Poll: Do you take backsights?

[Scott McCrea]

I like backsights and try to take them whenever I can.

[Jeff Bartlett]

Not taking backsights is just plain sloppy, isn't it? I can think of three different backsight "workflows":

(1) confirm that there isn't a blunder, record the data, and average the two shots
(2) confirm that there isn't a blunder, and record the data, but don't apply it to the line plot
(3) confirm that there isn't a blunder, and don't even record the data (i hate this one)

while opinions differ on what to do with the backsights once you have them, I can't think of a valid reason to not take them altogether if you're surveying with Suuntos... Sometimes, if a backsight is much harder than the foresight or the backsight reader is 3/4 underwater or something, we will end up discarding it and just going with the foresight, but these are specialized situations. In addition, I suppose if your cave is a two-shot-wonder, there's no reason to split hairs. But in a cave legitimately worth mapping, it's legitimately worth taking the time to shoot backsights, if for no reason other than to rule out survey blunders!

[shibumi]

Not taking backsights is just plain sloppy, isn't it? ... while opinions differ on what to do with the backsights once you have them, I can't think of a valid reason to not take them altogether if you're surveying with Suuntos...

If the survey matters a lot, like you are tying in to a surface point or another cave: yes
If it's a large cave: yes

If it's a small cave (100-200'), a single small pit, or a nasty nasty bathtub with a couple inches of
airspace and it won't really matter if we're off a few degrees: no

So, I don't think not taking a backsight is automatically sloppy, it just makes your survey
less accurate. But in science, as in construction, there's no point in measuring with a micrometer
when you are going to adjust with a sledgehammer. It's the difference between accuracy and precision.

I helped survey a cave running under an old toxic waste site (pcb) that had been capped and they
were still going from a few ppb of pcbs during normal flow to tens of ppm under flood pulse. The whole cave was
about two hundred feet long, O2 levels were about 18% WITH forced ventilation, and the airspace was
about 4" to 1'. We TOOK backsights because the accuracy mattered, they needed to be able to accurately
tie in to surface points. If it'd just been some little slimehole I found in the woods I wouldn't have bothered
other than a quick and dirty survey.

[Pat Kambesis]

I always do backsites because it helps minimize blunders (survey errors due to human error). Not to mention that it gives the sketcher a little extra time at the station. My experience has been that in a set of 50 survey shots instrument reader will make at least one blunder - which you catch with the backsite.

However, you gotta be careful when using two different instruments and instrument readers. If the instruments aren't calibrated then trying to make those shots agree would make for even more error in your survey. Ideally one should use a compass calibration course where the stations are calibrated to some set point like a star shot. If you can't do that, at the very least check both instruments before starting the survey. If they are off by more than a degree or so from each other for azimuth then this could be a problem. Same with clino.

pk

[Martin Sluka]

I draw the sketches in scale directly in the cave. I'll detect any blunder just there. No needs for backsights.

m.

[Pat Kambesis]

>I draw the sketches in scale directly in the cave. I'll detect any blunder just there. No needs for backsights.

How does sketching to scale assure that the reading you got is correct? If there is a blatant error like the reading results in a sharp turn of passage that is not the reality - then yes you'll see that on sketch. But if the instrument reader gives a number that is 10 or 20 degrees off of reality how does the sketch show that?

All the backsites do is help minimize human error - which is the the most common and hard to find after the fact. However, backsites alone don't show the accuracy (or not) of a survey line - the only way that is apparent is by closing loops. If one has a straight line survey, even with backsites, there is no way to check the accuracy of that line.

pk

[Jeff Bartlett]

I draw the sketches in scale directly in the cave. I'll detect any blunder just there. No needs for backsights.

[Anonymous_Coward]

Taking backsights is one way to check the accuracy of the readings the FIRST time, ensuring that some passages may only have to be visited once. This is important in very delicate caves that show impact from each trip. Some places really shouldn't be returned to.

Backsights for Conservation!!!!

[George Dasher]

I just noticed Xcathodex's March 13th post.

If you're going to take backsights, then write the number down. Not doing so is just throwing away data.

And besides, even when taking a backsight, it is still possible to blunder in the cave and write down a fore and backsight like: "100 and 180." But with both numbers written down in the book, then it is possible to figure out which azimuth is correct.

I usually try to take backsights in the main and side passages, but I certainly don't worry about them for splay shots or tiny passages. Sometimes I take backsights in small caves; sometimes I don't. I do try to pay attention and make sure that a 90° shot is to the right of a 0° shot, etc.

I recently was mapping the main passage in a major cave and we had to drop the backsights. The reason was we had an one-day permission to be in a closed bat cave, an one-day permission to cross the adjacent property to get to the cave, and a long walk to the cave (with a big hill to go over). There was no coming back, and we needed to pick up the survey pace. The foresights and backsights had been on the money, so we quit doing backsights. I kept a line plot of the survey though, and the passage was pretty straight--so any blunders would have jumped out at us. The real problem was that big hill--we just had to save enough energy to get over it, and since we couldn't legally come back...

[George Dasher]

Some passages shouldn't be returned to because they are yucky, painful, hateful, and just plain awful.

Those are the passages I usually end up in, not the pretty kind.