View Full Version : Good Article About Magnetic Declination
LostInSpace
12-09-2006, 19:08
Magnetic Declination - Frequently Asked Questions (http://www.geocities.com/magnetic_declination/)
Bookmarked for a more thorough review later.
RockyTrail
12-09-2006, 23:32
It's interesting that one of the magnetic anomaly locations he mentions is:
-Near the summit of Mt. Hale, New Hampshire (one of the 4000-footers, near the Zealand Falls hut on the Appalachian Trail) ; old AMC Guides to the White Mountains used to warn against it.
Thanks, that is an interesting read, I did not know they made compass needles counterweighted for different regions...
Gives you a greater appreciation for the ancient navigators doesn't it?
Some of the early compasses were just thin pieces of lodestone(?) floating on a cup of water.
ancient navigators and hikers are similar in that they were/are unable to steer a straight enough course to worry about declination.
In addition, little handheld compasses are only good for general direction in the woods and that's all you need.
If you're hiking cross country out West you'll surely want to set your declination or risk the chance of going over the wrong pass or walking the wrong ridge.
Even out west, declination will probably be the least of your worries, but set it anyways. Better things to worry about are your ability to follow an intended course, map reading, recognition of landmarks or topographical features, the list goes on.
Even out west, declination will probably be the least of your worries, but set it anyways.
Yeah, it's the least of my worries because I always set it! :rolleyes: Then when I orientate a map I can (hopefully) tell which peak or pass is which. ;)
The Old Fhart
12-11-2006, 00:12
The zero declination line (isogonic line) curves from about Mich. down thru the tip of Fla. That is where the declination is zero and you don't have to worry about correcting your compass, but in Maine the declination could be 21 degrees with magnetic north being west of true. On the west coast magnetic north is east of true. If you figure it out mathmatically, a 1.1 degree error in direction will put you off 100 feet in 1 mile. If the declination is 21 degrees and you don't correct, you'll be off about .4 miles for every mile you travel.
The difference between a GPS receiver and a compass is that a compass always points to magnetic north so if you aren't careful you can wander off course, still be going north, but not directly toward your destination. A GPS receiver will always point toward a marked waypoint (like your destination) so you can wander around a lake without taking backbearings or making other course corrections. A compass uses less batteries.;)
A compass uses less batteries.;)
Glad you finished your post with that tid-bit,
GPS ain't worth a f... down in the woods either.
The Old Fhart
12-11-2006, 08:48
Woodsy-"GPS ain't worth a f... down in the woods either."Actually the new generation of GPSs with the SIRF receiver chips are much, much more sensitive than the older ones. With my Garmin 60CSx I can get satellite lock inside some non-metal buildings and have never lost lock in the woods, under any conditions. Under poor conditions accuracy will suffer but I'm talking going from about 11 foot accuracy to at worse about 40 foot accuracy. This may not be the best for geocaching where you may be trying to find a film container in the woods but for finding a shelter or any hiking landmark, it can't be beat.
As an instructor in the NH chapter's mountain safety workshops showing hundreds of future trip leaders how to navigate through the woods using map and compass which they must understand, there is absolutely no comparison between the accuracy of the GPS and the compass. A GPS loaded with street routing and topo maps (I have the entire east coast loaded on the 1Gb card in mine) is impossible to beat with a compass.
The compass, as mentioned previously, has to have declination correctly set for the area you are in, the GPS can do this automatically, even though it isn't necessary because it uses the 24 satellites to tell you where you actually are, not just which direction you are heading. GPSs show your direction when you are moving by simple vector math. The better GPSs have fluxgate (real) compasses built in so you can still see which direction you're pointed as well as showing the declination for the area even when standing still. The GPS also shows the electronic 'bread crumbs' or trail of where you've been and that can be uploaded to your computer to show your exact route, time, elevation, delta distance, etc., every few minutes-a complete record of a trip. This electronic trail on the GPS screen also makes it easy to backtrack out of an area you have hiked into in much the same way as you could follow your footprints in the snow, only these don't melt or get obsured by blowing snow and work in the summer as well.
Comparing a GPS to a compass is like comparing a pencil to a computer for word processing (which is how I did my on-trail journal). The computer is far more capable and complex so it may have lower reliability. Having said all this, I’d never go into the woods without a compass for a back up to my GPS.
[quote=The Old Fhart;283267]Actually the new generation of GPSs with the SIRF receiver chips are much, much more sensitive than the older ones. With my Garmin 60CSx I can get satellite lock inside some non-metal buildings and have never lost lock in the woods, under any conditions.
Thanks for bringing me up to speed. You know what the say, In Maine, it takes 90 minutes to watch "60 Minutes" so it also takes longer to find out about latest technology. It takes even longer to be able to afford the stuff.LOL
Thanks for correcting me on this .;)
RockyTrail
12-11-2006, 12:57
The difference between a GPS receiver and a compass is that a compass always points to magnetic north so if you aren't careful you can wander off course, still be going north, but not directly toward your destination.
This is not true...a magnetic compass does not point towards magnetic north, but instead it aligns itself with the local magnetic field. The only time the compass points to magnetic north is when you are standing on the isogonic line.
An analogy: Think of a canoe in a giant whirlpool...the canoe will align and point in the direction of the local water flow, not towards the center of the pool. In the same way the compass points along the local magnetic flux lines, not towards magnetic north. The local declination figure is used to adjust for the difference between the local magnetic field and true north.
The Old Fhart
12-11-2006, 15:09
Rocky Trail-'This is not true...a magnetic compass does not point towards magnetic north, but instead it aligns itself with the local magnetic field. The only time the compass points to magnetic north is when you are standing on the isogonic line."This is not true and the first time I've heard anyone state that a compass doesn't point to magnetic north.
There is no 'isogonic line' but rather an infinite number of isogonic "lines". The specific isogonic lines where there is zero declination are the agonic lines. Note that I specifically said “The zero declination line (isogonic line) CURVES from about Mich. down thru the tip of Fla..” Isogonic lines are not straight but kind of “swirl” as they are plotted.
If you check out this site (http://www.centennialofflight.gov/essay/Dictionary/Poles/DI81.htm) It will explain it to you with two very nice charts. What it says there is this:
“A compass will point to the magnetic north pole. This is a point in northern Canada located close to 71§ N latitude, 96§ W that is about 1,300 miles (2,092 kilometers) from true north. It is where the Earth's magnetic pull is the strongest. The lines of force that lead to the magnetic pole are not straight like the lines that lead to the geographic poles but weave back and forth according to the magnetic fields in the Earth.”
Fannypack
12-11-2006, 16:50
Ole Fhart,
when can I get my GPS / compass lesson??? :sun
I hope it is before I get lost on the CDT....:mad:
-FP
Hey Fannypack, where yeah been? Are you hiking the CDT soon? I'll be in the area next spring/summer.
RockyTrail
12-11-2006, 19:50
This is not true and the first time I've heard anyone state that a compass doesn't point to magnetic north.
There is no 'isogonic line' but rather an infinite number of isogonic "lines". The specific isogonic lines where there is zero declination are the agonic lines. Note that I specifically said “The zero declination line (isogonic line) CURVES from about Mich. down thru the tip of Fla..” Isogonic lines are not straight but kind of “swirl” as they are plotted.
If you check out this site (http://www.centennialofflight.gov/essay/Dictionary/Poles/DI81.htm) It will explain it to you with two very nice charts. What it says there is this:
OF, sorry to burst your bubble, but check out the well-written 6th paragraph of the article that Lost In Space mentions above. I'll quote it here for convenience:
Do compasses point to the north magnetic pole?
Most people incorrectly believe that a compass needle points to the
north magnetic pole. But since the Earth's field is the effect of complex
convection currents in the magma, which must be described as several
dipoles, each with a different intensity and orientation, the compass
actually points to the sum of the effects of these dipoles at your
location. In other words, it aligns itself with the magnetic lines of
force.
The magnetic compass does NOT point to the magnetic pole, it simply aligns with the local magnetic lines of force. It's a subtle difference, but an important one. Look at the chart you referenced; the blue lines are not straight lines to the magnetic pole. If they were straight lines to the pole you would be correct, but they aren't. Yes, if you walk the lines with a compass you will eventually be led to the pole, but the compass will lead you around the countryside in the process and not always point "to the pole" at any given point...as you said, the lines "swirl"...the compass has no knowledge of where the pole is, it can only react to the local field it is immersed in. This is why you need to know the local declination to make the compass reading useful.
For example: In your link, look at the blue lines on the chart over the north shore of Alaska. The compass will align with the lines and point towards Greenland. However, the magnetic pole is some 60 degrees off to the right in northern Canada. It does not "point to the magnetic pole."
This is such a common misconception that even reference articles like the the one you linked to are incorrect when they state "a compass will point to the magnetic north pole."
Happy hiking:sun
The Old Fhart
12-11-2006, 21:11
Rocky Trail-"The magnetic compass does NOT point to the magnetic pole, it simply aligns with the local magnetic lines of force."And the local magnetic lines of force are lines that go from the south to the north poles. :rolleyes:
The purpose of declination is to correct for the local difference between the magnetic north and true north. If you were to look at the legend of any USGS map you will see an open triangle that tells you the difference between magnetic north, at the center of the map, and true north. The analogy you are getting sucked into is of academic interest only and of no practical use in the field. The concept and the understanding of magnetic declination and the earth being a giant magnet has worked for cartography for generations. Understanding that concept is quite simple.
A better analogy would be hiking the A.T. northbound. I’m sure you, of course, would first argue that the A.T. doesn’t go north but rather northeast. You would then also argue that the A.T. doesn’t actually go north at all because there are local areas where to get to Katahdin you are heading east or even south. Any person hiking the trail and not arguing semantics would continue to follow the white blazes north and get to Katahdin while others would be sitting around insisting that the trail doesn’t go north at all. :D
RockyTrail
12-11-2006, 23:39
And the local magnetic lines of force are lines that go from the south to the north poles. :rolleyes:
The purpose of declination is to correct for the local difference between the magnetic north and true north. If you were to look at the legend of any USGS map you will see an open triangle that tells you the difference between magnetic north, at the center of the map, and true north. The analogy you are getting sucked into is of academic interest only and of no practical use in the field. The concept and the understanding of magnetic declination and the earth being a giant magnet has worked for cartography for generations. Understanding that concept is quite simple.
A better analogy would be hiking the A.T. northbound. I’m sure you, of course, would first argue that the A.T. doesn’t go north but rather northeast. You would then also argue that the A.T. doesn’t actually go north at all because there are local areas where to get to Katahdin you are heading east or even south. Any person hiking the trail and not arguing semantics would continue to follow the white blazes north and get to Katahdin while others would be sitting around insisting that the trail doesn’t go north at all. :D
Well, go ahead and roll your eyes at me if you like OF, :rolleyes: but you don't have any business being condescending to others especially when you obviously don't understand the basic principles behind the magnetic compass (and apparently don't care to learn). As an instructor teaching mountain safety navigation to individuals I would think you should know this stuff by now. There is no "analogy to get sucked into" as you say, it's just the plain and simple truth. Actually, I thought it made an interesting topic as most people have never thought about how their lowly compass works. I've made my living as an electrical engineer for many years now and I have yet to see magnetic fields work in any other way than the way I described it to you.
What got this started was your assertion that the compass needle always points to magnetic north; it can (but rarely does) point to the specific location (currently in Canada) that is known as the magnetic north pole. The earth's magnetic flux lines vary greatly, if you don't believe me ask anyone in the Arctic regions that flies, boats, or hikes with a compass. The declination is the difference between the local magnetic field (you could call that the "apparent" magnetic north) and true north. In fact, the direction (i.e. bearing) to the actual north magnetic pole never even comes into play when using a compass for navigation. In over three decades of flying airplanes cross-country with a magnetic compass as a primary instrument I may have cursed at it a few times, but it has always been consistent.
I really didn't want to get into an argument about this, and I don't have any need to prove it to you, but before you go putting people down for trying to contribute to the conversation I suggest you go do your homework first. Start with Maxwell's equations and go from there... this stuff has been around for well over a century.
Peace, RT
The Old Fhart
12-12-2006, 07:20
Rocky Trail-“...but you don't have any business being condescending to others especially when you obviously don't understand the basic principles behind the magnetic compass (and apparently don't care to learn).”
-Boy, I’m glad that statement directed at me wasn’t condescending!:rolleyes:
-FYI, I understand exactly how a compass works.
-The USGS understands exactly how a compass works. Their maps and all the maps in the world are made with this simple understanding. Every map has the open declination triangle (you’d argue it can’t be a triangle) that clearly states “MN” for magnetic north (not Minnesota as you thought). Everyone except you seems to grasp this simple concept.
You made your understanding of how map and compass relates to declination perfectly clear when you made the erroneous claim that there was a single isogonic line instead of an infinite number, not knowing that zero declination isogonic lines are called 'agonic' lines. Do you homework, please.
Rocky Trail-“I really didn't want to get into an argument about this, and I don't have any need to prove it to you….”
-But that hasn’t stopped you from trying to do just that.:D
Rocky Trail-“before you go putting people down for trying to contribute to the conversation I suggest you go do your homework first.”
-I’ve done mine in the real world, perhaps you should join the rest of us there and stop your obscurantism.
Rocky Trail-"Start with Maxwell's equations and go from there... this stuff has been around for well over a century."
Your reasoning is all fluxed up but thanks for playing the “I’m the engineering expert” card. Maybe you can stop arguing semantics and understand what the entire world has been using and understanding for generations.
Perhaps you can come back after you've convinced all the mapmakers of the world to change all the maps to the "Rocky Trail NOT really declination method.:D "