So the other thread is totally derailed, but here is my thought on the subject. Yes, we use bi-pods to traverse with almost exclusively. Obviously it's better and more steady to have than either a steady stick, lath, or just holding it. I personally prefer then to the tripod because of the way that it collapses and is able to be carried. However, if I had no choice of a bi-pod and only a tripod, we would use the tripod.
Kent makes a good point as to why he prefers the tripod, and it just can't be argued with from a practical standpoint of quick adjustment in the field. However, there are other field procedures that allow for checking of the pole in the field. Typically, we check all of the poles at the office. 99% of the time, they require little to no adjusting.
I am not a fan of forced centering so the bi-pod is an excellent way to keep weight down and still not sacrifice accuracy through random errors of swatting at mosquito or just plain old fatigue.
Agreed. The way the rods are carried in the vehicle can minimize the adjustment needed for the bubbles, as well.
> I personally prefer then to the tripod because of the way that it collapses and is able to be carried. However, if I had no choice of a bi-pod and only a tripod, we would use the tripod.
Just as a point of terminology here, the prism pole tripod is something that looks like the SECO model supporting the pole with the GPS receiver on it in the photo below. The third leg is behind the pole. It really is a tripod.
That tripod is collapsible and is actually easiest to carry with the pole it it by means of a simple strap that loops around the feet of the tripod. It is no problem at all to carry two prism poles with tripods in that manner.
> I am not a fan of forced centering so the bi-pod is an excellent way to keep weight down and still not sacrifice accuracy through random errors of swatting at mosquito or just plain old fatigue.
As someone mentioned in the earlier thread, it is remarkably easy to measure the centering accuracy of a prism pole in a bipod. The problem, of course, is it depends entirely upon the state of adjustment of the bubble. The natural advantage to using a prism pole tripod is that you can easily verify the state of adjustment at every setup and can adjust the bubble whenever needed. It maintains a better process quality in better uniformity and lower centering errors than any other centering method using a prism pole that would be practical for most field use.
> > I personally prefer then to the tripod because of the way that it collapses and is able to be carried. However, if I had no choice of a bi-pod and only a tripod, we would use the tripod.
>
> Just as a point of terminology here, the prism pole tripod is something that looks like the SECO model supporting the pole with the GPS receiver on it in the photo below. The third leg is behind the pole. It really is a tripod.
>
>
>
> That tripod is collapsible and is actually easiest to carry with the pole it it by means of a simple strap that loops around the feet of the tripod. It is no problem at all to carry two prism poles with tripods in that manner.
>
> > I am not a fan of forced centering so the bi-pod is an excellent way to keep weight down and still not sacrifice accuracy through random errors of swatting at mosquito or just plain old fatigue.
>
> As someone mentioned in the earlier thread, it is remarkably easy to measure the centering accuracy of a prism pole in a bipod. The problem, of course, is it depends entirely upon the state of adjustment of the bubble. The natural advantage to using a prism pole tripod is that you can easily verify the state of adjustment at every setup and can adjust the bubble whenever needed. It maintains a better process quality in better uniformity and lower centering errors than any other centering method using a prism pole that would be practical for most field use.
Kent, I have used the tripod version for the range pole, and I'm not knocking, but but the way the bipod carries seems better for us. Like I said, I'd use the tripod if I didn't have a bipod.
I don't disagree with your procedure, just noting that there are others just as accurate. I'm not looking to fight. Agree with me or not, it's up to you. We use 10' bubbles and the instruments to verify plumbness instead of rotating within the housing. Both work. You're more solo than not and I'm more employees than solo, so both work.
> I don't disagree with your procedure, just noting that there are others just as accurate.
Well, it's something that can be readily tested. I doubt very much that if you ever do you'll find the centering accuracy of a prism pole in a bipod is equal to that of the same pole in a tripod.
> ... I doubt very much that if you ever do you'll find the centering accuracy of a prism pole in a bipod is equal to that of the same pole in a tripod.
Now take your East Texas non-sense home and get your @#$%^&! shinebox
> > ... I doubt very much that if you ever do you'll find the centering accuracy of a prism pole in a bipod is equal to that of the same pole in a tripod.
>
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> Now take your East Texas non-sense home and get your @#$%^&! shinebox
😛 Hilarious!!!!! What movie was that?
> > I don't disagree with your procedure, just noting that there are others just as accurate.
>
> Well, it's something that can be readily tested. I doubt very much that if you ever do you'll find the centering accuracy of a prism pole in a bipod is equal to that of the same pole in a tripod.
First, yes I've used your method and it works. We just have bi-pods.
Second, the centering error in either method of holding the pole is the same, if you use a center punch and punch the nail. The tip sits in a divot. The error is in your punching ability, not in setting the range pole up.
> Second, the centering error in either method of holding the pole is the same, if you use a center punch and punch the nail. The tip sits in a divot. The error is in your punching ability, not in setting the range pole up.
Kris,
The centering error is measured from the bottom of the pole to the center of the target. The punch mark just allows your positioning of the rod on the nail to be repeatable, but has little to do with the centering error Kent is talking about testing for.
Alan
I've never thought of it like that. When I set up my LSA, I understood (possibly incorrectly) the centering error of the target to be the same as the centering error of the instrument.
Our method for turning angles is to sight as low as possible on the rod, and pull straight up to the glass and not correct up at the glass, thereby lessening the error.
I suppose one could build a testing ground whereby the point placed in a divot, and set up multiple times and the angles wrapped from multiple observations, setting the rod up each time, one could build a statistical model for that error propagation. I've never done it though, we just keep the bubble in good adjustment and sight low on the pole and pull up. 🙂
Alan
Don't forget that the error in centering the target over the point also affects the distance. For what it's worth, we always use fixed targets on tripods.
Alan
> I've never thought of it like that. When I set up my LSA, I understood (possibly incorrectly) the centering error of the target to be the same as the centering error of the instrument.
No, the standard error of centering a theodolite or total station with an optical plummet that rotates should be about +/-0.24mm. That is significantly different from what is possible with a prism pole by any practical method of using it.
> I suppose one could build a testing ground whereby the point placed in a divot, and set up multiple times and the angles wrapped from multiple observations, setting the rod up each time, one could build a statistical model for that error propagation.
Instead of that, why not do it the easy way? just set a spike with a punchmark in the ground and set up your total station about six feet away from it in a position where you can see the actual punchmark of the spike. Now have whoever sets up the prism pole set it up (with a prism and target) on the spike. Tell them to use whatever procedure they following ordinarily in the field.
Measure the distance from the instrument to the prism pole station. Tape or EDM, either will work. Extreme accuracy in the distance isn't supercritical to the calculation.
Measure the horizontal angle from the point of the prism pole resting in the punchmark to the target above it. Calculate the off-line component of the centering error taking care to keep track of the sign (right = +, left = -)
Have assistant set up the prism pole on the spike again and repeat the angle measurement.
Do this 15 or 20 times and you'll have a series of basically exact measurements of the centering errors perpendicular to the line of sight to the instrument.
Calculate the standard error of the series and the mean.
If you want to get fancy, you can repeat the test with another pole to see how much variability there is from pole to pole with the bubbles differently adjusted.
Alan
> I've never thought of it like that. When I set up my LSA, I understood (possibly incorrectly) the centering error of the target to be the same as the centering error of the instrument.
>
> Our method for turning angles is to sight as low as possible on the rod, and pull straight up to the glass and not correct up at the glass, thereby lessening the error.
>
> I suppose one could build a testing ground whereby the point placed in a divot, and set up multiple times and the angles wrapped from multiple observations, setting the rod up each time, one could build a statistical model for that error propagation. I've never done it though, we just keep the bubble in good adjustment and sight low on the pole and pull up. 🙂
Kris,
As I understand Kent's position on the testing of centering errors, when running multiple crews, and subsequently, multiple rodmen, I'd think that Kent would probably advocate setting up a realistic centering error model per man, or woman.
For the centering test as Kent describes it, I would think a properly adjusted bubble to be a given. I suppose what you're truly trying to model would be the biases in the rodman's eyes, thumbs, and general ability to level a properly adjusted rod in a bipod or tripod.
While using nails, or spikes, with divots, punch marks, or whatever station mark you decide is a great practice, a realistic error model could be arrived at for whatever choice of control points you utilize.
Alan
> While using nails, or spikes, with divots, punch marks, or whatever station mark you decide is a great practice, a realistic error model could be arrived at for whatever choice of control points you utilize.
Yeah, someone who wants to use, say, unpunched 60d nails as control points should run the test on them, but instead of sighting the tip of the pole, sight the center of the nail since what you're interested in measuring is the centering error over the ground mark and the mark itself is the center of the nail, presumably.
My thought on testing is to determine how well the equipment should be performing in ordinary usage, so that staff who are consistently doing a poor job of basic tasks can be identified and properly trained.
Goodfellas