Magnetic Declination

General discussions on hiking in Oregon and the Pacific Northwest
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Koda
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Re: Magnetic Declination

Post by Koda » December 20th, 2011, 5:06 pm

Lurch wrote:1° is ~90' off at 1 mile. I usually round it up to 100' while I'm teaching just to make things easier.
thanks Lurch for the answer and good picture description. I think I do good navigating off trail because I split create route waypoints to get to my final destination. I don't think I would make it navigating to a waypoint as far away as 3 miles with all the terrain and vegetation involved.

retired jerry wrote: At some point, the magnetic North and South will swap.....
During the transition, it's not well known what will happen, maybe there'll be no magnetic field for a while.
yikes... I don't know what I would do if we lost our magnetic poles... I enjoy my GPS but would rather have the manual ability, there is something about relying on others technology 100% for my navigation that bothers me.
Lurch wrote: Now we just need to discuss map datums ;)
go for it, I have no idea how this affects my navigation... :shock:
lightweight, cheap, strong... pick 2

Lurch
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Re: Magnetic Declination

Post by Lurch » December 20th, 2011, 6:41 pm

Koda wrote:thanks Lurch for the answer and good picture description. I think I do good navigating off trail because I split create route waypoints to get to my final destination. I don't think I would make it navigating to a waypoint as far away as 3 miles with all the terrain and vegetation involved.
Yeah, I don't see ever really going 3 miles straight on a bearing, but even if you use multiple points and repeatedly error you'll end up off in the long run. The state requires SAR to demonstrate the ability to do an 8 leg compass course, through varied terrain that totals to a minimum of 1 mile. That isn't *that* hard, but it can be if your gear isn't set right!

For general navigation there are much better tricks than full on following a bearing, but if you're looking for a specific point it's still one of the best!
Koda wrote:
Lurch wrote: Now we just need to discuss map datums ;)
go for it, I have no idea how this affects my navigation... :shock:
Map datums really only come into play when you start plotting coordinates. Super layman version, the datum is the data set that makes the spherical model of the planet that the maps are laid on top off. Pretty much all USGS topo's use NAD27 as their primary datum. That would be the North American Datum - 1927.

The other dominate datum is WGS84, or the World Geodetic System - 1984 (sometimes people use NAD83, but they're essentially the same). Most modern technology, cellphones, vehicle GPS, google earth, google maps, blah blah blah, use WGS84. It makes sense.. It's newer, and more accurate to the world as a whole.

It's important to note that you can be just as accurate with the different datum for our needs. However, they are different! The exact same numbers, plotted in different datums will represent different spots on the ground. Around here, between NAD27 and WGS84 you're off by about 80meters. Which doesn't seem like a lot, but it can easily put you on the wrong side of a ridge or creek. In other words, if your GPS is set to WGS84, you cannot plot those coordinates onto a NAD27 map without converting them first. Luckily you can easily use your GPS to convert coordinates, and most people don't actually plot things in the field these days..

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Chip Down
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Re: Magnetic Declination

Post by Chip Down » September 22nd, 2019, 7:18 pm

Cripes, I feel so old. I knew it changed gradually, but I still use 22 degrees, figuring it's close enough.
I'm so damn ancient, the north pole isn't even where I remember it being! :lol:
Oh, based on Tom's post above, I couldn't possibly remember 22 deg. Presumably when I was younger I learned 22 deg from an old-timer who figured it was still good enough [edit: also possible he was from a different place, not a different time].

Some of you old timers should post and tell us stories about the days when it was 30 or greater.

Wow, just realized there's a line crossing the US where a compass points True North. I had no idea. I assumed that line was further east.

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aiwetir
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Re: Magnetic Declination

Post by aiwetir » September 22nd, 2019, 11:55 pm

If you are interested in complicating the process, if you have a really accurate GNSS receiver which usually gives you coordinates in WGS84, you will find those coordinates drift over time due to tectonic plate movement. So if you don't know the date your reference coordinates were collected, you may be off by over an inch per year here in PNW, much more in other places. If you aren't using a sub-meter accurate GNSS receiver, then you won't notice this, but I wonder what the future holds for this.

Couple plate tectonics with magnetic pole movement, and I just can't sleep anymore. :shock:

At least we're not talking about the geomagnetic north pole :lol:
- Michael

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retired jerry
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Re: Magnetic Declination

Post by retired jerry » September 23rd, 2019, 6:45 am

I'm not sleeping because of worrying about Cascadia subduction zone earthquake. Also large meteor striking earth and causing mass extinction event. :)

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