Guesstimating L-R-C-F

[Anonymous_Coward]

The NSS Cave Conservation Committe should be granting out laser rangefinders to active cave survey groups around the country!

Jeff, let the Conservation Committee know that I'll take a Leica Disto A3 with the distoX upgrade!

[ian mckenzie]

Should be a short article on the DistoX in the next Canadian Caver.

[Chads93GT]

The group, and myself, that I cave with records up to the ceiling directly above the station, and the total ceiling height in the cross section, such as 28/4, same with left and right and down. It doesnt matter if there is a ceiling ledge 7" off the ground but the passage cuts back another 20 feet to the actuall passage wall, we put it in the map even if you cant walk there. Seems if you are only putting in passage you can walk through, then you aren't depicting what is actually there.

[Ralph E. Powers]

The group, and myself, that I cave with records up to the ceiling directly above the station, and the total ceiling height in the cross section, such as 28/4, same with left and right and down. It doesnt matter if there is a ceiling ledge 7" off the ground but the passage cuts back another 20 feet to the actuall passage wall, we put it in the map even if you cant walk there. Seems if you are only putting in passage you can walk through, then you aren't depicting what is actually there.

True... it is important to show via cross sections and profiles tight stuff and other obstacles.

[cavemanjonny]

Well, to beat a dead horse....

I don't give a hoot about LRUDs once the sketch is made. I use the LRUD to scale my sketch properly. Once I've done that, the cross section, plan view, and profile record the LRUD graphically. If you're sketching to scale, you can always retrieve the LRUD from your sketch. This also clears up the very confusing "slash" dimension readings. A cross section/profile view of a big step in the ceiling is much clearer than "4 slash 10", imho.

[Pat Kambesis]

On thing to remember about LRUDs is being consistent in how they are estimated (or measured) i.e. perpendicular to the station or bisecting the angle between two stations or something else??. I have been on surveys where the sketcher thought one method was being used and those estimating/measuring dimensions thought the other was, and vice versa, and and even sometimes inconsistent within the same survey. So its important to know before starting the survey how the dimensions are determined. Even with a good, scaled sketch, the estimates/measurements recorded, and what the sketchs shows may not match. One would assume that the sketch is always correct - but maybe not. And even if it is, what about the recorded data? Its not uncommon for that data to be used to generate volumetric or 3-d computer plots (look at TopoRobot plots!). So, it is important to have some sort of consistency (that reflects reality) in how the dimensions are determined. This will be addressed as cave survey technology continues to improve.

My preference is to bisect the angle between two stations EXCEPT when passage dimensions exceed "estimatable distances" i.e. greater than 30 feet. In those situations I will either take a "real" shot to the wall, or with a disto I can take a measurement with a compass bearing (trying to keep it as horizontal as possible). This is the only way that I know for sure that the wall is really where I think it is.

pk

[Bob Thrun]

I was surveying in a cave with high canyon passages. I guesstimated the distance from the station to be about 50 feet. When I did the draft map, I found that I had a bunch of 53 and 54 foot high passages. I do not want to guess the height to be an overly precise 47 feet. I took to using LRFH. where H is the total passage height and F is the distance to the floor. Somewhere between 10 and 20 feet, LRFH becomes better LRUD.

Of course, with Distos, we can measure the actual ceiling height

[Jeff Bartlett]

Somewhere between 10 and 20 feet, LRFH becomes better LRUD.

I don't buy it. While I understand what you are saying -- that if you're guessing "50" for the total height, splitting that into exact fractions is a bit misleading -- your technique doesn't actually add precision. In other words, how is writing "50" for the H better than writing "47" for the C? They're both from-the-hip estimates. In my opinion, changing from LRUD to LRanythingelse just adds confusion to the survey.

If it's not more accurate, how is it better?

[cavemanjonny]

I took to using LRFH. where H is the total passage height and F is the distance to the floor. Somewhere between 10 and 20 feet, LRFH becomes better LRUD.

Then H-F = U and H - U = D then F = D. So you're recording the same data, you're just rearranging it. I'm not sure of the benefit either. I may be missing something though. What exactly is the advantage of this technique?

[driggs]

Somewhere between 10 and 20 feet, LRFH becomes better LRUD.

I don't buy it. While I understand what you are saying -- that if you're guessing "50" for the total height, splitting that into exact fractions is a bit misleading -- your technique doesn't actually add precision. In other words, how is writing "50" for the H better than writing "47" for the C? They're both from-the-hip estimates. In my opinion, changing from LRUD to LRanythingelse just adds confusion to the survey.

If it's not more accurate, how is it better?

The difference isn't in the accuracy, it's in the precision. Say that your Down measurement is 2.35 feet (taped) and you estimate Up as 45 feet; you cannot list the ceiling height as 47.35 because it implies a precision well beyond the actual measurement. Similarly, estimating ceiling height as 47 feet implies a precision of one foot; calling it 45 or 50 implies a precision on the order of five or ten feet. As long as you're consistent in the precision of your estimations locally, there's no confusion that the ceiling is 45.00 feet high.

Similarly, I take ceiling height measurements with a disto, but I don't record the height to two decimal places; I probably couldn't repeat the same measurement twice to two decimal places. Instead, for anything over a few feet, I round to the nearest tenth or half foot.

It drives me crazy when people report the length of a cave to be something like 1547.38 feet when they surveyed it with compass and tape with lots of slop!

[Martin Sluka]

Jeff, let the Conservation Committee know that I'll take a Leica Disto A3 with the distoX upgrade!

But if you not use the PocketTopo software with it you'll miss 90 % of joke. Two times faster than any other cave mapping method I know. At the end of the surveying trip one has in scale drew map, extended section, cross-section on each station (optional). Export of data to several softwares - Toporobot and Therion for example, export of sketches as background image for Therion's map editor.

The main advantage is possibility to measure as many as necessary splay shots to important points of cross-section or extended section or map and to important objects. And if you measure the same shot from station to station three times it automatically recognizes it as surveying shot.

Martin