Traversing

googled it
see http://www.state.nj.us/transportation/eng/documents/survey/Chapter3.shtm
3.6.3.2 B
I call it leap froging

Short BB answer

All stations in traverse occupied with instrument to prism, thus "leapfrogging" without pickets and plumb bobs.

With good atmosphere conditions, with 10 to 15 stations,we achieve a lineal error of closure .3 to .4 feet.

Cheers,

Derek

Thats the procedure we use, just had not heard the term before. I thought it might be something like when you move the instrument upnto the previous foresight and the tribrach is off a little, you adjust the setup back over the center again. I was always told not to do that.

I've always wondered what you were supposed to do when you eventually come back to your closing point.

Or on a multi-day traverse, how do you resume your forced centering?

Jeff

No expert here...

As I have grasped, the practice of "forced centering" emerges from the difference in the precision of the bulls eye level vial on a standard tribrach and the much more precise vial on a total station. When the forward tribrach is leveled by that vial and you put the total station in the forward tribrach, typically you'll find that the instrument is not necessarily even in range of the tilt sensors. But once you have tweaked the leveling of the total station, you are no longer over your point, so the thing to do it seems is to shift the instrument slightly to put it directly over the tack.

But once you move the tribrach, it really isn't over the point that was observed during the forward measurement. Very close perhaps, but still not the same.

A couple of solutions suggest themselves. One could either use a tribrach adapter with a plate vial of similar precision to the typical one on a total station. The problem is, those tend to be fairly expensive compared to the more typical puck style adapters. An alternative would be to purchase a tribrach adjusting cylinder with a plate vial in it. Use it to level the tribrach more precisely before putting the tribrach adapter and prism in the tribrach. Then you could expect to be level enough and over the point well enough to not need to relevel the tribrach and shift the instrument. This adjusting cylinders typically sell for under $100 so would provide and economical alternative to forced centering.

Forced Centering

One of the advantages of forced centering is that the instrument is locked in the same position as the foresight before it and the backsight after it.

Another advantage is that one is using fixed tripods which are heavier, more rigid and more stable. However if the tripod goes out of level it should be releveled and recentered. Yes there may have been some error during the observation, but it does not improve the traverse to continue on with that error. The traverse is based the points on the ground.

When one returns to continue the traverse, yes you reset up over several points, but higher precision is maintained because of the overal scheme of the process.

To get the most benefit from forced centering these are some basics:

1/ Use Topcon or equivalent see through plummet tribrachs, they should be adjusted under the instrument first using the instrument plummet and bubbles. Then the tribrach plummet and circular bubble are adjusted.

2/ Use Topcon or equivalent tribrach adaptors with a rotating plummet eyepiece and at minimum one vial bubble.

3/ Use prisms mounted in targets, use prisms with an aiming device or use prisms where the the visual center is the physical center (careful these have odd offsets). Aiming at the mirror instersects on a maligned prism is precise but not accurate.

4/ Use consistent equipment or make it consistent. For instance when on a small project, rule 2 is not followed. However my standard prism/tribrach setups all measure a rod height that is -0.31' less than my instrument height. If I forget to measure or more likely forget to immediately record the measurement I can still come up with a backsight height. If I get to the next setup and the difference is beyond 0.30-0.32' I remove the instrument, replace the prism and remeasure. When I am not forced centering I use multiple rods with bipod or triped stabilzers. Each of those rods comes out of the vehicle with a minimum 5.00' rod height. They can go higher but not lower.

I carry elevations on all my traverses, whether ot not I am taking any other shots with elevations. It is a very good check on traverse closure, and I have had traverses that closed vertically better than horizontally. Elevation is a required check should you occupy traverse points with GPS.

The object of a precise traverse is consistency, forced centering being one way to achieve it.

Paul in PA

Sorry About Your Tribrach, But...

...if you put your instrument in a tribrach, level it and you are not over the point, then that tribrach was never over the point in the first place. Recenter with the instrument over the point and hope you can live with the positioning error. Please note that when you move ahead the tribrach will still be over the point, so do not relevel it on it's own.

This is why you adjust your tribrachs under your instrument. In most cases when I set the instrument on one of my already setup tribrachs my instrument relevel is minimal. After observing the ground point through the instrument plummet at several rotations I decide if the setup is within my error budget. If not I readjust and note if my difference was still within my error budget. In almsot every case a readjustment is still less than the probable error of a rod sight setup.

Paul in PA

This "forced centering" is how I learned to traverse and SOP where I worked. When we fine-leveled over a point that had already been set up as our foresight, the tribrach level-bubble was still floating inside its ring. You need to keep your tribrachs in good adjustment. If it moves over a quarter-hundredth or a half-hundredth, I didn't worry about it. You are still turning through the same tribrach in the same position that the sight was on. Since you are more-precisely level, it just appears to be in a different position. The theory being that you turn through the same point you turned to. There is a half-hundredth slop if the fine-level proves that, but it is not slop in the traverse itself, and it is only in (how well your set up over) that point; making for better closer. We always got excellent closure using this method (with due care in solid setups, etc). (Back in the day) We always used a t2, a separate edm (we didn't have a built-in edm, and we remounted the edm on the donut vs. mounting it on top of the t2), and those topcon tribrachs that you can see your setup through the tribrach itself, and through the gun when you fine-level. One of the best inventions by topcon in my estimation.

Anyway, that's my take on the "forced-centering". I don't recall calling it that when we did it. And as someone pointed out, if you couldn't complete the traverse in a day, you had to pick up the beginning tripods before you were able to loop back around (that would be the original setup and the backsight setup. If we could do it all in a day, we would leave the first two tripods up all the way through.

P.S. mark those tribrachs that come off the point slightly and adjust them or send them in for adjustment.

This "forced centering" is how I learned to traverse and SOP where I worked. When we fine-leveled over a point that had already been set up as our foresight, the tribrach level-bubble was still floating inside its ring. You need to keep your tribrachs in good adjustment. If it moves over a quarter-hundredth or a half-hundredth, I didn't worry about it. You are still turning through the same tribrach in the same position that the sight was on. Since you are more-precisely level, it just appears to be in a different position. The theory being that you turn through the same point you turned to. There is a half-hundredth slop if the fine-level proves that, but it is not slop in the traverse itself, and it is only in (how well your set up over) that point; making for better closer. We always got excellent closure using this method (with due care in solid setups, etc). (Back in the day) We always used a t2, a separate edm (we didn't have a built-in edm, and we remounted the edm on the donut vs. mounting it on top of the t2), and those topcon tribrachs that you can see your setup through the tribrach itself, and through the gun when you fine-level. One of the best inventions by topcon in my estimation.

Anyway, that's my take on the "forced-centering". I don't recall calling it that when we did it. And as someone pointed out, if you couldn't complete the traverse in a day, you had to pick up the beginning tripods before you were able to loop back around (that would be the original setup and the backsight setup. If we could do it all in a day, we would leave the first two tripods up all the way through.

Joe Glidden

made the best statement about forced centering some years ago. I may be paraphrasing, but it goes like this. "Forced centering is great if you never need to use that hub again."

It produces excellent answers that to not bear much resemblance to the points in the ground, and as such, shouldn't be used in my opinion.

I don't have a problem with using tribrachs and traverse sets, but they get moved and not dropped in and out of tribrachs. Otherwise, your error estimates are grossly wrong with regard to instrument occupation.

>
> A couple of solutions suggest themselves. One could either use a tribrach adapter with a plate vial of similar precision to the typical one on a total station. The problem is, those tend to be fairly expensive compared to the more typical puck style adapters. An alternative would be to purchase a tribrach adjusting cylinder with a plate vial in it. Use it to level the tribrach more precisely before putting the tribrach adapter and prism in the tribrach. Then you could expect to be level enough and over the point well enough to not need to relevel the tribrach and shift the instrument. This adjusting cylinders typically sell for under $100 so would provide and economical alternative to forced centering.

Wild Heerbrugg had it figured out.

DDSM :beer:

The other thing wild figured out was to make the target hi the same as the instrument hi, I dislike using targets where that is not the case.

Kris Morgan, You And Joe Glidden Are Totally Off The Mark

However I cannot call you totally wrong, so please share what it is that you have learned from experience.

Paul in PA

>One could either use a tribrach adapter with a plate vial of similar precision to the typical one on a total station. The problem is, those tend to be fairly expensive compared to the more typical puck style adapters.

The “logic” behind dropping 20K-30K on a total station then trying to save $400 on cheap targets escapes me.

Joe Glidden

I guess I can't do it since I've NEVER used Topcon equipment, Wild/ Leica only.... Can you find high quality Topcon equipment? That sounds like an oxymoron to me 🙂

I cannot fight the urge to re-level the gun when you occupy the foresight tribrach. I don't understand the loss of efficiency if I have to level each setup with the gun first. so I always re-level the gun, and re-center the glass on the tripods.... Not true "forced centering" but my traverses close 1/40'000+ so I don't complain.

I leap frog, like Peter called it...

James

No total stations now are 30k. Robots are, and you don't get a cheap target with those.

Paul

I thought I enumerated the issues with it quite efficiently. If you adjust the tribrach, you're not over the same point. If you don't adjust the tribrach, you're not level.

It's a precision versus accuracy issue on it. That and, as I said, your error estimates of instrument occupation are out the window because they WILL change, not MAY change.

Joe Glidden, as usual, made a statement that caused the reader to really think about this practice and issue and to come to terms with it on their own level.

I don't force center, nor will I.

Donald

I agree that your method, while not forced centering, is an EXCELLENT method for traversing between traverse sets.

James

> No total stations now are 30k. Robots are, and you don't get a cheap target with those.

1. My supplier of choice still refers to their robots as total stations; I'll inform them of the error of their ways 😉

2. You'd be surprised what I’ve seen :-O