A quick question when sighting the prism: How does everyone align the cross hairs with the prism on "locked down" shots?
I usually lock down both vertical and horizontal screws at the lowest possible location at the point (striving for the center of the tip of the rod or even lower if I can see a monument). So in the initial lock-down I've centered horizontally, and I then scroll up with the vertical screw to the vertical center and take the shot.
I usually between an unadjusted closure of 1:30,000 to 1:40,000 with this method. The closure seems to be a lot worse when I sight just at the reflector itself during some traverse tests I've ran.
> A quick question when sighting the prism: How does everyone align the cross hairs with the prism on "locked down" shots?
>
> I usually lock down both vertical and horizontal screws at the lowest possible location at the point (striving for the center of the tip of the rod or even lower if I can see a monument). So in the initial lock-down I've centered horizontally, and I then scroll up with the vertical screw to the vertical center and take the shot.
>
> I usually between an unadjusted closure of 1:30,000 to 1:40,000 with this method. The closure seems to be a lot worse when I sight just at the reflector itself during some traverse tests I've ran.
Kind of elementary but if your closures are that much worse it might be a good idea to adjust your poles and your tribrachs.
Ralph
Well maybe not a whole lot worse, perhaps just "worse" would be the better description.
> Well maybe not a whole lot worse, perhaps just "worse" would be the better description.
The logical next test is to test the centering errors of the prism poles you're using. This is done at very close range with a total station to get an estimate of the standard error of centering by measuring the centering error of, say, twenty different setups on the same point with the same rig as you use.
> > Well maybe not a whole lot worse, perhaps just "worse" would be the better description.
>
> The logical next test is to test the centering errors of the prism poles you're using. This is done at very close range with a total station to get an estimate of the standard error of centering by measuring the centering error of, say, twenty different setups on the same point with the same rig as you use.
:good: I remember that document you put together, do you still have it? I've been through 3 computers since those days.
Ralph
>I remember that document you put together, do you still have it? I've been through 3 computers since those days.
It's probably easier to describe from scratch. You just set the instrument over some point about six feet away from another ground mark and measure the distance between the marks. Tape or EDM, either way will work fine.
Then you set up the prism pole on the other mark at least twenty times, each time measuring the angle from tip of pole to target above. Using the distance between the marks and the measured angle, calculate the offset of the target to line, paying attention to the sign of the offset (whether it's positive/right or negative/left).
From the twenty setups, you'll have twenty offsets. If there is no systematic error of plumbing the prism pole, the mean should approach zero. Calculate the standard error (RMS error) from the set of twenty. That should be a realistic estimate of one orthogonal component of the centering error.
> I usually between an unadjusted closure of 1:30,000 to 1:40,000 with this method. The closure seems to be a lot worse when I sight just at the reflector itself during some traverse tests I've ran.
Another thing to consider is the sensitivity of the pole bubble. Most poles come with a 40' vial, which allows a lot of slop -- even when the bubble is visually centered, it's just not sensitive enough to permit accurate plumbing. Switching to a 10' or 8' vial improves the centering ability considerably. Good adjustment, of course, is still necessary if you want accurate results.