I have been running a Topcon GPT series total station, recently I've noticed something strange happening with the vertical.
Whenever I traverse to CP #2, backsighting #1 is always around a tenth low, but if i set on CP #1 and backsight #2 it checks flat.
What could be causing that?
What are you using for foresights and backsights?
just a shot in the dark.
I assume you are looking above horizontal one way and below horizontal the other way. Maybe something is out of whack in the vertical circle? It checks flat from CP-1 because that's how you measured it in the first place but it doesn't check from CP-2 because you are seeing the vertical circle index error.
You can verify if the vertical circle is out by observing a target in both Face 1 and Face 2. The two angles should add up to 360°. If you take the mean of the two angles you will get the true zenith angle (subject to errors). If you do that from each station (F1 and F2) and compute your elevation differences then they should match and you will see whether or not your vertical circle is not level at 90 and 270.
Good point, Dave. If your vertical circle is reading 0.05' low at your traverse distance, then your traverse ahead, and back check, you're now reading a tenth low (two shots) ... and if you reoccupied the first point, and checked back to the TP, it would look fine. I think Dave nailed it.
not because I'm smart or anything, I learned that from the school of hard knocks.
Now any new-to-me instrument (e.g. used) gets a quickie vertical circle check. That will show past abuse in a hurry.
> You can verify if the vertical circle is out by observing a target in both Face 1 and Face 2. The two angles should add up to 360°. If you take the mean of the two angles you will get the true zenith angle (subject to errors). If you do that from each station (F1 and F2) and compute your elevation differences then they should match and you will see whether or not your vertical circle is not level at 90 and 270.
they do not match horizontally by 2" and vertically by 40". That would create the difference. I sadly do not have the manual to attempt a calibration. May need to get it calibrated at a dealer.
Thanks for the Help
Yes, 40" will do it.
I have a GPT-3000LW series manual.
It looks like a really simple procedure. I would be glad to scan it (probably tomorrow) and e-mail it to you. The vertical adjustment (software) is 1 page.
Let me know if I can help.
One of the best schools out there Dave..... I think they owe me a Doctorate degree.
Let me know the exact model and I can probably download a full manual from Topcon Support and send it your way.
Back in the days of brass transits, the advice given in some books was "keep your instrument in calibration, but use it as if it were out of cal." If you always do Direct & Reverse (both faces in modern speak) the 40" won't matter.
The procedure for the Nikon instruments is simple and I would imagine it's the same for Topcon.
I would check this routinely (maybe monthly) for any work you're doing you need elevations for.
Another thing to be careful of is that the compensator does not get turned off. There's usually a subtle hint in the display that it has (in the Nikon's, the display lines are shown preceded by a # sign). You might want to check the manual for that too or make a call to your service company and ask a tech about it. It can wreak just as much havoc on your elevation work.