Always looking for ways to reduce my equipment - often fly to jobs "up north" on small planes.
Have occasionally used a bipod and prism pole as part of a traverse, apparently with good results, but still a bit leary. Just wondering if any of you do the whole shebang that way? I'm sure some would frown on the idea as not quite the done thing.
I realise the bubbles on a prism pole are less sensitive than a tribrach, but I thought of using one of those orange 8" level rod bubbles that you hold against a rod - figure out a quick attach/release system - keep it bang on and use it rather than the attached bubbles - and obviously stop using the prism pole as a pry bar or digging implement!
Just a thought.
> Have occasionally used a bipod and prism pole as part of a traverse, apparently with good results, but still a bit leary. Just wondering if any of you do the whole shebang that way? I'm sure some would frown on the idea as not quite the done thing.
Sure, I've used prism poles in prism pole tripods for about 30 years. They work great as long as the bubble in the level vial is adequately sensitive and you carry an Allen wrench to adjust the bubble in the field as necessary. There is a simple test procedure to measure the uncertainty in centering of the target/prism over the ground mark using a prism pole in a tripod. I wouldn't consider using a bipod for the reason that you can't check the bubble on the prism pole using a bipod as you can a tripod.
The centering accuracies I've measured for the target/prism pole/prism pole tripod combination have been on the order of +/-0.4mm in orthogonal components (+/-0.4mm perpendicular to line of sight and parallel with it).
Thanks Sir,
The procedure you mention would be to rotate the pole and check the bubble stays centered?
> The procedure you mention would be to rotate the pole and check the bubble stays centered?
Yes, and if the bubble is slightly out of adjustment, to see that the bubble position is the mirror image of its position when reversed 180 degrees. For the best quality, you check the bubble by turning it through all four quadrants.
>There is a simple test procedure to measure the uncertainty in centering of the target/prism over the ground mark using a prism pole in a tripod.
Here's a link to a description of that simple test procedure:
[msg=182529] http://beerleg.com/index.php?mode=thread&id=182529#p182550 [/msg]
I generally use the tripods and tribrachs when I can drive the ATV or UTV to the setups.
Having said that, Chris and I completed a traverse yesterday that closed 1:78,620 with prism poles and bipods. It was wooded rural tract. I turned 3 sets, direct and reverse, shooting distances both forward and backward.
With properly adjusted equipment and proper procedures, you should be just fine.
Good luck!
It seems to be overkill to traverse tripod-to-tripod. I would assume that you are trying to achieve 1:5000 precision on rural surveys. If that is the case, prism w/bipod is more than sufficient.
> I realise the bubbles on a prism pole are less sensitive than a tribrach
It depends on the bubble. Most poles come with a 40-minute bubble, but Seco makes a 10-minute bubble that you can use as a replacement. They're a little spendy -- I think I paid about $90 for the one I bought a couple of years ago -- but well worth it in my opinion.
Another option, when conditions are right, is to use a very short pole. The Leica mini-prism (GMP101) comes with a snap-in point that gives you an HR of 0.39 foot. It also has a pretty sensitive bubble built in, which makes for very accurate centering.
> Most poles come with a 40-minute bubble, but Seco makes a 10-minute bubble that you can use as a replacement. They're a little spendy -- I think I paid about $90 for the one I bought a couple of years ago -- but well worth it in my opinion.
Dang, why don't you call me next time you're looking for a 10-minute prism pole bubble? I'll give you a brother-in-law price of $45 each and that will include the screws and backing pad. (And I sure hope you never find out what price I pay.)
The $90 or so number I mentioned was for the whole assembly that clamps to the pole and included shipping, but the APE price for a replacement vial looks great!
"The centering accuracies I've measured for the target/prism pole/prism pole tripod combination have been on the order of +/-0.4mm in orthogonal components (+/-0.4mm perpendicular to line of sight and parallel with it)."
How much error do you assume in the prism pole point repeating in a punch mark?
> "The centering accuracies I've measured for the target/prism pole/prism pole tripod combination have been on the order of +/-0.4mm in orthogonal components (+/-0.4mm perpendicular to line of sight and parallel with it)."
>
> How much error do you assume in the prism pole point repeating in a punch mark?
If the point is sharp and the punchmark is distinct, the centering error of the point in the punchmark is negligible. Obviously, replacing the point occasionally when it gets dull is important, as is keeping the bubble in good adjustment.
By the test procedure I described in the thread linked above, one can measure the centering of the point in the punchmark in the same way that centering of target over point is measured.
Kent
What type of Total Station are use that can measure sub-millimeter?
> What type of Total Station are use that can measure sub-millimeter?
Well, when you consider that a centering error of 1mm at 50m introduces an angular error of 0°00'04", any total station that can measure angles with uncertainties less than 0°00'04" can detect a centering error below the 1mm threshold.
For rural work, as traverse legs lengthen past 200m, centering errors at even the 1mm level become much less significant. At 200m, a centering error of +/-0.001m (s.e. of orthogonal components) introduces an angular error of +/-0°00'01" s.e. at each target, BS and FS, with the contribution of both BS and FS being +/-0°00'01.4" When you add (by root-sum-of-squares) that uncertainty to the fundamental uncertainty of the instrument, you get these results for different instrumental standard errors for total stations that can measure directions with standard errors of 1", 2", and 5", respectively:
[pre]
Inst. s.e. Targets Net s.e.
1.4" 1.4" 2.0"
2.8" 1.4" 3.1"
7.0" 1.4" 7.1"
[/pre]
Obviously, if one were using a 5" total station, there would, for all practical purposes be no net improvement in reducing target centering errors below 1mm.
You can't catch this guy out.LOL!
I did a great many section closures with prism poles and bipods. The bubble is just for getting close. I always followed up with a plumb line and refined the pole as required. Typical closures in the .2 to .3 foot in 4+ miles and 10 stations more or less. Typical angular error in the order of .3 seconds per station. One second direct reading theodolite and top mount EDM. Reducing the error to avoid time in error analysis always justified the extra few minutes to plumb the pole with the ancient plumb line.
Not sure how you would use the plumb line. Can you explain? Hold it in the centre of the prism and check the bottom, then from the side axis and repeat?
Stand back some 15 feet from setup. Use the plumb line to verify the edge of pole along it's full length, opposite each adjustment leg. Adjust as necessary. Just far more accurate than any bubble I ever saw.
Cool!
> Stand back some 15 feet from setup. Use the plumb line to verify the edge of pole along it's full length, opposite each adjustment leg. Adjust as necessary. Just far more accurate than any bubble I ever saw.
Using that test procedure described in the thread linked above, it would be pretty simple to actually measure the standard error of rod plumbing by eye with plumb bob string. I'd be very surprised if it worked better than can be done with 10-minute bubble in good adjustment on a prism pole in a prism pole tripod. The reason is because most unaided human eyes can't resolve angles smaller than a minute, which would mean that separations less than 1.3mm wouldn't resolve by eye at 4.6m (15 ft.).