How do I determine Clino error?

[jlillest]

We surveyed a beautiful quarter-mile long stream cave in central VA yesterday, and the end of the survey line is 15 feet below the cave entrance area. The problem is, the entire passage is in the stream and we were going upstream.

How do I set up a course to calibrate my Clinometer? I've taken a simple set of shots around the house to see if there are any gross errors in the Clino, but don't know how to interpret the data. How can I get the offset for my instrument without a survey course with known readings?

The cave is related to several others in the area and may someday tie in to create a multi-mile system, so somewhat accurate data is important to us.

Thanks,
-Jon

[Extremeophile]

I think a course might be the only way to determine this. You only need 2 stations and you have to do a front and back sight. If the stations are level (i.e. true reading of 0 degrees), then you're clino offset would be reflected in both readings equally. For example, it sounds like you're instrument might be reading low (negative error), so you might see -1 or -2 degrees on both the front and backsight. You can then apply that correction to your previous survey readings. Even if your course stations are not level you will have the same observed results with both readings being either high or low and you can still determine the offset.

The more likely problem is a sticky bearing. Suunto or Sisteco/Silva clinos are known to suffer from this more than compasses. For any given course shot you can aim the instrument up (like 45 degrees) and then slowly bring it down to your sight reading. Then repeat the measurement starting with the instrument held down at 45 degrees. If you get different readings (e.g. +2 degrees and -2 degrees) then you have a sticky bearing and it will need to be serviced. In this case there's no way to correct for the data you've already gathered.

I've also heard recently that clinos can be magnetically influenced. If there are steel or other iron containing materials in the wheel of the instrument then the reading could be thrown off by a magnet - e.g. Sten switch on your helmet. Most people, including me, have assumed that clinos are not sensitive to magnetic fields, but this might not be the case. I haven't tested this yet with my Sten and instruments but it should be easy to confirm.

Derek

[Extremeophile]

Your offset would be the average of the uncorrected front and back sight readings between any two points. E.g. a front sight of +6 and a back sight of -4 is (6-4)/2 = +1 degrees.

I'm curious if you took backsights during your survey, and if so did you use the same or different instruments?

Derek

[jlillest]

I'm curious if you took backsights during your survey, and if so did you use the same or different instruments?

No backsights.

I'm using my Suunto Tandems and just noticed a new bubble that has formed since I last checked them. I tried the sticky bearing test and it doesn't look like this is the case. I'll try the course again outside to check for the offset.

Thanks for the help.
-Jon

[MUD]

No backsights.

Why not?

[Extremeophile]

Without backsights there are a number of other possible errors that don't involve the instruments, e.g. reading the right side (% grade), reading the wrong way from the label (e.g. 8 degrees vs 12), calling or recording the wrong direction (plus vs minus angle), book person recording error, etc. Backsights catch most of these and would be recommended in any cave survey, especially a multi-entrance, multi-mile project.

[Scott McCrea]

In case you want to make a simple, level, 0 degree compass course to check the clino, stretch out a garden hose, use a pitcher or another hose to get it completely full of water. The ends will be exactly level. Make some marks on something—trees, fence posts, rocks, buildings, etc.

[Spike]

You could look at the data and try to tease out where you lost elevation. Blunders like reversed signs on the clino are most likey you're problem. Could be a clino that is a degree or so out giving you a systemic error(which if you had performed backsights, would have been canceled out), which you could try to correct shooting at water level in your back yard.

But at the end of the day, after you beat you head against the wall, trying all the tricks to massage the data and make it work, you still have a survey with no backsights and no real confidence in. Personally, would just start over. Even surveys with backsights have errors in them, we just happen to know they have errors, and often exactly where the error is. Those are OK cause we can reshoot the stations and fix the problem. Sometimes both front and back are bad somewhere in the survey and you still have to go back and fix it, you just may not know where the problem is.

[Crockett]

I would argue that you could set up a survey course to check your compass and inclinometer between any two points that are relatively level and within 50' of each other. Shoot front. Shoot back. Then thoughtfully compare. I am prepared to be beaten down on this but the beater should be prepared to explain why a flat level course is required.

More than two points would be better. Then you have front/back at more angles and loop closure too, but you gotta take the time to enter the data in a program (or sketch it out to scale?).

Finally, you might try adding +1 degree to each inclination angle and see if the water runs down hill. It's a hack but it is within the recognized error of the instrument.

Like Spike, I would resurvey with backshots, but most of the time I just want to be like Spike.

[jlillest]

I'm not interested in having a debate here on taking backsights or not, I'll save that for the next caver gathering. I'll buy the first round.

I have confidence in my data, I just need a quick technique for finding the offset. From playing with the data, I know that the offset is between 1 and 2 degrees. Now I guess I just need to set up a hydro level. Thanks for the suggestion Scott!

-Jon

[Extremeophile]

I still believe that you don't need a level course to determine the offset. Look at the equation I posted above again - in this example the true value of the shot is 5 degrees, but regardless, you don't need to know the true value.

I'd like to hear what you find. I've used a lot of different clinos and I'm not sure I've ever seen one with an offset - other than a Disto X.

[Aaron Addison]

I'm gonna agree with Spike. Mostly you are time ahead by taking the backsights instead of conjuring voodoo on data reduction. Especially since you think that this cave may tie in to other caves at some point.

Why not just set a level on something... well... level, and place your clino on top of it? Sight and see what the numbers read.

AA

[Extremeophile]

I think Jon is either going to always use back sights in the future, or never post a question to the forum again. Since we're beating the topic to death I'll just add that you can do back sights with a single set of instruments, and since the sketch is nearly always the rate limiting factor in any survey, it doesn't take any more time. I'm in the process of resurveying several miles of cave because they didn't do back sights (and there are significant loop errors) and didn't take any clino readings (because it was "horizontal" passage). It seems tragic that 27 survey trips produced data that is no longer useful.

I did a little experimentation with my Suunto Tandem and Sisteco Surveymaster. I couldn't get the clinos on either to budge with either my Sten light or a very strong hand-held magnet. I would say that the myth I heard about clinos being affected by iron or magnets is definitely busted.

[Scott McCrea]

I did a little experimentation with my Suunto Tandem and Sisteco Surveymaster. I couldn't get the clinos on either to budge with either my Sten light or a very strong hand-held magnet. I would say that the myth I heard about clinos being affected by iron or magnets is definitely busted.

I assumed that clino guts where aluminum (possibly stainless). Do you know different?

[Extremeophile]

I was following up on an earlier post where I stated that I heard clinos could be affected by steel or magnets. I just didn't want to spread false information so I checked for myself and it's not true.