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GPS data vs Compass Declination
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peter vacco

Locale: no. california
GPS data vs Compass Declination on 12/14/2013 20:50:48 MST Print View

ah.. just researched as to why my older and still apparently perfectly functioning garmin geko was giving me odd declination info.
it seems (Bob's op pending on this) that compass corrections come not from space (and how could they? ) but from old and immediately obsolete data and an elaborate formula stuffed into your gps when you bought it.
thusly, my vintage geko was running on 2005 data (at best), while my new (and better correction number giving) foretrex 401 is using data hopefully as new as 2010.
since i wander pretty far north sometimes, and just walking you can cross an entire degree of correction every few days, the declination amount looms larger than down south.
note : you can access your compass correction info by instructing the garmin to be in mag-north mode, and then it will show you the correction it's not using. but it won't cough it up on true-north mode.

moral : if concerned about exact corrective angles, sit on a spot that is Absolutely good and shown on the map solid. take a reading on another point also shown solid on your map, and calculate the angle off of that. you can normally enter this known exact number into the gps (as i understand them).

good luck,

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: GPS data vs Compass Declination on 12/14/2013 21:09:41 MST Print View

"moral : if concerned about exact corrective angles, sit on a spot that is Absolutely good and shown on the map solid. take a reading on another point also shown solid on your map"

That is what I do with my compass, since it doesn't have a declination feature. If needed, I can figure out the declination but rarely need to.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: GPS data vs Compass Declination on 12/14/2013 21:11:05 MST Print View

When a GPS receiver displays compass bearings based on North, those bearings are calculated approximations based on where Magnetic North used to be back when the receiver was new and its database was new. Yes, the magnetic declination drifts by a small amount. Normally that amount is small enough to be insignificant, except if you let the drift accumulate for a number of years (e.g. ten). It would be nice if the GPS receiver manufacturer would allow the receiver to be updated by the user, but I have never seen one that did. Instead, they allow the receiver to appear to be funky so that the user will go purchase a new one.

That is correct, a receiver gets no compass information from the satellites about this, never did, and never will. It is almost impossible for them to tell you which direction you are facing from a standing point, and it can only determine your direction of route once you start to move. The exception to this is if there is an electronic compass built into the receiver, but it has a different set of problems as well.

All a GPS receiver can determine, based on the satellites, are three things: position, velocity, and time. Some people call that a PVT solution. Some call it a fix.


Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: GPS data vs Compass Declination on 12/14/2013 21:21:53 MST Print View

"my vintage geko was running on 2005 data"

My oldest receiver is even older than that, 1998.

But, if you think about it, you can mentally think of where the Magnetic North Pole is and where it is moving, so you can mentally think of which way the error is drifting, and you can think of which way the error will force your compass reading.

I would think about this more if I were in far northern Canada.


Billy Ray
(rosyfinch) - M

Locale: the mountains
Re: GPS data vs Compass Declination on 12/14/2013 21:36:58 MST Print View

From Wikipedia:

"The magnetic declination in a given area may (most likely will) change slowly over time, possibly as little as 2–2.5 degrees every hundred years or so, depending upon how far from the magnetic poles it is. For a location closer to the pole like Ivujivik, the declination may change by 1 degree every three years. "

Marko Botsaris
(millonas) - F - MLife

Locale: Santa Cruz Mountains, CA
Re: Re: GPS data vs Compass Declination on 12/14/2013 21:38:33 MST Print View

Good topo maps give not only the magnetic deviation on a given year, but the drift rate per year at that location. So that should get you a reasonable offset going out several decades. On the geological time scale the deviation is actually chaotic, but fortunately not an issue for us mortals.

Guess the GPS example for older units is just another case, like the people who drive their cars out into the desert, past the road closed signs and get stuck in the middle of nowhere - because google maps told'em to.

Billy Ray
(rosyfinch) - M

Locale: the mountains
Re: GPS data vs Compass Declination on 12/14/2013 21:46:27 MST Print View

Using the Internet, you can now determine magnetic declination for any place on the earth since 1900. The National Geophysical Data Center (NGDC) offers programs for calculating magnetic declination.

To view a simpler graphical reference, the U.S. Geological Survey started a National Geomagnetism Program that produces maps and guides explaining the magnetic declination and annual change. The maps are revised every five years or so to compensate for the changes in the magnetic field.

Mike W
(skopeo) - F

Locale: British Columbia
GPS data vs Compass Declination on 12/15/2013 01:17:50 MST Print View

For a long time now, Garmin has allowed you to set your own magnetic variation. I'm not sure about the Geko (that's pretty old) but you can check the setting by going into "Setup/Heading/North Reference" and set the North Reference to "User" which allows you to set your desired degrees of magnetic variation.

peter vacco

Locale: no. california
Thank You Gentlemen. on 12/15/2013 09:31:22 MST Print View

thank you for your insights. i knew something was amiss in my perfect world of stellar navigation. i mean, i hit where i'm aiming, but it's not really the laser-like precision we'd hope for.
so, being a white guy, and the sort that blames the keyboard for bad spelled words, i predictably replaced the gps unit. and that was better ... but not the embodiment of technical perfection (and Spelling ! ) for which bpl members strive.
so, it's all good now. and Thank You. you've given me the information i need to do a better job.
on replacing the old GPS :
it's not all compass correction numbers. there is an article in wikepedia titled "Failure modes of electronics" and, as i work daily with electrical/electronic equipment that arrives to my bench already broken, it rang very sound and true. i was already knocking on the door of not expecting equipment/tools/gear/systems to last indefinitely, and budgeting/purchasing accordingly, as well and including the theory in my sales presentations. (let's don't load the customer up with equipment he's not going to profit from in a timely manner)
this led to thinking that a well proven, but vintage gps, vs the very minimal cost of a new one, is probably a sound investment if one is serious about having it working. this, even though the old unit showed no documentable signs of disfunction.

all that said. the older geko has a better interface and is a Vastly more appropriate color vs the foretrex 401.

thank you all,

Robert Blean
(blean) - MLife

Locale: San Jose -- too far from Sierras
Vendor? on 12/15/2013 09:57:28 MST Print View

As a non-GPS-owner I am surprised by this. I would have expected that:

*) This day and age you could download corrections for your GPS
*) At a minimum, you could send it back to the manufacturer to be updated

Evidently I am wrong. Sounds as if the GPS vendors are stuck in the bad old days. :(