I am wondering how tilt compensator really works. Is it relative to the current plumbline or to the initial plumb line? I am asking this simply because anxiety is really there when my TS tilts over point occupation time. This is a reality that we can't avoid. Our TS really tilts over time due to many factors. As it tilts, plumb line shifts. My question is where tilt compenstation is made relative to? If it is made relative to the initial plumb line then it is a great relief for me and I don't need to relevel my instrument in case a feeling of discomfort comes out. If it is made relative to the current plumb line, then there you go my problem. Any idea? Thank you guys.
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One of the first things I received a "Gibbs" for (clip on back of head) as a learner surveyor, was not treading on the tripod feet properly, to ensure that it is well set into the ground, so that nothing will move while working for anything up to the next 4-5 hours.
Firstly, if your instrument is moving while working, is it stable to start with?
Secondly, what accuracy are you working to? Having been involved in deformation work on a large dam, every small tweak or twitch that can improve the results MUST be put in place. When the circular (bulls-eye) bubble is within its circle, the automatic compensator may be operating, but is the instrument centred over the peg, marker, beacon? The only way to ensure that the instrument is centred, is to carry out a full levelling procedure with the aid of the plate bubble, confirm the instrument is centred properly, then let the compensator do its work for the duration of the setup.
The compensator in a surveying instrument allows the operator to set up the instrument more quickly because he must only level the instrument so that the bulls-eye level bubble is within its etched circle. The line of sight goes from the eyepiece to a mirror that deflects it to a prism suspended like a pendulum that refracts the line to a similar mirror on out to the objective lens. While within its operating range the prism-pendulum is aligned by gravity an amount that COMPENSATES for the imperfect leveling within the bulls-eye level bubble.
The compensator does NOT correct for tilts outside its limits (similar to the circle on the circular bubble level)! The direct answer to your question, the compensator is controlled by gravity both initially and eventually.
If your instrument is out of level to the point that the circular bubble touches the line - RE-LEVEL YOUR INSTRUMENT! Any observations made while out of level will NOT be correct!
Best wishes on your project.
JAC
P.S. Search for "compensator of survey instrument" on Google and study some of the images.
Dear Cavell...
It was the the way it works on the old school instruments (mechanical ones). If you have a Mechanical or electro-mechincal instrument, it was the way it works.
Actually, they are working in a different way. Take a look to this 2 attachments from Surveying Vol. 1 from S.K. Duggal (Mc Graw-Hill):
So, the idea is that you have an electronic component inside your total station that have some kind of electronic levels. There is a led that try to cross that level and depending on your tilt, you have a different value of the light intensity. A photo detector read it and gives a value of the tilt to your total station, so the final reading on your total station is the electronic reading plus tilt value from the electronic compensator.
So it is a better system than the mechanical, because on the mechanical, sometimes you are out of compensation range and you didn´t know that, except if the had a light block system, like Wild or Karl-Zeiss.
The electronical, besides, allows you to calibrate it. (On the mechanical, it there was any problem, you should go to Lab) Maybe that´s the only point that I don´t like too much: If you make your own compensator calibration, you can shift it very close to their limit, that´s why, when you "feel" that it is continuously error tilting, you should go to Lab in order to check if the compensator is out or nearly out of range.
Regarding your question, the electronical dual axis level follow the local gravity at your station point, so the plumb line. And it is not to the initial plumb line or the actual plumb line... it is always correcting it by your plumb line. Try this... put your TS on a tripod, level it properly with a good fixed leg... and then jump! the changing on the zenital angle is due to the vibration of the dual axis compensator. It is good for your continuous work but it is a nightmare for a windy day.
Regarding the "out of level" error with the time, it can be 2 main reasons: not a good station or temperature. Keep in mind that the actual TS are made, mainly, with plastics material so, when the support of the compensator gets hot, there is a small movement. You shouldn´t be worried about it, but, if your are observing in one way that the Sun hits you in only one side, then you should turn the station a little bit and this "out of level" error should dissapear. Even, for the actual TS, it is happening when somebody has calibrate the compensator and it is near its limit. Coming from Lab, it shouldn´t happen.
But the mainly issue, as Ian is saying, is basics: good tripod, good station, good levelling (not "just inside the circle", try to do your best), turn it quickly to remove inertia issues on bubble levels, try to work in the middle of the range of your levelling base...
Dear Mr. Bodego,
Thank you for the illustrations. There are, of course, more than one method of detecting tilt. It is important to keep an explanation simple enough to take a few words so that the principle is made clear. The principle is that gravity is the source of information used by the compensator.
How the compensator illustrates or acts upon the tilt varies with technique.
Bottom line: If one's instrument tilts noticeably during use it needs to be re-set and re-leveled.
JAC