Backsight zero drift in Trimble S series total stations

I can't address the Trimble situation, but I note that my Leica 1100 and GeoMax Zoom 80 are both remarkably stable when set up on stable ground. I'll often take topo sideshots for several hours and then check the backsight within a few seconds.

Jim Frame, post: 392721, member: 10 wrote: I can't address the Trimble situation, but I note that my Leica 1100 and GeoMax Zoom 80 are both remarkably stable when set up on stable ground. I'll often take topo sideshots for several hours and then check the backsight within a few seconds.

well, he was setup on a large concrete monolith, but he did say the temperature came up a lot during the survey.

Trimble has a tendency to pull towards reflectivity. If there was any around, ie vests, etc it certainly wouldn't lock on to one, but it will cause a drift.

cwlawley, post: 392724, member: 3114 wrote: Trimble has a tendency to pull towards reflectivity. If there was any around, ie vests, etc it certainly wouldn't lock on to one, but it will cause a drift.

I have definitely seen this as well, and in several different scenarios. One of the worse was when trying to get a backsight when a stop sign was about 4' away from the prism (the gun was about 200' away.

When I noticed the residuals were crappy I looked through the scope and saw that the aiming was off, and when the rodman threw his sweater over the sign it got the shot just fine. Don't worry, there was no traffic and plenty of time to remove the sweater had a car came down the street.

John Hamilton, post: 392720, member: 640 wrote: A client noticed last week that when he checked his backsight using a S9 after about an hour it was off by 57".

I'll bet dollars to donuts that the problem is the tribrach. More specifically, the instrument is not seating properly on the 3 contact points - usually because the contact points have become worn, or maybe because a speck of grit or something is on the points - and the instrument shifts ever so slightly as it is being used.

The next thing to check is the condition and adjustment of the tripod. If the legs are too loose at the head the whole works will shift in use.

Mark Mayer, post: 392731, member: 424 wrote: I'll bet dollars to donuts that the problem is the tribrach. More specifically, the instrument is not seating properly on the 3 contact points - usually because the contact points have become worn, or maybe because a speck of grit or something is on the points - and the instrument shifts ever so slightly as it is being used.

The next thing to check is the condition and adjustment of the tripod. If the legs are too loose at the head the whole works will shift in use.

Interesting but isn't a 3 points of contact arrangement inherently stable?

We had the ssme issue.We started setting up on a trimax with a reasonable height. The problem went away...

I would definitely try a different tripod first. If the problem still hangs around, I would call the local Trimble shop to schedule a clean'n'cal.

This assuming that the gun has been collumnated recently. And the level bubble hasn't changed from start to end of setup. S-series are known for quirky issues. Such as a steady wind and too-small of a tripod to cause the machine to error out due to a harmonic vibration. (It essentially created an infinite loop with the compensator).

I've seen a few people claim an S-series wasn't holding zero, but have yet to see it be the actual gun.

If it was set up through a big tempature change I would assume that is the culprit I don't know about Trimble but our Sokkia will go way out of level during temp changes, not enough to trigger the out of level alert.

What Mark said. S series guns exert a lot of torque. Any play at all in the tribrach or tripod and it's not going to hold zero very well. I usually get a little drift but nothing like 57", maybe 5-7", and that's after steady use for an hour or more. Trimax tripod is the way to go with that instrument.

Mark Mayer, post: 392731, member: 424 wrote: I'll bet dollars to donuts that the problem is the tribrach. More specifically, the instrument is not seating properly on the 3 contact points - usually because the contact points have become worn, or maybe because a speck of grit or something is on the points - and the instrument shifts ever so slightly as it is being used.

The next thing to check is the condition and adjustment of the tripod. If the legs are too loose at the head the whole works will shift in use.

I agree. Worn bushings, loose screws, and loose feet on a tripod can cause serious problems. Tribrachs get out of adjustment and contact points get loose. I would check all of the hardware before placing the blame on the instrument itself.

squowse, post: 392736, member: 7109 wrote: Interesting but isn't a 3 points of contact arrangement inherently stable?

There are those 3 contact points, but they aren't the only points of contact in the tribrach.

I will ask what kind of tripod they use. We use only trimax

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John Hamilton, post: 392755, member: 640 wrote: I will ask what kind of tripod they use. We use only trimax

I would have them check the tripod for anything loose, including feet, points, & bushings. IF that all checks out, the next thing to check is operating procedures and the tribrach.

Asymmetric heating of the tripod legs, especially with a quick rise in temperature, might well be causing the head to twist around. If the backsight angle was negative then this is a likely cause.

Another vote for a tripod as the first suspect.

Check to make sure all bolts are snug, and that the legs aren't rotted out inside the feet/points.

Drift is due to the tribach not clamping the instrument firmly enough. First check the instrument for slop in the tribrach before powering it up, if you can move it a second, the motor torque can move it seconds. The motion accumulates because of difference in the acell/decell torque. If the instrument always approaches a point fast on swing right, then readjusts on swing left and fine adjusts on swing right, it is that first swing right decell that causes the most drift which will eventually turn the base noticeably to the right. The motion in a sloppy tripod allows the decell force to impact the base clamping more.

Paul in PA

Plumb Bill, post: 392729, member: 226 wrote: I have definitely seen this as well, and in several different scenarios. One of the worse was when trying to get a backsight when a stop sign was about 4' away from the prism (the gun was about 200' away.

When I noticed the residuals were crappy I looked through the scope and saw that the aiming was off, and when the rodman threw his sweater over the sign it got the shot just fine. Don't worry, there was no traffic and plenty of time to remove the sweater had a car came down the street.

There's your solution....just cover up all the traffic signs in the area when you're working! :p

A concrete Instrumentation station is usually pretty stable but it is more susceptible to change in atmospherics especially if there is a large metal mounting plate for the tribrach. 57" in angle or tilt correction?