It's a tribrach, no op, with rotating adaptor, with bubble(s), so you can self check any time you'd like.
I always wanted one, but most folks got the one with op in the base.
Any equipment "experts" know why, and can provide advice?
I think I still want one.
Thanks,
Nate
Here is one, in sokia flavor.
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I use those exclusively. The advantage of the rotating adapter is that you are able to be completely assured of centering unlike a tribrach with an optical plummet.
People avoid using these because of the expense. You have that problem solved here and that is the real thing, not the "yellow" facsimiles you usually see on ebay.
Your second picture looks like my equipment except that those are re-branded Topcon tribrachs.
When I went out on my own 20+ years ago, I decided against using tribraches with built in optical plummets.?ÿ Instead I use either the instruments laser plummet, a Leica SNLL121 carrier with laser plummet or the GZR103 carrier with optical plummet.?ÿ With the Leica carrier, the prism and instruments heights are the same which is really nice for control.?ÿ Checking the plummet adjustment is as simple as turning it 180 and if it is out, adjusting is just as simple.?ÿ No need for a jig or a ceiling.
The SNLL2 is pricey, over 1K now days and the lasers tend to go out.?ÿ But it is really nice in a tunnel or at night.?ÿ The GZR103 is well worth the price.
The SNLL2 is pricey, over 1K now days and the lasers tend to go out.
I only use mine occasionally, so have never had a laser outage.?ÿ How expensive is the fix?
It was about half new.?ÿ The problem is they can never figure if it is the laser or the button going out.?ÿ So you get to buy both.?ÿ If I could get my hands on the parts I'd fix them myself.?ÿ I have gone through about 5 of them over the last 20 years.?ÿ I did have multiple crews at one point so its not like they fail that often but it still hurts.
On a side note, I do have two in which the height hook port is machined about 2 cm higher than is should be.?ÿ Not sure how that came about but is very obvious.
All of my tribrachs are without a built in optical plummet. We use Trimble rotating optical plummets to setup, and then remove them and use standard "hockey puck" adapters for GPS or prisms. The two total stations I use have built in rotating OP's.?ÿ
As others mentioned, more accurate, and self checking
What I mean is the o/p is in the adaptor, not in the tribrach.
And, it rotates.
They were expensive. But, the concept seems sound.
On another note, you can do good work with a mal-adjusted tribrach, as long as you orient it the same every time, and it does not have loose parts!
N
The Trimble traverse kits have the OP in the puck, with a 45" level vial. Pricey, but absolutely worth it for quality work or precise control networks.
Excellent piece of kit. But now with the advances in compensators, perhaps a bit moot. Though the compensators need checked and calibrated. I've seen some of the touted self-calibrating compensators, and saw in a factory how those are developed, built, and tested, but time will tell how well they hold up over time.
Yep, compensators these days are pretty impressive, but they won't help with centering and measure-up error, which is where a lot of TS traverses and networks fail. As much as I love the interchangeable tribrachs/prism assemblies, I hate that they encourage forced centering, which can absolutely blow a network if you're going for high precision.
I do not know that it is fair to say that they encourage forced centering, equipment is passive we make our own choices.
I hate that they encourage forced centering, which can absolutely blow a network if you're going for high precision.
How does force centering blow a network?
When using forced centering, each time the TS moves up, if the tribrachs are not relevelled and recentered, centering and measure-up errors at two of the stations (backsight and instrument point) are exactly the same, and thus not independent. And of course, when measuring sets with a total station, it doesn't matter how many angles you crank, those centering and measure up errors won't change.
Because two of those tribrachs are not being moved, the measurements between the TS and backsight will be almost identical to those of the previous setup.
An old-school traverse closure such as for a compass rule adjustment will look really, really good. But it is more representative of the tribrach positions than it is of the points over which you were set up.
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And that's because only about one-third of the error components which have the largest impact on propagated error (centering and measure-up) are actually independent.
As far as least-squares analysis software is concerned, those setups are composed of independent measurements. Because the majority of centering/measure-up errors are not independent, when least-squares analysis is run, those near-identical measurements between previous foresight and current backsight will look really good, statistically speaking, and thus make your final propagated errors far smaller than they actually are.
It is possible to fool the analysis by setting your centering and measure-up errors to zero, but that's not really best practice, and of course when you do that your final error ellipses will be far larger - because they are, at least with respect to those points on the ground.
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Again, if you're never coming back to those points again, or it's just control for topographic measurements, it's not that big of a concern - we still use forced centering for a decent portion of our traversing
But if you have to quantify the precision of your control points and develop a tight network, forced centering is just not best practice.
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Many people argue over the benefits of force centering. My main observation is that both camps are completely sure they are the best way...and certainly the ONLY RIGHT WAY.?ÿ
I would also submit that if you are using the equipment shown in the OP in the correct way, your error in your centering should be little more than background noise in the loud banging of the tripods and refraction and all the rest.
Essentially, we are getting awful close to politics and religion in this sort of rabbit trail.
I agree that in theory a forced center network can have perfect closure at the center of the instrument but not accurately represent the control monuments on the ground because of setup error.?ÿ But I would argue that errors in measure-up and to a lesser extent centering are more blunder than systematic or random error.?ÿ Those blunders can be minimized by proper field techniques and well adjusted equipment.
For a typical control setup I check the centering and level with two (2) devises, the precision carrier and the instrument.?ÿ The instrument/target height is checked on at least two (2) occasions, when it first set-up over and when the instrument is over the point.?ÿ During the network adjustment phase I input a realistic error budget for my centering and measure-up based the equipment I use.
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The simple (and cheap) answer is to be kind to your tribrachs. All ours travel in comfort, foam cut-outs holding each one.
They are checked regularly. In the office we have a tripod with a deep stencil on the top, shaped to the exact foorprint of the tribrachs. It's a matter of a couple of minutes to drop the tribrach in, point the o/p at a gridded target underneath, then turn it through two lots of 120 degrees to check. Three readings: adjust to mean if necessary. The set of 6 we keep for precise traversing takes less than half an hour to do. Checking them on a regular basis fills in those gaps when waiting for somebody to "phone back in 5 minutes", or waiting for the kettle to boil!
We also use separate plate bubble carriers to check the exact levelling on precise work. A few miniutes in preparation is a lot better than having to go back to site.
Always preferred having the OP in the adapter vs the tribrach.?ÿ Love the Topcon and Sokkia ones, but even the knock-off brands aren't bad, since it's easy to see if they're in or out of adjustment simply by turning it.
Tribrachs with the OP, you can't tell just by looking at them whether they're good or not.?ÿ So you're either checking them every day or going on faith that everyone is treating them the way they should.
I agree with this your outlook on this. The biggest issue that I have with force centering is actually to do with the measure-ups.?ÿ
An experienced field crew knows the offset (or lack of one) between the mirror and the instrument, and if they get lazy the instrument operator may not actually measure the instrument height off the mark. Essentially, this is a force centering on the vertical...and it will often create a blunder in the dark that is waiting to be stumbled over like a kind of a land mine.
The same is true of the horizontal aspect. That means that the setup needs to be completely checked, even if forced centering.
Always preferred having the OP in the adapter vs the tribrach.?ÿ Love the Topcon and Sokkia ones, but even the knock-off brands aren't bad, since it's easy to see if they're in or out of adjustment simply by turning it.
Tribrachs with the OP, you can't tell just by looking at them whether they're good or not.?ÿ So you're either checking them every day or going on faith that everyone is treating them the way they should.
This is the same as my thinking.
Why they were rejected, based on the forced centering issue seems obscure to me.
They seem to me, to be identical. (On the forced centering issue).
I'm either dumb, or I'm out of this line of logic.
After setting up a tripod, and tribrach, they are not moved.?ÿ
N
