Traverse network - leveling instrument after change of station
I have a question regarding leveling (making horizontal) of the Total Station instrument in traverse network.
Is it necessary to level instrument after changing the station - means when I transfer SNLL121 to the new station and instead of this I put TS16? Is it good practice to leave bubble compensation on or it should be off? I think I read somewhere I shouldn't level instrument and compensator should be off - in this way we preserve the same conditions and minimizing errors?
We are using the following equipment:
- Leica TS16 1"
- Traverse kit Leica SNLL121 - http://leicatotalstation.org/tag/snll121/
Thank you in advance
Compensator should be on and the instrument should be level before making measurements. There is a school of thought to not recenter the instrument over the point if it has had small apparent departure from the point, this approach is called "forced centering" but does extend to levelling of the instrument.
The only time to turn off the compensator is when you are on an unstable surface such as the deck of a boat, heavily swaying structure, top of a hay pile etc.
I will level the instrument, compensator on, but never move or recenter. You are traversing through the center of the tribrach some called forced centering as opposed to the center of the point on the ground.
If you look through the optical plummet and discover that you are considerably off center from the point on the ground (you should not be) I know of some folks who have carefully adjusted the point to be coincident with the plummet.
A tripod is not a fixed pedestal, it is a multi faceted mechanical device with adjustable parts. In fixed centering you are assuming the best condition and that you had not made a mistake, which may not be true. The typical variation is that the clamps are not tight and/or that the leg tips are settling. The best you can assume is that the movement was slow and steady and that at the time of observing that location of the tribrach was more likely closer to where it should have been and moved mostly in the interim. Therefor you must relevel and recenter so that you are getting the best observations possible at the position you are at from the position you were to the position where you are going. Not doing so is a case of hoping that gross errors cancel out. Please note I said relevel and recenter which are done separately. After releveling the instrument you observe your ground point and note and record any visual differences then take a backsight distance and record it. Compare it to your previous foresight distance. Now you recenter the instrument, take a backsight distance, record and evaluate any differences. You now have three probable distance for that leg.
When the traverse is complete you re-evaluate the differences in the bad leg before any other adjustment. First what is your raw error? Can it be reduced by using a different or meaned traverse distance? Once any obvious distance errors are compensated one can check for angular errors to those questionable traverse points. Having done both the you can then expect your adjustment to get you closer to the Truth. In general an averaged leg distance getting you halfway from perfect is the better solution.
Paul in PA
Bojan, measuring in two faces compensates for instrument errors not setup errors.
Experiment with it yourself. Turn a set of angles without re-levelling. Then fine level and do it again (just relevel, not re-center) without . I expect that you will find that, as long as you are within the instruments compensation range, the angles will be the same.
The question is how well centered over the point you are. After re-levelling you may appear to no long be over the point. If not, perhaps the thing to be doing is adjusting your tribrach. Then you won't have to deal with this problem.