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#278230 - 12/27/15 11:02 AM Re: A new compass thread [Re: Mark_F]
Tom_L Offline
Addict

Registered: 03/19/07
Posts: 690
A few posts earlier I wrote about the kind of accuracy that can be achieved in the field with various types of compasses. 2-3 degree accuracy is achievable with a mirror compass. Most of the time it isn't really necessary. Sometimes however, it is - and unless you have a working GPS at hand, a precision compass is the only tool that can help you out.

10-degree accuracy, which you've been referring to in your last post, would be a very poor performance by anyone's standard. Translated into mils, that's 178 mils or a deviation of 178 m at 1 km / 1.8 km at 10 km. More than enough to miss your mark completely in any kind of difficult terrain and/or low visibility.

For any kind of precise "triangulation" (I assume you actually mean resection) you'd need a highly accurate compass. Ideally a prismatic one, such as the British Army issue Francis Baker M73:
http://www.ravenlore.co.uk/html/prismatic_compass.html

These are very expensive toys, but note that they're not fitted with a spirit level or even a tripod mount. BTW, I've never had any issues using a compass on steep slopes. The magnetic needle itself is a good enough indicator if you're holding the compass level - if not, the needle won't swing freely anyway and it will be impossible to take a bearing.

As for a tripod mount, I can't see any utility in that unless you're a land surveyor using a professional surveying compass (the $5 eBay special isn't going to cut it). Keep in mind that pretty much any photo tripod out there uses a steel screw to secure the camera. Not exactly a good idea if you intend to mount a compass...

P.S.: If you want to check the accuracy of your compass I would recommend a better method instead. Go some place with a clear field of view to easily identifiable landmarks (such as mountain tops) as distant as possible. Take bearings to each individual landmark. Now take a good map (Google Earth works, too!), identify your location and measure the angles to each landmark in question. They should correspond more or less exactly with the bearings taken with your compass, correcting for the magnetic declination obviously.

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#278236 - 12/27/15 06:22 PM Re: A new compass thread [Re: Tom_L]
Outdoor_Quest Offline
Enthusiast

Registered: 08/17/09
Posts: 305
Loc: Central Oregon
Sorry, I am really late to this thread.

I appreciate TomL's comments on the issues surrounding a bubble's impact on acompass.

For my use a mirrored compass is the only way to go.

Blake

www.outdoorquest.blogspot.com


Edited by Outdoor_Quest (12/27/15 06:23 PM)

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#278365 - 12/31/15 07:17 PM Re: A new compass thread [Re: Tom_L]
jamesraykenney Offline
Enthusiast

Registered: 03/12/04
Posts: 315
Loc: Beaumont, TX USA
Originally Posted By: Tom_L
IME a small bubble should not affect the accuracy at all. However, a bubble larger than 1/4" or so could already prevent the needle from swinging freely, introducing a completely random error that might be 5 or 180 degrees depending on how exactly you hold the compass and how the bubble moves inside the capsule.

A bubble this size is pretty bad. That compass needs to be repaired or replaced, either way it cannot be expected to give accurate, consistent bearings:
http://tasmania.bushwalk.com/forum/downl...4&mode=view

While a small bubble does not necessarily mean the end of a compass it's still a cause for concern. A small bubble is likely to grow bigger in cold temperatures and high altitudes.

Is it possible to repair a copass with this sort of defect in the field? I'm not sure. If there is really no other way you could poke a small hole and drain the liquid from the capsule. Unfortunately, that does not seem to work well in my admittedly limited experience.

Generally speaking, an accuracy of 2 degrees would be considered excellent, 3 degrees still acceptable from what I've been taught about land navigation. Keep in mind that even a compass in perfect working condition isn't really capable of perfect readings. IIRC, a top-notch prismatic compass has an ideal accuracy of +/- 0.5 degree. A good mirror or lensatic compass somewhere between 1 and 2 degrees. A baseplate 3 degrees or so.

This is the inherent accuracy of the compass and its sighting system (or lack thereof). It does NOT take into account user error or slight mechanical defects (loose bezel, slightly offset sighting mirror/sight line etc.).


All of this is why I was always taught NEVER(when posible) to navagate directly to where you want to go, but to one side of a wide landmark(like a road). That way, when you reach the landmark you are SURE which way to go to reach your destination.

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#278366 - 12/31/15 07:23 PM Re: A new compass thread [Re: Mark_F]
Montanero Offline
Veteran

Registered: 10/14/08
Posts: 1374
Loc: North Carolina
It is called deliberate offset. It works best when you have a linear terrain feature (or manmade feature) so that when you encounter this feature, you know which direction you need to go to hit your objective.

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#278367 - 12/31/15 07:26 PM Re: A new compass thread [Re: BruceZed]
jamesraykenney Offline
Enthusiast

Registered: 03/12/04
Posts: 315
Loc: Beaumont, TX USA
Originally Posted By: BruceZed
Its not really how old the Topo Map is, it how accurate it was when it was made and how much has changed in the area since. Accurate Map with very few changed = Good Useful Topo. Still given a choice I would take a Topo updated last year in comparison to one made 30 years ago.


The only other MAJOR problem with an old Topo map is that the magnetic variation changes over time... Sometimes dramaticly!

The magnetic poles move around quite a bit...

Here is a very good link showing some rather EXTREME changes...

Magnetic Declination
http://www.compassdude.com/compass-declination.php

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#278370 - 12/31/15 08:32 PM Re: A new compass thread [Re: jamesraykenney]
hikermor Offline
Geezer in Chief
Geezer

Registered: 08/26/06
Posts: 6540
Loc: southern Cal
At least for most of the western US, changes in declination are fairly minor and predictable. What we haven't discusses is variation, the change in the compass needle orientation brought on by local magnetic bodies - everything from your neck knife or eye glasses to ore deposits. I have experienced variation of 35 degrees, which I am sure we will all agree, is just a tad too much...
_________________________
Geezer in Chief

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#278372 - 12/31/15 08:57 PM Re: A new compass thread [Re: Mark_F]
wildman800 Offline
Carpal Tunnel

Registered: 11/09/06
Posts: 2810
Loc: La-USA
I saw a variation of up to 90deg in the UP of Michigan because of old iron ore mines.
_________________________
QMC, USCG (Ret)
The best luck is what you make yourself!

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#278384 - 12/31/15 11:49 PM Re: A new compass thread [Re: wildman800]
jamesraykenney Offline
Enthusiast

Registered: 03/12/04
Posts: 315
Loc: Beaumont, TX USA
Originally Posted By: wildman800
I saw a variation of up to 90deg in the UP of Michigan because of old iron ore mines.


Don't tell me a little thing like being 90deg off is going to get you lost... grin

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#278385 - 01/01/16 12:00 AM Re: A new compass thread [Re: jamesraykenney]
wildman800 Offline
Carpal Tunnel

Registered: 11/09/06
Posts: 2810
Loc: La-USA
It certainly didn't help me find myself until I reasoned out the amount of error.
_________________________
QMC, USCG (Ret)
The best luck is what you make yourself!

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#278386 - 01/01/16 12:27 AM Re: A new compass thread [Re: wildman800]
Outdoor_Quest Offline
Enthusiast

Registered: 08/17/09
Posts: 305
Loc: Central Oregon
Another good source for declination info of an area is http://www.ngdc.noaa.gov/geomag-web/

I normally check the correct values before going into the field.

Blake

www.outdoorquest.blogspot.com

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