Joe Glidden
We always got at least 1/100,000 closer on a regular basis, including in the mountains. Your tribrachs have to be in good adjustment. I don't know about all topcon equipment, but they sure made good tribrachs in my estimation. I always check that I am over the point in the beginning, middle and end of a setup. That is my experience. If a person is concerned about "fine-leveling" when using a sight on a tribrach, they can get the precise-leveling tools as someone else posted above.
If you fine-level and find yourself not over the point, your tribrach is not in good adjustment. That is a problem whether you "force-center" or not.
Just my 2¢
Sorry for the double-posts above.
James
LOL! You may be right!!!! 🙂
Releveling does not move the point you are over significantly, although it will change the point you see through the optical plummet. So releveling with the finer bubble of the instrument does not invalidate forced centering.
Forced centering (and not shifting the tribrach) may improve your overall traverse data (as compared to re-centering) because you are taking your backsight and foresight from essentially the same point.
However the results are not as good as the closure or least squares statistics would indicate, because of the neglected errors between the points defined by the tribrach and the points on the ground.
If you use a least squares program, you should assign a reasonable centering error to take into account the error between the tribrach and ground. However, the program will not know that the centering errors are almost perfectly correlated between foresight and backsight. It will probably assume random centering errors in all cases, and this may affect the statistics a little.
level
Well hold on a moment here, am I to follow that one does not fine level the instrument when "forced centering", because that would be crazy talk. I have only heard the term in print and have always found it a bit confusing because either the folks online or on the page always assume it is a given as to the definition.
Popping the top and moving up is how I was trained to traverse, as far as whether or not to fine adjust over the point after fine leveling I usually do not unless the set up was disturbed or the distance is more than say a half to one.
I have always thought of it as traversing "through the plate" and going back on decades of work performed that way I have not discovered any sort of gross systemic error. If I were to build my own system from the ground up I would do things that way with Wild gear for the sights with the level vial adapters.
In the end though I kind of think of it as splitting hairs for the purposes of most survey work, if adjusting up over the center of the tack from the rim gives you more confidence then go for it. Like many things in surveying I have seen passionate defenders of both sides of the argument.
Neglecting the equipment is a far more common offense than adjusting or not adjusting up a couple mil.
Of Course Not, But You Were Never Over The Point To Begin
There is no need to occupy every point with the instrument first. The need is to adjust your tribrachs such they are within reasonable agreement of your instrument.
Leica's most precise tribrachs do not come with an optical plummet. They assume if you are interested in precise work you will center with the instrument or use a tribrach adaptor with an optical plummet. Leica charges more money for that adaptor than a tribrach.
Leica tribrachs with a plummet cannot be seen through, so no matter how precise they may be, you cannot verify it.
Topcon tribrachs with optical plummets can be seen through by an instrument or tribrach adaptor plummet. In addition one can readily adjust the circular level and the plummet in the field.
A prudent rich surveyor would send a Leica tribrach in annually for checking and adjustment.
A prudent frugal surveyor can check his Topcon tribrach every time he puts his instrument on it and adjust it himself if necessary.
Quality work does not require the most expensive equipment, it requires quality use of the proper equipment.
Paul in PA
I forgot to put in the approximate area of 100 acres traversed.
0.4 in 1000' is 1 min 23 sec.
But the total angles may be out that much and the shorter sight may/will accommodate the LEC "error".
Try and get the identical results the next time.
Not possible ?
0.4 LEC in a 100 acre parcel is about 1:27,000.
In Ontario, the allowable LEC is 1:5000
As one of my supervisors said about 40+ years ago: "We ain't buildin' a watch !"
Cheers,
Derek
I learned
"Leapfrogging" or "Forced Centering" on an old Kern outfit. Fourth leg was set into the dimple in the top of a tack (or on a pin). When the instrument was set on the tribrach it was ALWAYS fine leveled and the bubble on the fourth leg didn't move. Of course the Kern leveling screws didn't have a whole lot of play so the tribrach had to be really close to level before the instrument was placed in it.
Andy
Lots of good discussion. I have always been told not to adjust the level or slide the instrument around on the plate to make it sit over the point again. It never made sense because if your traverse takes more than a day you will start with brand new setups anyway. I have tested in the field adjusting the instrument back over the point and re-leveling and have still had great closures. Older guys seem to freak out about it, but I think it is just a case of that's how we used to do it so that's how we still should do it. My knowledge and experience with surveying tell me we obsess about the accuracy of points but yet on our important control we have points to set over, but claim to be traversing through the tribrach and not the point. It just never made sense to me.
After reading some of the comments, I wonder that if you are relying upon the position of the instrument and target that is not occupying the position over the nail, why set and leave a nail for reference.
The point is for stakeout later or to locate other things. Thats why setting a point but saying the location was actually where the tribrach was never made sense.
[sarcasm]Hi everybody. I'm Archie Bell of the Drells of Houston, Texas. We don't
only sing but we dance just as good as we walk. In Houston we just started a
new dance called the "Tighten Up."
Tighten it up. You can get up.
Move to your left.
Move to your right.
Tighten it up, now.
Everything will be out of sight.
Come on and tighten it up.
Tighten up now.
You can do it...
[/sarcasm]
B-)
> [sarcasm]Hi everybody. I'm Archie Bell of the Drells of Houston, Texas. We don't
> only sing but we dance just as good as we walk. In Houston we just started a
> new dance called the "Tighten Up."
>
> Tighten it up. You can get up.
> Move to your left.
> Move to your right.
> Tighten it up, now.
>
> Everything will be out of sight.
> Come on and tighten it up.
> Tighten up now.
> You can do it...
> [/sarcasm]
>
> B-)
ya put yer right leg out...ya put yer left leg back...
ya walk around the tripod like ya know what yer about...
ya do the Wild Heerbrugg and ya turn yer self about...
that's what it's all about...
[sarcasm]MC hammer...Can't touch this...[/sarcasm]
lol
DDSM:beer:
Robot. Handheld rod. 8 minute bubble.
3D free stationing with HI=0.00 achieves what forced centering only does in theory: Eliminates the instrument centering error. This contains the bubble/plumb error to the target end. Put the best rod and bubble there you can and shoot it in two faces from at least two places.
Check the rod for plumb, figure out a way to check the bubble(s) daily. Treat it like a precision measuring instrument.
With 3D free stationing, you're never going to occupy that point in space ever again. QED there is no centering error. The next time you set up it will be another 3D free station using ground points and the rod. And when you do there will be none of this superstition about whether you are over the point or need to re-level: The rod is on the point and you are leveling it right now with your best intent.
And it won't matter if the tribrach plummet is out of whack or the tribrach bubble disagrees with the instrument bubble. You're only using the instrument bubble (and the compensators) at the instrument. And no plummet. The tribrach is just a platform for leveling with the instrument bubble.
Never trust a tribrach, even if you keep it under your pillow.
Love it!:-)
Over here it's called constrained centering.
It's a very useful technique to minimise errors in a traverse. It requires well adjusted gear, but then that's par for the course. A wonky bubble in a tribrach is a problem whether using this technique or not.
First I'll cover the issue of relevelling. Even with well looked after equipment it's common for the instrument to be not perfectly level when it's put on the forward tribrach, especially with electronic bubbles. Relevelling is not usually necessary (I've never seen our instrument outside it's compensator range when put on our tribrachs) but if you do so it will generally not introduce any significant error in the position of the instrument (Usually in the tenths of millimetres or less).
This can be shown thus: The instrument axis is around 250mm above the tribrach and the tribrach is around 1.5 above the ground control point. If relevelling introduces an error of 1mm in the intrument position then the ground point will "move" by 6mm (which should be pretty obvious). This is a sign of poorly adjusted equipment. With well adjusted gear the intrument position shift is pretty well insignificant.
Now, as for constrained centering vs relevelling and shifting the tribrach consider this thought experiment. We'll ignore all the errors other than those introduced by unlevelled tribrachs.
With constrained centering you're minimising the errors introduced to the traverse as the instrument and targets always occupy the same points in space and are constrained by the tribrachs. Now, if the tribrachs aren't perfectly level then the control point under the tribrach may not be perfectly under the target/instrument position, however, that error is localised to that control point and does not propagate with the traverse.
If you relevel and shift the tribrach onto the control point, then you're introducing a random error into the traverse that will now propagate forward through all new stations. As the traverse progresses the error ellipse will increase, and become more variable as you "tweak" each tribrach. Rather than having a small error around each control point, you end up with a larger and larger error ellipse for each control point as the traverse progresses, until finally the traverse is closed. Then you get to adjust the traverse to bring it back to "accurate".
Constrained centering is a very useful technique. We use it for all of our traverses and with our Leica 1203 (and professional Leica targets and traverse kit) we normally close at around 1:175000.
It's so consistent that we were able to identify a bad instrument because it was closing at 1:40000 - 1:60000. That instrument was replaced and now it closes better than the 1203. (It should too, being a TS15 1" robot. :D)
I've always preferred to do traversing this way. I never knew the term for it....we just always called it leapfrogging. I always felt it was much more reliable to use full set ups (tripod, tribrach, adapter, and prism) vs bipods and rods. In practice I've always gotten better results running a traverse with three tripods.
I remember traversing like this back in the day. We had a Topcon ITS1B, 1" instrument and the Topcon traverse tribachs. I always fine tuned the instrument after leapfroging, the tribrach fisheye was 10’/2mm level sensitivity while the spirit tube was 30”/2mm. It was never even a consideration, if your equipment is in good shape whatever movement encountered would more than likely be negligible.
Today with my S8 (+/- 0.5"), if my compensator says it's in range I'm off to the races. Then again I don't traverse anymore either.
Ralph