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#81519 - 12/29/06 03:45 AM Bearing repeatability with a sighting compass?
haertig Offline
Pooh-Bah

Registered: 03/13/05
Posts: 2250
Loc: Colorado
How repeatable are others with a sighting compass? The wife just got me a new one for Christmas and like a total dork, I've been sighting bearings around the inside of the house because we're pretty snow-bound here north of Denver at the moment.

I seem to be getting about a 2-3 degree repeatability using my new Brunton 8099 Eclipse (it has graduations down to a single degree). Indoors is probably not a fair test since I'm only looking at 10-12 feet total distance. I'm also trying to find the best compass-to-eye distance to allow me to focus these older bifocal-equipped eyes on the target and the compass dial simultaneously.

Just wondering what repeatability others are able to achieve. I'll keep practicing.

BTW, the 8099 is a very nice compass. I've been researching other models and didn't even realize I mentioned a compass to the wife, so the gift was somewhat of a surprise. She investigated and found the 8099 at a really cheap price locally. She did good ... I really like it!

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#81520 - 12/29/06 04:16 AM Re: Bearing repeatability with a sighting compass?
KenK Online   happy
"Be Prepared"
Pooh-Bah

Registered: 06/26/04
Posts: 2141
Loc: NE Wisconsin
Now that is funny. I thought I was the only goof that did that kind of thing. Just tonight I was sitting doing repeated sightings with my Brunton 8099 and Brunton 15DTCL - partially to remind myself why I like the 8099 better (I do!! the ciircle-in-circle thing really is MUCH easier to use than the arrow box on the 15TDCL) and to look at the repeatability. The 8099 was repeating within 1.5 degrees. The 15TDCL was within 3 or more degrees.

Tomorrow morning I'll repeat this some more to give you a better answer.

By the way, another reason I like the 8099 is that rubber boot that many don't seem to like. I store the 15TDCL in a plastic bag to keep it from getting scratched up, but I don't have to do that with the 8099.

I actually have two 8099's. A while back my first one got a bubble in it. I tried to fix it by putting it under a lamp ... and promptly melted the capsule (argh!). I liked it so much I bought a new one. My wife e-mailed Brunton asking if they could repair the old one - they did - for free.

The new one has different printing on the dial. I can't remember the details - I think they switched the green/black numbers. I'll check both tomorrow and report on the difference. I'll also check the repeatability on both compasses.

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#81522 - 12/29/06 06:01 AM Re: Bearing repeatability with a sighting compass?
MarshAviator Offline
Marsh Aviator
Journeyman

Registered: 11/18/05
Posts: 70
Loc: Baton Rouge, LA, USA
My experience with mirror compasses is that 2 degrees are typical for repeatability.
Keep in mind that the 8099 and 15TDCL are indirect reading, so not only do you have optical issues (boxing the needle), but also
the manipulation of the circle as well.
Toss in some vertical misalignment too.

True prismatic's (Sirs G150,PG M-73 or Suunto KB14, KB-360') can be read repeatability to about 1/2 degree.
Never mind any claims to "1/4 degree" or "1/6 of a degree".
You would need a surveyors compass and tripod and then still use repetition and averaging to get 10 min of arc.
That assumes that any local attraction can be compensated by shooting both sides of the line.
Anything hand held is really difficult to read to 1/2 degree, nearest degree is realistic.

A number of caving organizations use true prismatic compasses for underground surveys and have published
a lot of experimental data on accuracy and repeatability.
It surprised me to learn that most of these still had routine errors of several degrees (offset).
http://www.chaos.org.uk/ has a number of articles on the topic.

The old lensatic compass is actually specified as 2 degrees of accuracy and 1 degree of repeatability.
Most people are fooling themselves if they are getting better results with either a lensatic or a mirror compass.
Don't get me wrong land navigation can be done with 2 to 3 degrees of accuracy, it's surveying that requires
better results, that's why the military mortar and artillery units need the M2 (military version of pocket transits)
or the G-150,M-73, even then these are backups only.

Try going to a field and mark an irregular shape with cane poles or other markers, make sure the distances are at least 100 FT.
Now walk from point to point and shoot both forward (forward sight the next pole) and reverse (back sight to the last pole).
Then reverse the process, record all the results and then plot them out.
The results will speak for themselves.



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#81525 - 12/29/06 05:11 PM Re: Bearing repeatability with a sighting compass?
KenK Online   happy
"Be Prepared"
Pooh-Bah

Registered: 06/26/04
Posts: 2141
Loc: NE Wisconsin
... that's what the foil hat is for isn't it? <img src="/images/graemlins/grin.gif" alt="" />

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#81526 - 12/29/06 05:28 PM Re: Bearing repeatability with a sighting compass?
haertig Offline
Pooh-Bah

Registered: 03/13/05
Posts: 2250
Loc: Colorado
OK. You guys got me. I guess it IS incredibly assinine to go messing with a compass indoors, at least in determining repeatability and/or accuracy. It's just so hard to get a new toy an NOT be able to play with it. I guess I'll have to go back to non-magnetic uses like measuring the incline of our stairway banisters - woohoo! I just have to be a little discrete in my dorky endeavors so the wife won't roll her eyes at me so badly that I have to take her to the ER to get them fixed. That almost happened during the last dork episode when I was using a white noise generator and a dB meter to adjust our sound system. Testing the directionality of my homemade Yagi TV antenna from the middle of the livingroom was another recent episode, come to think of it. BTW, that's another item that is best tested OUTSIDE - much like a compass ;-)

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#81528 - 12/29/06 06:37 PM Re: Bearing repeatability with a sighting compass?
haertig Offline
Pooh-Bah

Registered: 03/13/05
Posts: 2250
Loc: Colorado
At tip from experience here ... use barbless hooks whilest casting inside. They come out of the drapes MUCH better! And if you pre-saturate the carpet with a viscous fluid (honey or pancake syrup works) you can get enough drag to practice your roll casts too. <img src="/images/graemlins/grin.gif" alt="" />

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#81529 - 12/29/06 08:37 PM Re: Bearing repeatability with a sighting compass?
oldsoldier Offline
Old Hand

Registered: 11/25/06
Posts: 742
Loc: MA
HAHAHA! man, thats a good one. I can almost see that one happening. I was taught to flyfish in a parking lot, on a military base. Casting, retrieving. We did it for like 2 hours. I never did get to go flyfishing. Ah well.
I carry my old trusty military lensatic compass, and, IIRC, within 3* was acceptable for navigation. That was for compass to cheek method. Center hold method was like 10* or so. Personally, I ALWAYS use compass-cheek. I have tried to use civilian type compasses in the past, but cannot seem to get the grasp of them. Someday, I will run across someone who has the patience to show me, and I will dump the heavy one I carry. Until then, I am stuck using my old, beat up (but still accurate and glowing), lensatic.
_________________________
my adventures

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#81530 - 12/30/06 01:05 AM Re: Bearing repeatability with a sighting compass?
tfisher Offline
Member

Registered: 01/29/01
Posts: 186
Loc: Illinois, USA
When I was teaching navigation we kept a record of a couple different compasses on succesive readings and it seemed to be about 2 degrees was the average.

We sighted targets at 100 yards and about 1000 feet.
_________________________
If you want the job done right call "Tactical Trackers"

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#81531 - 12/30/06 03:45 AM Re: Bearing repeatability with a sighting compass?
kevingg Offline
Addict

Registered: 10/21/05
Posts: 442
Loc: NH
I've got the same compass, had it for several years. Love it. I've found that all the color in the base (scales, blue line etc.) will fade right off in the rain if you carry in a pocket without the rubber shield. This has happend to me twice. Both times Brunton has sent me a replacement. Another time it developed a big bubble and they sent me a new "dial" part so you can pop out the old and pop in the new. Great company.

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#81532 - 12/30/06 04:29 AM Re: Bearing repeatability with a sighting compass?
haertig Offline
Pooh-Bah

Registered: 03/13/05
Posts: 2250
Loc: Colorado
Quote:
I've found that all the color in the base (scales, blue line etc.) will fade right off in the rain if you carry in a pocket without the rubber shield.
Thanks for the heads-up on this. You'd think they'd put some type of protectant/sealant over the printing. I've never had a problem with any other compass, but to be honest I take very good care of them (even the cheapies). I don't remember ever using one in the rain or getting one wet in any other way. I was planning on leaving the rubber boot in place anyway, so it's probably a non-issue for me. Thanks for the warning though.

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