I started a post recently about something else and it led to wondering "why have a Tribrach with an optical plummet"
My current tribrachs have optical plummets and I check them now and again but I worry about them becoming out of perpendicular when I'm out in the field. Surely its much better to have the plummet on the tribrach carrier and to be able to rotate it round to check it for perpendicular.?ÿ
So it makes me wonder "why have a Tribrach with an optical plummet"
Ideally I'd agree with you, but traverse kits with the O/P in the prism adapter are expensive, and with GPS there's not as much need for specialized traverse hardware.
I think you could also make the argument that if you're changing out the instrument and prisms between tribrachs without breaking setup then you don't have independent observations.
Ideally I'd agree with you, but traverse kits with the O/P in the prism adapter are expensive
yes but so are tribrachs with an optical plummet. Having the plummet in the prism carrier means you can have cheaper tribrachs with no optical plummet
I think you could also make the argument that if you're changing out the instrument and prisms between tribrachs without breaking setup then you don't have independent observations.
Do you really want independent observations?
Imagine (for the sake of argument) the old days with a four man crew. On his first day the new guy is sent to a foresight to set up a target. The instrument man shoots it and on arrival sees it is not over the point. He moves to the point and continues the traverse. What good is that for an independent observation?
I feel the same way about sliding the tribrach after its initial set-up. I would rather traverse through a point within a couple hundreths of the nail than introduce dozens of independent random errors.
Lee makes a valid point about the GPS - many users won't have a separate base with O/P for the GPS.
We quite often need to set up a number of tripods at a time and leave them up all day, but we wouldn't necessarily want to have prisms in place all day. (Good example is our instrument baseline where we have 8 tripods set up while all the gear is checked, but only one calibration prism set, which moves back and forth along the 1km. line for each instrument).
Having the O/P on the tribrach means that somebody can go around setting up tripods in advance without taking a target with them. On a monitoring job we might need, say, 8 tripods placed, but no set-up needs more than 4 targets to be observed. Minimising the number of targets used cuts down one source of possible errors.
I suspect that tribrachs probably get handled with a little more care than prism carriers. Our tribrachs are all Wild GDF6 - still in excellent trim after 40 years of hard work and probably good for another 40 years. You get what you pay for.
I prefer the OP in the prism holder/adapter rather than the tribrach, but I see the point in having it on the tribrach at times.?ÿ I'd get some of both, and check/adjust often.
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If your goal is an artificially high precision that will get the job done. What we are after is an expressed relationship close to true with high confidence. That demands procedures that will introduce random errors while minimizing most sources.
I started a post recently about something else and it led to wondering "why have a Tribrach with an optical plummet"
My current tribrachs have optical plummets and I check them now and again but I worry about them becoming out of perpendicular when I'm out in the field. Surely its much better to have the plummet on the tribrach carrier and to be able to rotate it round to check it for perpendicular.?ÿ
So it makes me wonder "why have a Tribrach with an optical plummet"
Why have a tribrach with optical plummet? Because it works well, can reduce the weight and space of the kit, has less parts that have to be square, and hence less components that can be fouled, and in my experience (the leica gear at least) is more than robust enough to cope with normal field use and remain in good adjustment for years.
if you are worried about your tribrachs then check them as often as you like in the field, and then stop worrying. I??m sure you could devise a quick procedure to field check them that is as accurate as you??d need.
if you are worried about your tribrachs then check them as often as you like in the field, and then stop worrying. I??m sure you could devise a quick procedure to field check them that is as accurate as you??d need.
that's the point. You can't check a Tribrach with otical plummet in the field.
if you are worried about your tribrachs then check them as often as you like in the field, and then stop worrying. I??m sure you could devise a quick procedure to field check them that is as accurate as you??d need.
that's the point. You can't check a Tribrach with otical plummet in the field.
Well, you can check them in the field if you like, but it takes time.?ÿ And you usually only do it if you think something has happened to it.?ÿ But I get your point.
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If your goal is an artificially high precision that will get the job done. What we are after is an expressed relationship close to true with high confidence. That demands procedures that will introduce random errors while minimizing most sources.
Not sure who you're replying to there, it's not obvious on my screen.?ÿ But you're right after me, and if you're replying to me, I've no idea what you're talking about.
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Almost all of our tribrachs do NOT have an optical plummet. I prefer to use the instrument (rotatable) and a separate optical plummet (also rotatable) which has the same HI measurement as the notch on the instrument. We do have some with plummets, they came with the scanner and target kits. The great thing about this setup is that all of the HI measurements are the same. I just add 0.042 m to the slope measurement to get the HI of the prism, or subtract 0.041 m to get to the tribrach plate. I can input that slope measurement into the data collector which converts it to true vertical to the optical axis. I find trying to measure a slope HI to the prism is inherently inaccurate.
The tribrachs are less expensive, and I don't worry as much about them getting jolted out of adjustment. We carry the rotatable optical plummet in a padded bag. And, as others have mentioned, the rotatable plummets are able to be field checked, and, if out of adjustment, can be compensated for in the field (rotate 180/split the difference). I feel that we can get 1 mm centering accuracy this way consistently, and also the same accuracy for HI measurements.?ÿ
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Hi John - what is the mysterious separate optical plummet you refer to? Do you have a picture or a link?
if you are worried about your tribrachs then check them as often as you like in the field, and then stop worrying. I??m sure you could devise a quick procedure to field check them that is as accurate as you??d need.
that's the point. You can't check a Tribrach with otical plummet in the field.
That??s not ??the point? at all, because I can check one in the field, quite quickly and accurately using a total station, mini pole and the backlight setup. I??d hoped my post might make you think about what the tribrach problem would look like and how you??d solve it with the equipment you have on hand
We use a Trimble S6 total station (and an SX10 now), and "Trimble" tribrachs (from Hungary). The plummet is made to have the same HI measurement as the lower notch on the S6. Here are a few pics....the measuring point swings out and pops into place. We don't usually use it as a base for a prism or GPS, just to setup over the point and then we use a standard rotating adapter (0.032 m thick). Prisms are all 0.05 m from the bottom of the holder to the prism, and then 0.05 m to the top of the holder, where we have a spacer bolt and then the GPS ARP. So everything is a consistent height, once we measure to the notch all of the other HI's are easily calculated. We have a spacer for our scanning spheres that make them the same height as the S6 (0.196 m above the tribrach plate), and the TX8 is also a known height (0.265 m above the tribrach plate). Actually I have a program that you enter in the measured notch height and it computes and displays all of the possible other heights, I just need to make it an android app to have on my phone.?ÿ
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Interestingly, when we bought the optical plummet I did not know it was the same HI as the notch on the S6, we discovered that at a later time.?ÿ
?ÿAnother note about tribrachs and costs...I have told this story before...We were setup on a lock wall when they were dewatering a lock chamber for repairs. A sudden storm came up, and we grabbed the S6 and took it inside, but left the backsight tripod/tribrach/prism sitting on the wall. It was blown off the wall, struck the vertical lock wall face 60 feet across the chamber, and fell to the bottom (rock/dirt) about 35 feet below. The only damage was to the tribrach, but the tripod (Crain) and prism (seco mini) were OK. So I had to replace a relatively inexpensive tribrach without a plummet. Lost a similar setup but with an R8 on top into the river off of a lock wall in 2016 during a storm, never recovered that. Now we tie a safety line to a railing for GPS setups on the lock walls.?ÿ
wonderful thanks a lot. Looks like a great setup
if you are worried about your tribrachs then check them as often as you like in the field, and then stop worrying. I??m sure you could devise a quick procedure to field check them that is as accurate as you??d need.
that's the point. You can't check a Tribrach with otical plummet in the field.
I take it you don??t have any tripods that look like this or know what it means....
Most of mine look like that
Hi Conrad. Okay, what I should have written was that in my opinion you can't check a tribrach with optical plummet easily in the field. I've tried it by tracing round it and have never got on with it or felt it was very accurate.
I think using a carrier with a rotating optical plummet is easier and more accurate - in my opinion.