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NZcaver wrote:Of course the survey crew will get cranky if there's big borehole and they can easily get 100-200 foot shots (or anywhere up to the 650 ft range of my Disto) but they're told not to. On the other hand, the book person has to sketch the passage anyway. No sense in the rest of the crew being idle/bored for too long.
Cavemud wrote:I personally think you should keep all shots uniform. With as little "slop" as possible between fore and backsights. Calibrated instruments should read the same regardless of which way you're sighting them. So both instrument readers should have the same readings, 180 degrees different. Why should there be any "slop" to them? Maybe someone has some cave mud in their eye?!
jharman2 wrote: Are you telling me that your survey standards do not allow for any error between fore and back sights? I bet those trips are painfully slow, cold and long without much footage at the end of the day.
Scott McCrea wrote:So, a 50' shot with 1 degree of accuracy will potentially produce the same amount of potential variance (2.5') as five 10' shots, right? However, the chance of a blunder is 5x less on the 50' shot.
jharman2 wrote:Scott McCrea wrote:So, a 50' shot with 1 degree of accuracy will potentially produce the same amount of potential variance (2.5') as five 10' shots, right? However, the chance of a blunder is 5x less on the 50' shot.
This is a very good question. The MAXIMUM coordinate change would be the same, but in real life I doubt that the surveyor reproducing the the 50' shot in 10' increments would consistently be off by 2 degrees in the same direction. The averaging effect of the 10' shots may actually make it more accurate. I also don't know that you can say the probability of a blunder would be 5x more, only that there are 5x more chances for blunder.
jharman2 wrote:Most survey projects that I am involved with use the "standard" +/- 2 degree error between fore and back sights. I realized over the weekend that a 50' shot that is off by 2 degrees impacts the accuracy of the survey much more than a 5' shot that is off by the same 2 degrees.
Does anyone's survey standard require increasing agreement between fore and back sights as shot length increases? Has anyone done a mathematical study to show that short shots can tolerate more slop without impacting the overall accuracy of the survey?
Imagine how much easier it would be to survey in a tight nasty canyon with 3 degree slop instead of 2 degree slop!
driggs wrote:The real devil is systematic error, which compounds over many shots and leads to overall inaccuracy and poor loop closure. Systematic error is the result of problems such as using two compasses which are out of alignment, a reader using both eyes to sight the instrument with parallax angle error, applying an incorrect compass correction value. If the forward and backward shots continually disagree, even on "easy" shots, this should send up a big red flag! In this case, stop and examine the instruments and readers to determine where the error is coming from. Unfortunately backsights aren't enough to catch all sources of systematic error, for example I once discovered that the point-man had been placing the folding metal pull-loop of my fiberglass tape on station rather than the 0.0 tape end, which added 3/4-inch to every distance reading.
Dawn Ryan wrote:If it makes any difference, we always go for a less than +/- 1 degree error. Most of the time the error ratio is nothing (we have some good instrument readers that are used to working with each other). However, if the shot is difficult ie: You're hanging over a 50 ft canyon, doing a back bend with water pouring down on you, I will allow for a 2 degree error. Anything over that is not acceptable.
Dawn Ryan
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