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Gps Setup Question


cachinfools

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As a neophyte, I do not know whether to set my GPS (Garmin ETrek Vista) to magnetic north or some other available setting. I have an 11 degree compass deviation in this area.

 

Are the coordinates of the cache Magnetic or otherwise?

 

(I hate being the "new kid on the block")

 

Thanks , Jim

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Hi & welcome to the forum :mad:

 

Coordinates themselves have no magnetic influence. Regarding which setting for your GPSr, it depends on how you want to use it. If you want to use a mechanical compass alongside the GPSr while searching caches (recommended) then use Magnetic North. If you want to use your GPSr with paper maps then True north is a better choice.

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Personally I would suggest Magnetic North.

 

There are several ways to locate a cache, but two of the most common are as follows:

Method 1) Keep walking towards the cache site, until the distance reads zero. Unfortunately 99% of the times just as you get to, or approach, zero your GPSr suddenly says the site is 30 feet in another direction. If you're in a wooded area you soon find yourself doing the dance of a drunken duck as you run towards zero in a dozen different locations. All the while cursing your #@##@% GPSr.

Method 2) Walk towards the cache site and stop when you're about 50-60 feet away. Now read and remember the Magnetic North bearing to the site. Put away the GPSr, get out your compass (which you should ALWAYS have with you) and project a path to the site along this bearing. Finally walk this bearing the indicated distance to the site. If it's not there, remember this spot and repeat this procedure from another location.

 

With all that said, this is why I recommend setting it to Magnetic North for geocaching. If there's a practical reason to support the setting of True North for geocaching I'd like to hear it.

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With all that said, this is why I recommend setting it to Magnetic North for geocaching.  If there's a practical reason to support the setting of True North for geocaching I'd like to hear it.

Working with maps is a practical reason to use true north.

 

In any event most compasses actually intended for navigational use (as opposed to the toy types) have a means of adjusting for magnetic declination and this results in the compass showing true north. Even without such a feature if one knows how many degrees they are off from true north it isn't difficult to do the math in one's head.

 

Ultimately, for the purpose of geocaching, it really doesn't make any difference and is purely a personal preference matter.

 

Quite honestly I couldn't tell you offhand how my GPSr is set for north. It just isn't important 99.9% of the time I use my GPSr for something.

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Over the short distances usually involved in a cache search, it really shouldn't make that much difference. The USGS publishes a chart showing the magnetic declination (difference between true and magnetic). The farther north (or south) you are the greater the difference will be. I don't worry about it until my offset gets more than half a mile away.

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It doesnt' matter until you use some function that needs it. Latitude and Longitude are based on a grid that uses true north. However for geocaching your Pointer points at the cache coordinate.

 

In 870 caches I've never used a function once that needed True vs. Magnetic north. The one time I did try to use my GPS as a compas I went out and bought a real one and it works so much better.

 

Somewhere around 800 I looked at what my GPS was set at and I think it was true north. But not I've forgotten.

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My two pesos

 

1. If you are using your GPSr to get you within 50 feet or so of a cache then using your compass to finish up, the degree of declination isn't going to make any difference. I'm not good with the math but an 11 degree difference over a 50 foot path is going to put you off by, what, maybe a foot or so. Since your GPSr is probably at best accurate to about 15 to 20 feet you're going to be searching for the cache anyway. Now on the other hand, if you are going several miles then you better know what you are dealing with.

 

2. I'm no expert, but I don't think how far north or south you are has a whole lot to do with declination, short of being on the Arctic Circle. I have seen a wide variation in declination within a degree or two of the same latitude.

 

Yours aye,

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This is a question that will never die. But if you have to ask which setting you need, you don't need to worry about it. But I would like to address some "information" posted in this thread.

 

Spencersb Posted on May 10 2005, 11:39 AM

The farther north (or south) you are the greater the difference will be.
There are NO definitive rules. There can only be LOCAL rules, that do not apply to most people in a widely dispersed audience.

 

Here is a Year 2000 World Magnetic Declination Flux Line Chart

 

theprospectors Posted on May 10 2005, 12:15 PM

I'm not good with the math

Amen to that Brother.

 

At a 50' Radius, a Circle has a Circumference of about 314' 1.91" making each degree of rotation about .87266' of arc on the Circumference. Thus .87266' times 11° = about 9.6'

 

And don't forget that Magnetic Declination also shifts over time. If you are using a Magnetic Compass with a 30 year old map, you must take that into consideration and not just rely on the Magnetic Declination indicated on the map.

 

Here is a Geographic Magnetic Calculator you can download (and use off line).

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At a 50' Radius, a Circle has a Circumference of about 314' 1.91" making each degree of rotation about .87266' of arc on the Circumference. Thus .87266' times 11° = about 9.6'

 

Thanks Cardinal Red. One thing I've learned in my short time on this forum is that there are alot of really talented people out there and I knew somebody could come up with the answer.

 

So since magnetic declination only operates in one direction ( + or - ). If you start at point A and walk 50 feet using your compass, the MOST you would expect to be off is 9.6'. That would put you close enough to find almost any cache I would think.

 

Now here is another thought I had. If say, just for discussion, your GPSr was showing accuracy of 22', you could be standing anywhere within a 44' diameter circle relative to your true location. If you then took your compass out and started to walk toward where you think the cache should be, allowing for those 11 degrees of declination, does that mean the cache would lie somewhere within a circle 53.6 feet wide (44 plus 9.6) ?

:D

I think I'm getting a head ache.

 

Yours Aye.

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You should know how your GPSr is set- True or Magnetic. For caches that have projections, or bearings to do, you will have to know what the hider is using and what your GPSr is set to. Many multi-caches require you bearings to the next waypoint. 3 of the caches I hid, and several caches I have found require you to know which reference you are using.

 

Your manual compass should be of the minimal quality that has at least the declination marks. You can put a black mark on the under side of the clear compass at the declination point, and put the needle over that to read true north on your compass (the azimuth ring.) Its that simple.

 

I recommend and use True North for my GPSr and manual compass. Its easier to use True north for everything (even with the 13.5E here.)

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Spencersb Posted on May 10 2005, 11:39 AM
The farther north (or south) you are the greater the difference will be.
There are NO definitive rules. There can only be LOCAL rules, that do not apply to most people in a widely dispersed audience.

 

Okay, so I should know better than to believe everything I read! I got that from some egghead's thesis at Cal-Berkley! :D

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If there's a practical reason to support the setting of True North for geocaching I'd like to hear it.

I use True North in Urban Settings as most streets run True North/South

 

I use Magnetic when in woods and when kayaking - Then the GPS'r and Compass agree.

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From what I can see from the previous posts, everybody has come close to proper use of a compass, but not quite there. To fully utilize your compass, you must understand declination, sometimes refered to as variation. Is it east or west?

The amount of declination will change every year, as the magnetic pole is not stationary. If you purchase a high quality compass and learn to use it properly you will find it to be quite accurate when used in conjunction with a paper map. Contact a local orienteering club, Outward bound or the Boy Scouts for instruction on compass use. It is possible to use a paper map regardless of wether your compass is adjusted to True or Magnetic, just ensure your gpsr is the same.

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I carry two compasses at all times, one is adjustable and the other does it the old fashion way, sticks to your refrigerator thingy. Both my GPS and compass are set to mag. I teach my students when I teach land navigation to learn how to adjust for declination using their noodles. Map bearing is True and field bearing is magnetic and learn how to convert. Even though they have adjustable compasses they have to learn the old way, just in case the adjustable compass gets lost or broken. One important thing to know about declination is what is the current value, since most maps are out of date and that Mag Pole has moved. Their is a link for that solution and it pays to be prepared before you go. Now that Nat Geo has some good software for maps one can make an updated map to go with them.

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I keep my Vista on TN so it matches any paper maps (quad topo, etc) that I might have with me.

 

I'm also using a Pocket PC to autonavigate on the road and as a supplememntal navigation system with its own compass rose and arrows and direction (Mapopolis). You reallly have to keep the Vista on TN because Mapopolis can only be used on TN. I want both the PPC and Vista to match.

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Compass rules from my pre GPS days using a Chart and a compass

 

True = True North on chart

Variation = from True North varies by location

Magnetic =T =/- V

Deviation = caused by local metal varies by heading

Compass =course to steer

 

Moving down the list subtract east add west

Moving up the list add east subtract west

 

Always record true directions they match the chart

 

Compass least, error east

compass best, error west

 

We used to sail a steel hulled sailboat. The deviation was as great as 23 degrees when heading west. If this was not taken into consideration, we might not even see our destination, or run aground.

 

Jeff

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