Forum: Questions And Answers About Geocaching
Topic: Magnetic Deviation
started by: Geo G

Posted by Geo G on June 25 2002,9:42 pm
I'd like to get more information on how GPS units compensate for magnetic deviation.  I have an older model Magellan Pioneer (it still gets me there).  How good is the correction, and how much error can still be expected?  Just getting started in the hobby and I found the link to the Trimble.com site to be a great tutorial..
Posted by Hinge Thunder on June 26 2002,4:58 am
I am not quite sure what you are asking? Do you mean when you select magnetic north, as opposed to true north? The GPS probably has a simple table in it to tell it what the declination is based on the lat/lon.


Posted by Road Kill on June 26 2002,6:21 pm
The GPS is not a magnetic device (although some do have an active compass). It is nothing more than a specialized calculator. It has a reference table in it that knows where the poles are as well as where the satellites are. In the set up of the GPS you select which pole (magnetic or true north) you want the compass ring to point to, in much the same way you select which cache you want the bearing arrow to point to. It doesn't see the global magnetic field direction and make corrections, it just calculates the direction of the selected pole and turns the compass ring to point to that location.

Someone else will have to step in to describe how those units with active compasses work. It is probable that they do the compensation thing. If so precise corrections can get very complex because magnetic deviations vary from location to location. That is to say a magnetic compass doesn't precisely point to the magnetic north pole. But we're talking ±2 degrees here for the magnetic correction to the true deviation.  I hope I haven't thoroughly confused anyone, but the bottom line is don't worry about it. The active compass units will still point to the cache correctly even if your not moving.


Posted by Geo G on June 26 2002,7:35 pm
Thanks for the info..  I'm ok with setting the GPS up for magenetic vs true north, and with the deviation correction being fed in from a table.  I was just fishing around for how accurate this really is, and I think Road Kill's response set me straight on that.  Ready to go cachin'!!  Thanks guys..
Posted by barrington on June 26 2002,8:44 pm
The magnetic pole is moving continuously, so the magnetic declination from true north changes slightly every year, increasing or decreasing depending on your location.  For 2002 in Western NY it's 11* 37' W., and increasing.    Go to: < http://www.geolab.nrcan.gc.ca/geomag/e_cgrf.html#MIRP >     for a calculator to find the declination from your location.   There's also some very good tutorial pages at that site.


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