Is +/- 2 degrees enough?

[TomJewell]

I was looking at the <a href=http://www.brunton.com/product.php?id=349>Brunton Nomad V2 digital compass</a>.

The specs say it is accurate to +/- 2 degrees with a readout in 1 degree increments.

Is that good enough for a good cave survey? My gut says no, but I do not have any real idea.

[cob]

+/- 2 degrees is the excepted margin of error between foresights an backsights.

If the FS and BS are sepreated by 2 degrees, and the 2 instruments are each off by 2 degrees (in opposite directions) one could end up with a six degree error (in theory) How would one know? By checking the 2 instruments against each other above ground.

In an ideal world all inst. read dead on. In this world, I suggest checking them above ground (esp. if your cave is lengthy, where errors pile up)

tom

[George Dasher]

Yes. 2 degrees is fine for cave surveying.

The least of your problems is going to be the instruments.

Your real problems are going to be nasty passages, mud and water, blunders, tired people, and things like that.


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Also, being two degrees off is usually two degrees in the same direction. That means that a -2 degree "wrongness" will be the same in the foresight and the backsight. The instrument will thus appear dead on. It won't be 6 degrees off; it'll be 2 degrees off.

"Off" instruments are a good reason to have an aboveground compass course; however, my experience with such courses is that you are usually calibrating the instrument reader, not the instrument. However, I guess calibrated instrument readers can be a good thing to.

Mmmmm.... I guess you could also calibrate the instrument reader per number of beers. You could make charts and graphs and publish articles. Maybe I did that once... I can't remember.

[Phil Winkler]

Many years ago I accompanied my 4th grade daughter's school class to a compass course in a graveyard in Huntsville. It was sponsored by Federal agency. Well, they had a big display compass to explain how it works. The goal was for these kids (9-10 yrs old) to follow a compass course in teams. When they reached a destination they were to write down the name on the tombstone. The team with the most correct answers at the end would win. I thought this was quite cool and educational.

Well, the guy doing the talking got up in front by the display compass and introduced himself and the program about how to determine direction. Then he asked the class to all point to the North. As a group they all pointed straight up.

I almost wet my pants.

[Grandpa Caver]

"Your real problems are going to be nasty passages, mud and water, blunders, tired people, and things like that."


amen brother

[bsignorelli]

my experience with such courses is that you are usually calibrating the instrument reader, not the instrument.

Instruments have serial numbers and you can figure out the "correction" to what they read on the compass course and apply it to that days surveying.

Or if you are just referring to the tendancy of your instrument reader to "remember" what the compass course answer is and report that back then yeah...calibrating the reader sucks

(tounge in cheek) The best instrument readers have horrid short term memory problems (c:

This is another good reason to use one instrument (and reader) for front and back sights as much as practicable (especially in longer caves).Don't forget t ohave your compass course on a dead level setup so you can check your clino too.

These are just brief thoughts from me....please talk to others more experienced with compass courses than me for more details. Jim Borden posts here and can explain it better than I just did

[Bob Thrun]

The only way you could tel the difference between a survey that
is accurate to +/- 2 deg and a slightly more accurate is to lay them
on top of each other. You could probably not tell the difference
if they were shown side-by-side. Both are more than accurate
enough for roadmap use. Neither is accurate enough for drilling
an entrance far from the original entrance.

I looked at the Nomad 1 in a store. I could get the reading to
change by 10 or 15 degrees by tilting the compass side-to-side
or back-and-forth despite the claim that the magnetic sensor
was gimballed. The Nomad 2 seems to have fixed this problem
by adding a bubble level. But I do not see any sights on the
Nomad 2! I do not know how I would take an accurate reading.
Is there an eyepiece on the end? Then why the big dial on top?

Bob Thrun

[TomJewell]

My plan was to mount it on a small piece of wood, along with a laser pointer, then calibrate to match the pointer.

[Bob Thrun]

Try to find a glass rod about the diameter of the laser beam.
If you shing a laser beam on a horizontal glass cylinder, it
will be spread out into a vertical fan. You need to keep the
digital compass level.

You can easily make a laser clinometer from a laser pointer
and a SmartTool digital clinometer. I will have one in my
car when I go to the PSC meeting Monday.

Bob Thrun

[driggs]

How about +/- 0.5deg compass and +/- 0.2deg clino for a few hundred dollars and a bit of DIY elbow-grease more...

True North Technologies Revolution

This is apparently supported out-of-the-box by the Auriga PDA software for the serious high-tech caver.

[Bob Thrun]

The True North website does not do a good job of explain the
company's products and why I might want to buy them. I run
into a lot of poorly organized websites.

Let's price things out:
Nomad digital compass, supposedly gimballed $70
SmartTool digital level $100
Disto laser rangefinder $350

Laser Atlamta and MDL combind these elements into
one package for $4000

I forgot to mention that I have a Disto set up as a laser
pointer with my SmartTool.

Bob Thrun

[NZcaver]

Let's price things out:
Nomad digital compass, supposedly gimballed $70
SmartTool digital level $100
Disto laser rangefinder $350

Laser Atlamta and MDL combind these elements into
one package for $4000

If you're going to spend that kind of money, just get something like IKE - http://www.surveylab.co.nz/


Meanwhile back in the real cave-surveying world - shop around and you can probably find some good deals. In my case, eBay was the answer:

Suunto Twin compass/clino - about $100, virtually new
Leica Disto Lite 4 - about $175, barely used

Good used Suuntos, especially combination ones, are frequently available from those in (or leaving) the satellite dish business. And Distos become available as people upgrade their older "dumb" models (like I have) to Bluetooth ones.

I like the idea of a digital clino too - might have to try that myself one day...

[TomJewell]

Try to find a glass rod about the diameter of the laser beam.
If you shing a laser beam on a horizontal glass cylinder, it
will be spread out into a vertical fan.

[...]

Bob Thrun

This seems like a good way to make the laser more eye safe too.

[George Dasher]

Back when I was ran the OTR compass course (which is eight stations long with the distances that are already measured), people would always need to borrow a set of instruments. I tried really hard to loan out only one set of Suuntos.

The end result was that I ended up with a lot of "calibration" data for one set of instruments. Maybe 20 or so for one of my sets and six or so for my other set.

The results were all over the place. I think from about -5 to +5 degrees for both the compass and the clinometer. I published the results somewhere. In The West Virginia Caver I think.

The bottom line is that I don't know what the course was "calibrating," but it sure wasn't the instruments.

I might have been calibrating the users, but I don't have any redundant data to support that.

Anyway, I have my doubts whether compass course calibrate anything at all.

[Bob Thrun]

Back in March 1981 DC Grotto ran a compass course with
participants sighting to and from 10 posts. Over 1600 sightings
were made. The course was level and reading conditions were ideal.
I put the finishing touches on an 11-page report a few months ago.
I will submit it to a newsletter only if the editor promises not to
edit or reformat it. I will have some preprints at the NSS Convention.

If a random person used a random compass to make a sighting,
the standard deviation was between 0.8 and 1.3 degrees for Suuntos.
For Brunton pocket transits, the standard deviation was in the
range 0.3 to 1.0 degrees. The Suuntos disagreement in the direction
of magnetic north had a standard deviation of 0.6 degrees.

Bob Thrun