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Compass Confusion


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I bought a Silva Engineer Bearing compass recently to use the bearing given by my eXplorist GC to better guide me to the cache. Obviously the compass points to "magnetic" north, so that is the setting I selected for my GPS. However, driving down a straight road (as a test), my heading from the GPS was 0, so why when I stopped to use the compass was my bearing 346? When I set the GPS to use "true" north, both heading & bearing agree. Perhaps I've answered my question, I just thought they both should be on magnetic.

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I bought a Silva Engineer Bearing compass recently to use the bearing given by my eXplorist GC to better guide me to the cache. Obviously the compass points to "magnetic" north, so that is the setting I selected for my GPS. However, driving down a straight road (as a test), my heading from the GPS was 0, so why when I stopped to use the compass was my bearing 346? When I set the GPS to use "true" north, both heading & bearing agree. Perhaps I've answered my question, I just thought they both should be on magnetic.

 

If your compass has an adjustable plate, it may have been adjusted for true north. Thus you GPS need be on true north.

 

What compass is it, can you provide a link to it? Most have this adjustment. Make sure the N on the adjustable ring is aligned with the arrow or hair of the compass and it will be giving you magnetic readings. It may have been possible that it was set to true at the factory.

 

EDIT: As well the amount of declination changes from area to area, and as well things such as iron deposits can effect the way a compass works in that area, which is some times noted on high quality maps.

 

It could be that in your area there is no declination to take in account, there for your GPS should be set for true north.

Edited by mchaos
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I don't really understand why you would want to get a compass. The GPS arrow will point towards the cache and one of the data fields shows how far away. The compass reading is irrelevant and could actually be removed from the screen as unnecessary.

 

Secondly I can not remember any time when I have told the GPS to find a cache and then walked in a straight line.

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Most GPSr's need to be moving at a certain speed to keep the arrow updated (try stopping and figure out which way the arrow is pointing, rotate the unit and tell me which way is correct). When you are close to GZ a compass is easier and quicker to get a fix on the proper direction the arrow is pointing.

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If your compass has an adjustable plate, it may have been adjusted for true north. Thus you GPS need be on true north.

 

What compass is it, can you provide a link to it? Most have this adjustment. Make sure the N on the adjustable ring is aligned with the arrow or hair of the compass and it will be giving you magnetic readings. It may have been possible that it was set to true at the factory.

 

EDIT: As well the amount of declination changes from area to area, and as well things such as iron deposits can effect the way a compass works in that area, which is some times noted on high quality maps.

 

It could be that in your area there is no declination to take in account, there for your GPS should be set for true north.

The Silva web site doesn't show my compass as a product they carry, but here is a link to a photo of it. http://www.kaboodle.com/reviews/silva-lensatic-360-compass

This compass is intended to be used by a surveyor to get a bearing on a distant point, it doesn't have the plate for aligning with a map, it only provides the bearing when viewed through the 2 sights, you rotate yourself if you want a specific bearing. I live in So. Cal so the difference in my readings was about right at 13-14 degrees.

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I don't really understand why you would want to get a compass. The GPS arrow will point towards the cache and one of the data fields shows how far away. The compass reading is irrelevant and could actually be removed from the screen as unnecessary.

 

Secondly I can not remember any time when I have told the GPS to find a cache and then walked in a straight line.

On my eXplorist GC it points to the cache in relation to North, I can be facing South and the GPS "compass" still has north at the top so I find it helpful and it saves me time to be able to sight the bearing through a real compass.

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Most GPSr's need to be moving at a certain speed to keep the arrow updated (try stopping and figure out which way the arrow is pointing, rotate the unit and tell me which way is correct). When you are close to GZ a compass is easier and quicker to get a fix on the proper direction the arrow is pointing.

 

One has to be careful to distinguish between an electronic compass display on GPS with a compass...

and the compass like bearing indicator that is common especially on Garmin units...

 

The compass should show the direction the compass is facing and will change as you turn the GPS (should be horizontal and usually indicates the top of GPS as pointer). This can be set to indicate whatever you want... including corrected for declination. It normally defaults to TRUE, can show Magnetic. I prefer to use magnetic adjusted manually for local declination which gives TRUE or Grid if you choose that correction for map use.

 

The bearing indicator shows the correct bearing from where you are to where you are going when in GOTO mode.

 

Heading indicator shows the direction you are travelling (track made good) when you are moving. This is what most people mean when you say have to move to get a result. It is neither of the first two but is useful when no compass is present... I like a hand compass instead of electronic.

 

Doug 7rxc

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Oh yeah, I didn't think of that either, but year, anything metal too close will mess it all up. When I was a scout, I was taught to hold the compass in the palm of your hand, away from your body and any jewelry or metal accessories you may be wearing.

 

That makes so much sense LoL.

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Watch out for any electro magnetic fields of any sort... I often watch trainees lose things when they have a track take them under power lines, near ground mount transformers etc. This can also happen when a line parallels a road or path. A strong field can permanently ruin a compass... at least without major work.

 

Local magnetic anomalies can throw you as well... such as happens around iron ore deposits, or meteoric remains occasionally. Some are small and some really large areas...

 

Good advice... never put your trust in a single reading... move and try it again... especially hand held magnetic compasses... watch the level too... one advantage to an electronic compass, but regular ones don't require batteries.

 

Doug 7rxc

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I don't really understand why you would want to get a compass.

 

 

If you do any wildernass caching... even in county, state, or regional parks, you really owe it to yoursef to not only carry a compass, but have a pre-determined panic azimuth each time you leave your car. Beside loss or damage to your receiver, or batteries, there are stll other reasons that your GPSr may fail you.

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