation using a variety of Nav instruments and systems I have used several types of compass' for many hundreds of hours and am very familiar with their advantages and disadvantages.

For Geocaching I can easily shoot a true course using the GPS compass display. Usually though I will hike along an existing trail or path and turn toward the cache at the CPA (closest point of approach) This I determine in advance using topo SW on the PC or real time on the trail using the GPS map display (with or without topo data)

As a sidew note:

Shooting straight cross country compass headings without any regards to geography (maps or familiarity with the location) one runs the risk of crossing property lines or difficult/hazardous terrain.

December 27, 2001

Of course your Track compass goes invalid when you pull up to a stop (usually near the cache) Now it's time to forget all that fancy navigation mumo jumbo and look around for that stache.. no need to shoot compass bearings for that. TREASURE HUNT !

December 27, 2001

Clayjar...

No miff intended.

I actually appreciate your passion and knowledge for navigation and geocaching.

I was just jerking on your compass lanyard.

I hope someday our compass bearings and track lines will converge with high dilution of precision and microscopic EPE an then I will treat you to a cold adult beverage of your choice.

December 27, 2001

quote:
Originally posted by garminpilot:
I was just jerking on your compass lanyard.

No problem. I can take a jerk or two... Um, maybe that didn't come out quite right.

December 27, 2001

quote:
Originally posted by garminpilot:
I was just jerking on your compass lanyard.

No problem. I can take a jerk or two... Um, maybe that didn't come out quite right.

December 27, 2001

jfitzpat,

I agree with you regarding the human error problem. The accidents in North American Mountaineering though I have a diferent take on. The single most comon excuse I get, from one of the folks new to the profession, when they wind up some where other than where they are suppose to is "Local Atraction". This is an excuse used to dodge responsibility. The reality is that they have not fully developed thier map and compass skills. Even if there is local attraction it is usually easy to identify and correct for. "So how come you didn't notice that you were on the wrong side of the ridge?"

I have heard this excuse before and there is absolutely no reason for anyone to get lost because of local attraction. They are just trying to save face and not look bad.

It is quite a believable excuse though.

December 27, 2001

ta's declination adjustment abilities?

Tks

Alan2

December 28, 2001

quote:
Originally posted by Alan2:
1. How does the software/hadware know what the declination is no matter where I am at. (I've only been in NY with a declination of 13 1/2 and PA with a 13).

It uses a mathematical model of the earth's magnetic field. Since the magnetic field does change with time, the model includes a time dependant factor. The GPS receiver manufacturers take that model and extrapolate forward in time, but eventually (a time scale of years), they will either have to release new firmware to update the model or their receivers will creep away from real magnetic north.

(So, eventually, if you use your receiver for many, mnay years after the last firmware update, you will undoubtedly be required to use true north and manually adust your declinations. Well, or you'll have a new receiver, I think.)

January 1, 2002

ClayJar, what are your thoughts on the posting of bearings or when bearings & distances are given as clues in multi-part cache hunts? Depending on location and the projected distance, the difference between TRUE and MAGNETIC can be very significant. Should the poster or cache hider be expected to specify which North Reference was used? Thanks for your insight.

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

~Rich in NEPA~

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

? A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ?

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

January 1, 2002

quote:
Originally posted by Rich in NEPA:
ClayJar, what are your thoughts on the posting of bearings or when bearings & distances are given as clues in multi-part cache hunts? Depending on location and the projected distance, the difference between TRUE and MAGNETIC can be very significant. Should the poster or cache hider be expected to specify which North Reference was used?

When a bearing is involved in a cache, the cache page should *definitely* include the north reference to use. Not indicating north reference is like not saying the bearing at all.

Note that magnetic north does move. It probably wouldn't change enough over the life of a cache to make it important, but if a cache is going to be set in bedrock somewhere with an equally permanent offset marker a long enough way away, you'll definitely want to use true north.

January 1, 2002

quote:
Originally posted by Rich in NEPA:
ClayJar, what are your thoughts on the posting of bearings or when bearings & distances are given as clues in multi-part cache hunts? Depending on location and the projected distance, the difference between TRUE and MAGNETIC can be very significant. Should the poster or cache hider be expected to specify which North Reference was used?

When a bearing is involved in a cache, the cache page should *definitely* include the north reference to use. Not indicating north reference is like not saying the bearing at all.

Note that magnetic north does move. It probably wouldn't change enough over the life of a cache to make it important, but if a cache is going to be set in bedrock somewhere with an equally permanent offset marker a long enough way away, you'll definitely want to use true north.

January 2, 2002

quote:
Originally posted by ClayJar:

Note that magnetic north does move. It probably wouldn't change enough over the life of a cache to make it important, but if a cache is going to be set in bedrock somewhere with an equally permanent offset marker a long enough way away, you'll definitely want to use true north.

ClayJar, I'm still curious, though. Does it really matter as long as the actual North reference is given? (Which I believe it should.) I would think that in most cases a compass is going to be used to take the bearing, especially if the point is line of sight to begin with.

In either case, though, if the change in declination was significant at this location, then over the course of a couple of years, a person would need to know either the rate of change, or the difference between the two values in the time period. A compass would only indicate the current magnetic north (which has now moved). With a map, it would also be a matter of how current it is. Is this right, or am I still missing something?

Here in NEPA, for example, the supposed shift in declination is approximately an increase of 1 minute per year. When projected over a short distance, say a few hundred yards, the error may not be enough to worry about, especially when compared to GPS positional errors. But, if projected over a mile or more, the error could become a problem. No?

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

~Rich in NEPA~

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

? A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ?

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

January 2, 2002

quote:
Originally posted by ClayJar:

Note that magnetic north does move. It probably wouldn't change enough over the life of a cache to make it important, but if a cache is going to be set in bedrock somewhere with an equally permanent offset marker a long enough way away, you'll definitely want to use true north.

ClayJar, I'm still curious, though. Does it really matter as long as the actual North reference is given? (Which I believe it should.) I would think that in most cases a compass is going to be used to take the bearing, especially if the point is line of sight to begin with.

In either case, though, if the change in declination was significant at this location, then over the course of a couple of years, a person would need to know either the rate of change, or the difference between the two values in the time period. A compass would only indicate the current magnetic north (which has now moved). With a map, it would also be a matter of how current it is. Is this right, or am I still missing something?

Here in NEPA, for example, the supposed shift in declination is approximately an increase of 1 minute per year. When projected over a short distance, say a few hundred yards, the error may not be enough to worry about, especially when compared to GPS positional errors. But, if projected over a mile or more, the error could become a problem. No?

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

~Rich in NEPA~

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

? A man with a GPS receiver knows where he is; a man with two GPS receivers is never sure. ?

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

January 2, 2002

ng the plane in a Compass Rose, then turn on all the radios and crap.

Although my air navigation is mostly old fashioned (VOR, etc.), my friend's oh-so-sweet T206 has a Garmin moving map in it that reads off in magnetic as well. Presumably because ATC instructions are magnetic.

Granted, the standard way to work out a flight plan is to start with true course, adjust for wind to get true heading, use the isogonic lines on the chart to get magnetic heading, then finally use the deviation on the card to get a compass heading...

But, there are some other methods. Again, as Forester noted, the principal benefit is the minimization of human error. Fewer steps can also be helpful if you need to plan a new course (say around a thunderstorm) in mid trip.

Which leads me to the second thing that surprises me about your posts. I've never been a CFI (never needed/wanted to build hours towards a commercial rating ), but I don't know a CFI or, for that matter, a seasoned pilot who has not seen his/her share of instrumentation failures. I'd bet good money that chart and compass have the lowest failure rate of any aircraft system, including the ashtray.

Likewise, I don't know an experienced backpacker or mountaineer who would wholely trust his/her navigation to a high tech device. If working forever without changing batteries and working when it is 50 degrees colder than a GPS or PC craps out isn't enough, a chart and compass are a lot lighter!

Since you are so experienced at both land and air navigation, I'm surprised that you think that high tech solutions will replace M&C any time soon. High tech is certainly convenient, but low tech mastery (and availability) would seem to be essential in the air and on foot for a long time.

-jjf

January 2, 2002

ng the plane in a Compass Rose, then turn on all the radios and crap.

Although my air navigation is mostly old fashioned (VOR, etc.), my friend's oh-so-sweet T206 has a Garmin moving map in it that reads off in magnetic as well. Presumably because ATC instructions are magnetic.

Granted, the standard way to work out a flight plan is to start with true course, adjust for wind to get true heading, use the isogonic lines on the chart to get magnetic heading, then finally use the deviation on the card to get a compass heading...

But, there are some other methods. Again, as Forester noted, the principal benefit is the minimization of human error. Fewer steps can also be helpful if you need to plan a new course (say around a thunderstorm) in mid trip.

Which leads me to the second thing that surprises me about your posts. I've never been a CFI (never needed/wanted to build hours towards a commercial rating ), but I don't know a CFI or, for that matter, a seasoned pilot who has not seen his/her share of instrumentation failures. I'd bet good money that chart and compass have the lowest failure rate of any aircraft system, including the ashtray.

Likewise, I don't know an experienced backpacker or mountaineer who would wholely trust his/her navigation to a high tech device. If working forever without changing batteries and working when it is 50 degrees colder than a GPS or PC craps out isn't enough, a chart and compass are a lot lighter!

Since you are so experienced at both land and air navigation, I'm surprised that you think that high tech solutions will replace M&C any time soon. High tech is certainly convenient, but low tech mastery (and availability) would seem to be essential in the air and on foot for a long time.

-jjf

January 2, 2002

Jfitzpat

Good questions and thoughts...

Some points to ponder

Geocaching and flying are two different activities. So it's OK to treat them differently in regards to equipment requiremnts

We do have Mag compass in aircraft. Unfortunately as you fly across country you are constantly dealing with changing Mag Var and having to do some math to figure out your actual True course on an air chart.

Most folks will use a GPS in the aircraft these days to determine True Course (a must for plotting on an air chart) If they bother to plot their course on the charts at all. If they use a compass to solve some nav variables they have to convert to TRUE HDG to calculate True Course, Winds aloft, Ground Speed, Track, Etc.

Of course we keep the Compass up and running because it is a necessary back up and it's required for most aircraft. It is however prone to many errors and is marginally reliable compared to all the other Nav systems, especially GPS.

Yes they do give flight vectors in Magnetic because it is an established method and there are still aircraft (believe it or not) that have only a compass to navigate by. ATC could give True Vectors and it would not make any difference to them. They have to deal with crosswind and will provide heading corrections until the desired TRACK ACROSS THE GROUND is achieved anyway.

The point we try to make about geocaching is that most people don't want to bother with good land navigation techniques. They don't really want to invest in a good quality compass, buy USGS topos, etc. And thats OK ! You should be able to get close enough to a cache using only the GPS to find it.

Our goal is to eliminate Magnetic variation and use one constant around the planet (TRUE NORTH) modern equipment allows us to do this easily. If we have to use the compass then let's use a mag var corrected compass so it mates up well with maps, charts, roads highways, property lines, GPS receivers, map software, etc.

Happy geocaching

Be True to yourself and your compass

January 2, 2002

Garminpilot:

I'm not even going to touch a compass being less "reliable" than other instruments. You probably meant 'precise', but despite things like northingly turning errors, and ac/dec errors, that is still another debate...

But more to the point, if we both agree that most cachers don't give a hoot about learning navigation, then it would seem magnetic is the way to go. First, if cachers rarely use maps, then declination is largely moot.

Second, when they do use maps, are they really plotting bearings like a pilot? Probably not, since they are spending most of their time on an trail. More likely they are trying to find their current position (as reported by the GPSr) on a map. The problem then isn't declination, but compatible datum, an entirely different issue.

Third, if they set either their GPSr to True, then they are much more likely to have to learn that declination exists and deal with it. Magnetic means, look at the display, and follow your dime store compass when your batteries are getting low (gee, that never happens ) and you want to save them, or the bearing indication on your GPSr is leading you in circles (I've seen threads on that).

True means, uh, how far do I swivel this compass to make it match the GPS? And, uh, which way? How many degrees does each line mean again?

Last, even if you spring for a $40 compass, learn about declination, and apply it, you may need to toggle your GPS to magnetic and back to to be able to use it as a bearing aid for the GPS. If you don't determine what declination adjustment the GPS is actually using, your book adjustment on your compass can be 2-6 degrees different.

But, most of the above has been stated before by others. You indicated then that 'pros do...' Now you indicate that the point is that cachers are not pros and don't want to be...

That seems a bit circular. I think you hit it on the head before. Some people simply view this as a compulsive issue not really open for meaningful debate.

Me, I say, pro or not, keep it simple, keep it safe, keep it fun. Beyond that, whatever spins your wire...

-jjf

[This message has been edited by jfitzpat (edited 02 January 2002).]

January 2, 2002