Hello rfc,
If you are enthusiastic enough, don't take anyone's word for it, try it out yourself. Calibrate the instrument properly. Read a bunch of angles around you to targets, at sufficiently differing elevations, in two faces, with the compensations off. Read several rounds. Reduce your angles. Turn the compensations on and re-read the angles and reduce. Make the same number of pointings in the one face rounds as you did when you were using both faces. Compare. See if your differences are statistically significant.
> Hello rfc,
>
> If you are enthusiastic enough, don't take anyone's word for it, try it out yourself. Calibrate the instrument properly. Read a bunch of angles around you to targets, at sufficiently differing elevations, in two faces, with the compensations off. Read several rounds. Reduce your angles. Turn the compensations on and re-read the angles and reduce. Make the same number of pointings in the one face rounds as you did when you were using both faces. Compare. See if your differences are statistically significant.
That's exactly where I was headed in "Total Station Direction Accuracy--Chapter 5" before I got side tracked into "Calibration". I don't really need to know if the compensations "work" or not; all I need to know is the ultimate practical accuracy the instrument is capable of, which the "Kent Test II" (5 targets, 150'distant, FL/FR measurements all around, then Starnet the daylights out of the results), will tell me.
That said, if I have time before the "soccer kids" show up and take over my test range, I might shoot a round with all compensations off or set to zero, just to see. I don't doubt they'd make a difference; why would the manufacturer have them in there? It is, after all, an electro-mechanical optical instrument. They can't all be identical and perfect out of the factory.
Stay tuned for Chapter 6. 🙂
> Hello MightyMoe,
>
> Once a total station has been correctly calibrated, what instrument errors do you suppose 2-face readings will eliminate that will remain otherwise uncorrected for in observations of the one face?
I know of one: Centering Errors. My manual says that even though the compensators can correct horizontal angles for vertical axis errors, it won't correct centering errors. If the instrument is set up with the vertical axis tilted by 1', that could result in a centering error of about .4mm depending on how high the instrument is set. At 10 meters, that's about 8" of horizontal error.
Here's what the manual says on the issue:
Technically, if the instrument has been (and remains) correctly calibrated, you shouldn't see a difference between the measurements taken with both faces, but there's one way to know for sure: Just DO it!
I know when I did the test with my last instrument, at the beginning, FL and FR readings were the same almost always. As the day wore on (instrument heated up?), they got further apart.
Other than the note above about centering errors, I'm seeing now that routinely using both faces is really a safety net.
Hello rfc,
I'm curious. How will reading 2-faces correct a centering error? Perhaps we have a different understanding of centering error?!?
I was only referring to errors that 2-face readings can correct verses properly compensated 1-face readings.
> Hello rfc,
>
> I'm curious. How will reading 2-faces correct a centering error? Perhaps we have a different understanding of centering error?!?
>
> I was only referring to errors that 2-face readings can correct verses properly compensated 1-face readings.
I partially mis-understood the question and you mis-understood (or I mis-stated) my answer.
1. Reading both faces will not correct centering errors.
2. Calibrating the instrument won't either.
3. Even if the instrument is calibrated properly, reading both faces provides additional surety in the accuracy of the readings, especially because they're done in real time, through out the day; unlike going through the compensation process, which is typically done occasionally.
I'm certainly not an expert on the matter, but it seems easy enough to test how much consistently reading both faces will help, and that's on my list of things to do.
After calibrating my total station and storing the error in the instrument, I still notice that when I check the instrument for error I have 17" error on the vertical reading. That is when i sum the FL and FR readings. Is this proper? I though after calibration the instrument automatically applies these corrections to subsequent observations?
Its good to note that 17" was the vertical error during calibration that was saved in the instrument.
Hello MightyMoe,
Once a total station has been correctly calibrated, what instrument errors do you suppose 2-face readings will eliminate that will remain otherwise uncorrected for in observations of the one face?
Trunnion axis error is the one error that only CL/CR will allow you to compensate for. The trunnion axis is the axis about which the telescope tilts, and if this is not perpendicular to the vertical axis and the collimation axis, then essentially your telescope will list to one side tilted up and the other side tilted down. Taking CL/CR readings (the same number of readings on each side, mind you) will allow you to take a mean of these readings, which essentially balances the error out.
