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Magnetic North keeps moving- erratically

Posted: Mon Feb 04, 2019 6:01 pm
by CAMERONM
Might be time to give up on that map and compass and just submit to GAIA.
Link to NY Times article:
https://nyti.ms/2DTZ7Ep

Re: Magnetic North keeps moving- erratically

Posted: Tue Feb 05, 2019 2:53 pm
by Wandering Daisy
I use maps only but do not take a compass, so it does not change my navigation method. You really do not need a compass in most of the Sierra where landmarks are pretty obvious.

Re: Magnetic North keeps moving- erratically

Posted: Tue Feb 05, 2019 3:35 pm
by longri
You need to know the declination and you should understand that what's printed on the map is what the declination was at the time the map was produced. The declination in the Sierra has changed by about 1° per decade in recent times. That means that in the last thirty years the declination in the Sierra has changed by less than the accuracy of a typical backpacking compass, even one of those fancy mirror sighting compasses. Of course it would matter somewhat more in other places... so you just look up the declination before your trip.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 9:14 am
by Wandering Daisy
I read the article to mean that declination move the last 10 years has NOT been same as the historical average- rather a dramatic increase. Declination shown on all maps become outdated. I do not see this as a problem if you simply look up the current declination and use that to set your compass. Just curious- if you download USGS map, do you get a PDF of the original paper map or do they update the declination and print that on the download?

I would think that if all you were doing were taking a sight on a feature, then using that bearing to get there, it is all relative anyway so the "real" north would not matter. Correct me if I am wrong on this.

The point is: you should check and know today's declination if you plan to use a compass to navigate.

Another caution: if you in an area with rock that has magnetism, the compass will be useless. For example, it may not be that accurate if navigating through some non-granitic rock with iron, or "crazy colorful" areas in the Sierra. I have been standing on some very magnetic outcrops where the compass needle was wildly unstable- it is quite obvious because you just cannot get a stable reading. At that point, you know your compass is useless.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 9:57 am
by bobby49
If you go north to the Lava Beds NM, the lava tubes there are particularly bad for having magnetic anomalies.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 12:59 pm
by longri
Wandering Daisy wrote:
Wed Feb 06, 2019 9:14 am
I read the article to mean that declination move the last 10 years has NOT been same as the historical average- rather a dramatic increase.

Yes, historical, as in hundreds of years. But you have to put it in context. The absolute change in the Sierra is modest in the last 30-40 years. It's small enough to be irrelevant to a backpacker.

Here are the declination values for Fresno (calculated by a model):

yeardeclinationchange/year
-----------------------
201912°46'E5'W
200913°44'E6'W
1999 14°42'E5''W
1989 15° 8' E2'W
1979 15°35'E4'W
1969 16°10'E1'W

Unless you go back to 1969 it's very unlikely you would be able to discern the difference even with careful use of a mirror sighting compass.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 2:28 pm
by Wandering Daisy
A declination change from 12 degrees 46 minutes in 2019 from 15 degrees 35 minutes in 1979 (when many of the USGS 7.5 minute maps were made) is 3 degrees 11 minutes. This IS significant! As anyone who surveys knows, a small change in degree measured as two diverging lines on a map, will put you more and more off with distance. If all you are doing is roughly lining up your map to find nearby topographic features, then probably OK. But if you are following a bearing to a distance location, you will end up in the wrong location. Current declinations are published information; it is just smart to adjust your compass to the correct declination if you are going to bother with a compass in the first place.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 3:48 pm
by longri
Wandering Daisy wrote:
Wed Feb 06, 2019 2:28 pm
A declination change from 12 degrees 46 minutes in 2019 from 15 degrees 35 minutes in 1979 (when many of the USGS 7.5 minute maps were made) is 3 degrees 11 minutes. This IS significant! As anyone who surveys knows...

Who is using professional surveying equipment when backpacking?

Typical baseplate compasses have an accuracy of ±2-3°. Mirror compasses are only slightly better. And that figure assumes you are using it in an ideal manner. There are other sources of error that are difficult to eliminate in the field. So a 3° change in declination is, at best, barely significant. For most people it's lost in the noise.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 6:00 pm
by bobby49
longri wrote:
Wed Feb 06, 2019 3:48 pm

Who is using professional surveying equipment when backpacking?
Just about nobody. For a different sport, I use a survey instrument with magnetic accuracy of about 0.5 degree and better resolution.

Re: Magnetic North keeps moving- erratically

Posted: Wed Feb 06, 2019 7:09 pm
by Wandering Daisy
I am not implying that backpackers use professional survey gear. I simply mentioned surveyors because sometimes the general public is not aware that small angle errors add up to a lot at distances. The real point is why not just KNOW what the current declination is and adjust to the current declination? Although cheaper compasses have errors of that magnitude, at least you are starting from the actual declination. Errors can be additive.