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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 Offline
Moderator

Registered: 02/05/03
Posts: 2968
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 Offline
member

Registered: 02/03/06
Posts: 6580
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: 2806
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 Offline
member

Registered: 02/03/06
Posts: 6580
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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#201830 - 09/30/18 08:14 PM Re: Altitude and compass [Re: OregonMouse]
Jim M Offline
member

Registered: 11/23/03
Posts: 294
Loc: Kitsap Peninsula, WA
Yes. I find I can get topo maps on MyTopo.com free and use them often. I "photoshop" out the unnecessary areas, figure out the scale and send a digital copy to my friends who are also going to be on the trip. Even if you have a GPS (I sometimes do take mine) with a map application; I like to locate my self on a larger map so I can see the relative position, distances and such. The GPS map is too small, even though i know I can zoom in and out. Besides you can write notes on the topo map and save them for next time. Perhaps just an old habit, but I don't need no stink'n GPS.


Edited by Jim M (09/30/18 08:16 PM)
_________________________
Jim M

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#201835 - 10/01/18 01:08 PM Re: Altitude and compass [Re: Jim M]
Lonerock Offline
member

Registered: 12/10/15
Posts: 37
Loc: Southern Oregon
I still have some of my old topo maps and yes, topography doesn't change but roads, trails and other featues do. Driving several miles to find out the access road you were going to take is now overgrown and abandoned can be a real bummer. Downloading current maps onto a device (gps,phone,tablet etc) can save a lot of time and frustration plus provides some additional safety. The national forest where I live is divided up into districts and each district has their own map with current info plus elevation contour lines. You can puchase the paper version or download the exact same map in pdf form at about the same cost through the Avenza App. Avenza also has topo maps that can be downloaded, many of which are free.

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#201838 - 10/01/18 03:02 PM Re: Altitude and compass [Re: Lonerock]
OregonMouse Offline
member

Registered: 02/03/06
Posts: 6580
Loc: Gateway to Columbia Gorge
I've never relied on topo maps as road maps. I've found that even the newer ones often label as roads those which have been abandoned or at least no longer are safe for lower-clearance vehicles. I use either Forest Service maps or (more often) the latest Benchmark atlas. A call to the appropriate USFS ranger station just before the trip is also a good idea, especially in this era of frequent forest fires and weather effects such as washouts, etc.

For example, in SW Washington, many of the USFS roads are labeled by name instead of number. Somewhere just south of the latitude of Mt. St. Helens, only names are used. Only the Benchmark Atlas has both names and numbers!

The road maps, of course, stay in my vehicle.
_________________________
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey

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#201856 - 10/03/18 02:19 PM Re: Altitude and compass [Re: OregonMouse]
Lonerock Offline
member

Registered: 12/10/15
Posts: 37
Loc: Southern Oregon
Do they have Forest Service district maps for your area or the places in SW Washington? I have friends in Washington and would like to do more hiking there.I'm not sure how many National.Forests have district maps available like they do for my area. They're really nice detailed maps with contours and current roads and landmarks info but I know that some areas, like in Northern California don't have them. I have downloaded all of the district maps for my forest plus adjacent Forest Vistors maps for national forests that don't have districts. I've also downloaded OpenStreetMaps which have fairly detailed info, including contours,for the entire state of Oregon,California,Washington and Nevada so at least I'm somewhat covered no matter where I go. For hikes in Oregon I like William Sullivans books for hiking/backpacking suggestions.

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#201857 - 10/03/18 04:07 PM Re: Altitude and compass [Re: Lonerock]
JustWalking Offline
member

Registered: 01/12/16
Posts: 193
Loc: PNW
I use Green Trails maps almost exclusively for backpacking in Washington. Not the online ones, the paper ones. Best maps I've found for Washington.

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#201860 - 10/03/18 08:22 PM Re: Altitude and compass [Re: JustWalking]
Lonerock Offline
member

Registered: 12/10/15
Posts: 37
Loc: Southern Oregon
Thanks. Maps look nice but a little spendy for the coverage I need.

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