Hi there
I've been surveying for two years now. I use a Trimble 3 sec robot with Leica round traverse prisms on rotating prisms carriers. I consider my set up and procedures to be very tight.
When I've finished surveying or setting out from a station I always use the "check backsight" feature on Access to check the set on station hasn't moved during the surveying or setting out period for that occupied station.?ÿ This quality control procedure was something drummed into me at college.
Last week I used the check backsight feature at the end of a session and it said the backsight was out by 0.05 foot (backsight was 150 ft away).?ÿ I checked the backsight's optics and levels - it hadn't moved. I check the instruments optics and level vial - the set on station hadn't moved either.
Same thing happened yesterday twice. I set up over a known point with a known backsight and got deltas of 0.001 foot at the start of the session. I did check backsight at the end of the session and it said it was out by 0.03 foot (the backsight was 280ft away). I checked the backsight and instrument optics and neither had moved.
I have rotating prism carriers and I center them over a nail, turn 90 degree and check the centering. Instrument and prism are always perfectly centered.
I'm not sure what's going on but I'm quite concerned. I've never noticed errors of this magnitude before when using check backsight. On all occasions when I got the error I've changed face and hit checked backsight and still got the same errors.
At the weekend I did a Horizontal, Vertical and Autolock calibration. Today check backsight showed a delta at the end of a session of 0.022ft.
John
Not all routines in Access use the same prism types and definitions. Doublecheck your styles. I havent run access in a few years so I can't remember how to walk you to all settings?ÿ
Goid luck, Tom?ÿ
Did you perform the compensator calibration and collimation??ÿ With the S6, Trimble recommends that those functions are performed daily.
Have found horizontal drift to be a disappointing occurrence with the Trimble S series. And heard it mentioned by others.
A couple of things to check - you're using top quality tribrachs and legs? Try giving the head of the tripod (and the tribrach)?ÿ a twist when looking through the instrument (auto lock off). See if it reliably returns to the same place.
The check backsight routine didn't work that well with resection. I think they may have fixed it. Before, it used the adjusted orientation instead of the original HA to the backsight.
Do you check backsight in both faces, same as original backsight observation??ÿ
Instrument temperature has acclimatised to the surroundings before taking backsight? I had a lot of trouble with this when trying to do precise monitoring in the depths of a cold winter. I could perform a calibration and get a different answer 10 minutes later. In that case the instrument was cooling down from having been in a heated office or car. But could happen the other way round eg early morning on a sunny day.
Legs expanding due to sunshine on one side? Had this one plenty when working in Australia.
Apart from that can only recommend taking backsight check at regular intervals and seeing if that helps understad when and how the drift occurs. I doubt the problem is with your backsights although you could verify this by checking to a pencil line on an adjacent building (or something similarly fixed).
Drift of this sort is rarely a fault in the instrument. It is usually
- a setup on shaky ground,
- temperature effects
- a loose & wobbly tripod,
- worn tribrach.?ÿ?ÿ
I suggest that you check your backsight about 20 minutes into your session and reset 0 if it seems appropriate. Quite possibly it will stay fixed after that.
When the drift occurs I look through the optics of the backsight and set on station. Both are perfectly centred so surely wobbly tribrach or tripod can be discounted??ÿ
I regularly check the backsight nowadays, about every half hour. When it has drifted i've noticed it after a while?ÿ say one hour. Within half an hour it always fine.?ÿ
My tribrach are Leica precise range and my tripods are wooden Leica ones too.
So do you think the drift is due to the set on station's tripod or tribrach twisting??ÿ It would only need to twist a tiny amount to cause a drift to the backsight. I can verify this in the field by checking to another backsight or foresight to see if that has drifted to.
How could temperature be affecting this? Other than twisting the tripod??ÿ
How could temperature be affecting this? Other than twisting the tripod??ÿ
The fact that this is happening in the heat of summer makes strong sun on one side of the instrument a strong candidate.?ÿ Yes, that can definitely cause this.?ÿ You have the top brand of tripod and tribrach there, but that doesn't preclude them from being the source. Are the tripod legs properly tightened at the head? Has the tribrach been roughly handled or dropped at any time? You have a baseline a few inches in length that you are projecting out over a hundred feet. Microns make a difference.?ÿ?ÿ
Yep i tighten everything up good but i know my setups are moving. I only have to look at my prism carrier's level bubble after a while. Half the time the bubble has moved. Not much but a bit.
Is this just the way it is or should i be checking my gear? What do I check for?
My tribrach and tripods are Leica but came from ebay. I thought I was being clever saving some money!?ÿ
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If your backsight is off level then it has almost certainly moved off the point? Maybe not enough to be causing the errors you describe though. Is that a wooden tripod as well?
Temperature affects the instrument by warming the components and maybe they go out of alignment?
I think its more the instrument tripod that is moving a tiny bit that is making a largish (0.04 foot) change to the backsight.
I'm going to check my setups more from now on. I'll check there is no play in the tripod head and/or the tribrach.
