How do I make Compass Course?

[Scott McCrea]

How do I set up a compass course for checking instruments for accuracy?

Getting two level spots is easy, but how do I determine the real azimuth between the two points?

[Jeff Bartlett]

The one at Mammoth Cave used an actual star shot between the brass caps. Aaron, were you involved in that?

[Phil Winkler]

Scott,

Are you asking how to determine true north or declination for your compass so as to add or subtract degrees to any observed reading? It's not clear what you are asking, but airports nearby usually have a true north setting for pilots to adjust their compasses.

Some marinas do, too. And, nautical charts (and I assume aviation charts) also have this.

[Scott McCrea]

Are you asking how to determine true north or declination for your compass so as to add or subtract degrees to any observed reading? It's not clear what you are asking, but airports nearby usually have a true north setting for pilots to adjust their compasses.

Some marinas do, too. And, nautical charts (and I assume aviation charts) also have this.

I mean if I take an azimuth reading with my compass, how do I know that my compass is accurate? I can find declination online for where ever we are surveying.

The airport or marina is a good idea. I'll check on that.

[shibumi]

We used 2 PVC posts set in concrete and filled with concrete about 50' apart. Then we had a precision compass
(1/10 degree resolution) give us our initial measurement. That compass was a certified one from Indiana Geological Survey.
We check it every year or so to make sure declination numbers match. Since Suuntos give us about a half-degree
resolution, it's used to check all compasses with each other before we go in to survey. Serial numbers are recorded
so error can be tracked, but for Buckner our loop closures were really good.

(note: I am NOT an expert by any stretch of the imagination, just reporting on what we did for our
course)

[Phil Winkler]

Mariners learn to use a chart and two known points to check/calibrate their compasses. A topo map can be used just as well. Find two points you can see together, plot their bearing on the topo and see if your compass provides the same reading. Front and back sight, etc. Water towers work well since they are usually visible over long distances and often marked on charts and maps.

But, checking compasses periodically reminded me of a team trip into Anvil Cave during the Annual Rally when it was held. Paul Boyer had driven up from Ft Walton as part of the F.R.O.G. team. Both he and I had forgotten our usual compasses so we each stopped to pick up cheap ones somewhere.

In the cave we came to a spot where we had to go south for some distance. We each pulled out our compasses, took a reading, and then pointed in opposite directions to where south was.

So much for cheap compasses.

[ian mckenzie]

Could you not simply take the same reading with two good instruments? If the readings are identical, chances are both are accurate; if they aren't then at least one of them isn't. And with three instruments...

Just make sure you are wearing the same metal-rimmed glasses for all of the readings

[Crockett]

Hey Scott,

You need two points, a theodolite, a clear view of the north star, and an instrument operator.

The two points should be stable. They should not be made of or be near materials that might effect the compass like iron, automobiles, pipes in the ground, electrical lines, electrical transformers, strong batteries, motors or magnets. Concrete without reinforcement and an open field are good options. The orientation is not critical but close to north/south is best. The points should be level. You might use a laser to get level. I have never seen one but you could have more than two points. That might add some entertaining symbolism but would substantially increase the complexity. Beer and lawn chairs are also useful.

After the concrete sets, the sun sets on a clear night, and you sober up the operator should set up the theodolite along the axis of the two points. Turn the instrument to the north star and record that direction along with the time. Turn back to the axis of the two points and record the difference. Repeat this until you are satisfied, tired, or otherwise ready to quit. If you are a real freak you could set up and do it from the other end. Theodolites can be expensive and complicated. The operator will likely know how to do this part without much direction.

When you get some numbers consult the charts that allow you to adjust your readings. The north star is not always north due to precession (wobbling planet) but it is really close. There are charts that will allow you to adjust your observations based on the time you looked at the star. Adjusting for precession will yield a level of precision well beyond the abilities of a cave compass but this should be done so you can brag to others...but also so you know you are using north and not some made up direction for your course.

Another option for you would be to ask Dan Henry to help you with this. He is the most knowledgeable person I know on this subject.

After you have your course ready you should read about using it in some sort of cave survey book or manual. You know most of this already but the person who will operate the compass should run the course with it. The instrument person should be dressed for caving and use the method they will use in the cave. This might reveal that they have a helmet, light, watch, jewelry, or plate in their head that is going to hork the readings. It might also reveal that they don't have some essential knowledge, skill, ability or inclination needed to operate a compass during a cave survey. This is best discovered on the surface prior to the trip. A good course is devalued if not used properly. That would include applying the results to the making of the map.

A number of studies have been done regarding cave survey blunders related to reading instruments. Jim Borden has done very detailed studies. He knows a lot and is an accomplished instructor deft at explaining what can go wrong and how to avoid it. Some of this might be available on line but I can't find a link on the first try and have run out of time.

If this is all too much you could do a turning survey. That might be the wave of the future.

Hope this points you in the right direction.

Mike

[LWB]

I'm not sure what you mean by testing for accuracy. The main purpose of most compass test course is so you can accurately compensate for the wandering of magnetic north over periods of time. The published data may or may not be good enough for your area.

If that is what you mean, you need to find a spot without magnetic interference (and open fields are not always good), and set two "stations" All I've used were PVC filed with concreted with a brass cap on the top. We used to have more than two stations - I think this was related to determining the eccentricity error correction for Brunton Pocket Transits, but I don't know the details.

Once you have your stations you get a Theodolite and do a number of star shots (the North Star) over defined time intervals and plug the numbers to some formula to get the angle of your survey line between the stations from truth north. Then before a cave trip you do the FS/BS on the test course with your Suuntos to get the correction factor. You put that into Walls or whatever program you are using so the surveys done in 1995 and in 2009 are corrected to true north. So even through magnetic north may have changed 4 degrees in that time, your survey will not be screwed up. In some areas magnetic north can change significantly over the course of a year, so this is very important for on-going projects.

I'm don't know the accuracy required in the Theodolite for this. I have a 10 second digital Theodolite, but I have never attempted a star shot. Bob Osborn did the Hamilton Valley Compass Test Course (and proved in the process that open fields are not always void of magnetic interference) and was involved in the star shots - he will know the details. By the way, digital theodolites are not hard to use, downright simple compared to the old days.

If you only want to know if the compass is bad, you can use a test course without knowing "truth". Just compare the Foresights / Backsights. If they will not agree something is wrong with the compass or the reader (or the site if there is magnetic interference). Of course you will find this out in the cave when your FS/BS numbers don't agree, but it is better to find out on the surface. It is important to test inclinometers too. They can have problems too.

[Spike]

Correcting for Declination is only part of the purpose of the course. You are also compensating for error inherent in the accuracy of the instrument. How close is 0 on the dial to the magnetic pole of the compass? Since declination is hard to determine for any given place at any given time (the calculators are approximate). The difference of the true azimuth of the course and that of the compass is declination plus the error in accuracy. Think of it as D+E=C where D is declination and E is error and C is the correction factor. We don't know either D or E but we can measure C directly using a compass course and this is the number we are interested in anyway.

Where folks get messed up is when the value for E on 2 different compasses is not the same. This appears to be a result of the labeling on the dials not matching the poles of the magnetic field of the compass and some slight inconsistencies in manufacturing. There is about a 2 degree spread in numbers reported for the compass course at Hamilton Valley on any given expedition. But over the course of a year there is enough outliers that show up to make a 4 degree spread.

People often make the mistake of assuming that a compass that returns a different value than say 3 others is "bad". It isn't "bad" it just has a different E and therefore correction factor. The value for E on the lone compass could be smaller than that of the other 3. A "bad" compass is one that has inconsistent readings, where E changes from read to read. Think sticking compass.

Often people will take 2 sets of insturments that had different correction facotrs on a cave trip and try like hell to get them to agree in the cave. Often the party will declare that one was bad and needs fixed. If 2 sets are going to be used. Make sure they agree first, at least to .5 degrees.

Good Luck!

[Crockett]

What Spike said and if you take more than one set of instruments on a trip be sure to clearly note which set is used. That can be as simple as: "Used Suunto 72843 from B100 to B122. Used Suunto 99832 from B122 to B198.". ...making a compass course is probably easier than using one.