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#201477 - 08/01/18 08:54 AM Altitude and compass
Billy02 Offline
newbie

Registered: 05/18/18
Posts: 9
Loc: Dallas, TX, USA.
I would like to know will a compass be effected by altitude or it does not matter? thanks


Edited by aimless (08/01/18 02:56 PM)
Edit Reason: removed link to commercial site

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#201479 - 08/01/18 02:56 PM Re: Altitude and compass [Re: Billy02]
aimless Online   content
Moderator

Registered: 02/05/03
Posts: 2964
Loc: Portland, OR
elevation has no effect on a compass

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#201481 - 08/01/18 08:10 PM Re: Altitude and compass [Re: Billy02]
OregonMouse Online   content
member

Registered: 02/03/06
Posts: 6561
Loc: Gateway to Columbia Gorge
You may be confusing a compass (which points to the earth's magnetic poles) with an altimeter, which measures altitude. They are two different instruments.

A compass, because it depends on the earth's magnetic field, is not affected by anything unless there is a large power source or large mass of metal nearby.

An altimeter actually measures air pressure, which of course varies with altitude. The altimeter needs to be adjusted to actual altitude (as found on a topographic map) frequently because the weather, as well as the altitude, affects air pressure. The standard saying is, if your altimeter shows that your campsite has lost considerable elevation overnight, a storm is on the way.

The problems with air pressure for an altimeter are why most people now rely on a GPS for altitude measurement. I understand that the GPS isn't 100% accurate, either.
_________________________
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey

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#201643 - 09/03/18 12:28 PM Re: Altitude and compass [Re: OregonMouse]
wandering_daisy Offline
member

Registered: 01/11/06
Posts: 2797
Loc: California
Someone just told me this summer, that magnetic declinations are shifting quite a bit. If you have an older paper USGS map, say pre-1970's, the declination shown on the map may be incorrect. Probably best to "google" the most current declination for your trips.

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#201646 - 09/03/18 05:13 PM Re: Altitude and compass [Re: wandering_daisy]
OregonMouse Online   content
member

Registered: 02/03/06
Posts: 6561
Loc: Gateway to Columbia Gorge
Thanks for the reminder, W_D! Many of us who are older are still using older topo maps! The topographic features (other than roads and trails) may not change, and may even be less accurate on newer maps, so why buy new ones? The magnetic declination may have changed considerably. though.

Per this reference:

Quote:
The magnetic declination in a given area will change slowly over time, possibly as much as 2-25 degrees every hundred years or so, depending upon how far from the magnetic poles it is. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2800 to 5000 km below the Earth's surface) causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors. Unfortunately, the annual change corrections given on most of these maps cannot be applied reliably if the maps are more than a few years old since the secular variation also changes with time in an unpredictable manner.


It's easy enough to search online before a trip for the current declination and pencil it in on our older maps if it has changed. That should be part of our pre-trip planning routine.


Edited by OregonMouse (09/03/18 05:19 PM)
_________________________
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey

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