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> True Or Magnetic North, What should I use?
Ormley Gumfudgeon
post Sep 15 2004, 06:31 PM
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When posting the loc of a cache, do most people use True North or Magnetic North as their reference, or does it make a difference? What should I use when hunting or placing a cache?

Thanks for your time. unsure.gif

Ormley
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Prime Suspect
post Sep 15 2004, 06:58 PM
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Magnetic is easier, if you're working with a compass.
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strikeforce1
post Sep 15 2004, 07:02 PM
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I have my two gpsr's set to "true north". I guess which ever you choose, is up to you. I've used both settings, and haven't had any problem finding caches. SF1
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KG7JE
post Sep 15 2004, 08:16 PM
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If posting directions be sure to note whether you are giving it as a Magnetic or True direction. In some parts of the county it makes no difference. In other parts (where I live) it is about 19.5 degrees different.

Be sure you know how to convert from one to the other for the location in question.
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Tahosa and Sons
post Sep 15 2004, 08:18 PM
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Some swear by true, and others want mag. I'll never say on my caches what it is, you'll have to figure it out.

Mag is easier, learning how to work with the declination is the hard part for some.
And there is a difference between a field bearing and a map bearing when you work with mag, And there is no difference when you work with True and an adjustable compass.
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Thot
post Sep 15 2004, 08:42 PM
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Just in case it wasn’t clear from the replies. True and Magnetic north have to do with compass readings. Latitudes and longitudes have nothing to do with compass readings. So, whether you set your GPSr to True or Magnetic north makes no difference whatever in finding a cache using Lat/Lon.

It does affect the bearing (and heading) reading the GPSr gives, so if you intend to use the device in combination with a compass it makes a difference. As Prime Suspect said, magnetic is easier to use with a compass. That’s because the compass points to magnetic north -- not true north. So, the answer is choose the magnetic option unless you intend to use the device in combination with maps while wandering around in the field on a hunt, or want to learn a lot of unnecessary stuff about declination from true north.

This post has been edited by Thot: Sep 15 2004, 09:12 PM
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Major Catastrophe
post Sep 15 2004, 09:11 PM
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You can use whichever you prefer. As someone mentioned, magnetic may be easier if your compass is not adjustable for declination.

However, when you tell others a bearing, it has to be assumed to be referenced to true north unless you specify that it is a magnetic bearing...
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Thot
post Sep 15 2004, 09:23 PM
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QUOTE (Major Catastrophe @ Sep 15 2004, 09:11 PM)
As someone mentioned, magnetic may be easier if your compass is not adjustable for declination.

Assuming your compass has correction marks; if you’re in an unfamiliar location how do you know the declination so you can correct for it?

I know how to look it up on the internet, etc. I mean when you're on your own in Timbuktu.

This post has been edited by Thot: Sep 16 2004, 05:51 AM
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Ormley Gumfudgeon
post Sep 16 2004, 08:03 PM
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Thanks for the info. That was JUST what I was looking for-when & why to use each. Plus I never thought about magnetic vs. true not being relevant to Lattitude & Longitude readings. huh.gif

Very informative.

Thanks again, smile.gif

O. G.
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KG7JE
post Sep 16 2004, 08:53 PM
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QUOTE
if you’re in an unfamiliar location how do you know the declination so you can correct for it?


That's why you purchase aeronatical charts for the area into which you will be going. The declination is marked on each chart with a line showing roughly the locus of points that should exactly match. However, there is usually a note at the bottom stating that it is changing at such-and-such a rate per year. Why they post that note I don't understand since the charts are "not current" 6 months after publishing (provided that the new chart is published on schedule).

But then, if you don't fly your own plane, then you may not want all the other information shown on the chart. And it isn't really set up for back county navigation -- hence the title "aeronatical".

The other way is to keep track of the lat/long for magnetic north. Then do spherical trig to calculate the declination based on your current GPS suppllied lat/long.

Of course, if your GPS will show both True and Magnetic then switch between and note the difference in bearing to the destination.

This post has been edited by KG7JE: Sep 16 2004, 08:53 PM
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Renegade Knight
post Sep 16 2004, 09:02 PM
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QUOTE (Ormley Gumfudgeon @ Sep 16 2004, 09:03 PM)
Thanks for the info. That was JUST what I was looking for-when & why to use each. Plus I never thought about magnetic vs. true not being relevant to Lattitude & Longitude readings. huh.gif

Very informative.

Thanks again, smile.gif

O. G.

Ongitudes convirge on True North. But LL are clearly defined and all you ever work with (in geoaching) is where you are (Your present LL) and where you want ot be (your waypoint) and the arrow points towards the waypoint from where you are.
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Thot
post Sep 16 2004, 09:05 PM
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QUOTE (KG7JE @ Sep 16 2004, 08:53 PM)
QUOTE
if you’re in an unfamiliar location how do you know the declination so you can correct for it?

Of course, if your GPS will show both True and Magnetic then switch between and note the difference in bearing to the destination.

Now why didn't I think of that? I did wonder why GPSrs don't have a feature that gives it since they obviously know it and many people use compasses in conjunction with there GPSr.
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Renegade Knight
post Sep 16 2004, 09:09 PM
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QUOTE (Thot @ Sep 16 2004, 10:05 PM)
QUOTE (KG7JE @ Sep 16 2004, 08:53 PM)
QUOTE
if you’re in an unfamiliar location how do you know the declination so you can correct for it?

Of course, if your GPS will show both True and Magnetic then switch between and note the difference in bearing to the destination.

Now why didn't I think of that? I did wonder why GPSrs don't have a feature that gives it since they obviously know it and many people use compasses in conjunction with there GPSr.

Your bearing has to be measured from something. That something is "North" by custom, be it True, Grid, or Magnetic. It doesn't change that the arrow will point at the cache. wink.gif

Oh and my personal favorite north "Plan North" which is what an Architect will use to adjust their plans to an orientation that they like completly ignoring that some poor surveyor will have to figure out just what in the heck they did later.

This post has been edited by Renegade Knight: Sep 16 2004, 09:09 PM
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WeightMan
post Sep 16 2004, 10:26 PM
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QUOTE (Thot @ Sep 16 2004, 10:05 PM)
QUOTE (KG7JE @ Sep 16 2004, 08:53 PM)
QUOTE
if you’re in an unfamiliar location how do you know the declination so you can correct for it?

Of course, if your GPS will show both True and Magnetic then switch between and note the difference in bearing to the destination.

Now why didn't I think of that? I did wonder why GPSrs don't have a feature that gives it since they obviously know it and many people use compasses in conjunction with there GPSr.

I don't know about others, but my old eTrex Yellow and my new 60c will show the declination when I set it to magnetic.
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=BB=
post Sep 17 2004, 07:37 PM
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This is silly.

When someone posts a bearing, it is assumed to be true north (accounting for magnetic variation [declination]). When you need to make a projection on your GPSr for a cache, you should ensure you are using the proper reference (ie. true north vs magnetic).

This is a VERY important topic in geocaching as using the wrong reference can throw you off by an increasingly large value as distance to the projection increases.

That said, anybody who does not understand magnetic variation should take the time to familiarize themselves with it - it could save your life one day!
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Major Catastrophe
post Sep 18 2004, 09:52 PM
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QUOTE (Thot @ Sep 15 2004, 09:23 PM)
QUOTE (Major Catastrophe @ Sep 15 2004, 09:11 PM)
As someone mentioned, magnetic may be easier if your compass is not adjustable for declination.

Assuming your compass has correction marks; if you’re in an unfamiliar location how do you know the declination so you can correct for it?

You read it off the map. USGS quads have a spot in the legend that gives the declination for that map as of some particular date.

Someone mentioned using aeronautical maps and those are probably the most accurate. In some parts of the world, declination changes rapidly as you move about, so this can be important.
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Alan2
post Sep 19 2004, 07:52 PM
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QUOTE
When someone posts a bearing, it is assumed to be true north


I don't think you can assume that with cachers. Maybe with hikers who use maps all the time.

Many cachers use magnetic compasses and search using magnetic north never thinking when they post, they are giving mangetic reading. I'd be more comfortable if the poster includes whether its north or magnetic just to be sure..
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Thot
post Sep 19 2004, 08:08 PM
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QUOTE (Alan2 @ Sep 19 2004, 07:52 PM)
QUOTE
When someone posts a bearing, it is assumed to be true north


I don't think you can assume that with cachers.

I think you're right.
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FeralBoy
post Jan 23 2008, 10:48 AM
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There is a NOAA web page that will give your magnetic declination
when you enter your latitude/longitude, and also for a particular
date (the default is today):

http://www.ngdc.noaa.gov/seg/geomag/jsp/st...calcDeclination

(It also has a zip code search!)

The only format that seems to work there is degrees/minutes/seconds,
separated by blanks. This converter will help:

http://boulter.com/gps/



Feral Boy

This post has been edited by FeralBoy: Jan 23 2008, 10:52 AM
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Thot
post Jan 23 2008, 03:41 PM
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No that's what I'd call a delayed response. biggrin.gif IPB Image
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