squowse, post: 425449, member: 7109 wrote: If you move your tribrach when occupying you are not doing forced centring.
I would rather be a couple of mm out on each point than mess up the tightness of the traverse. The errors could compound and then you adjust out the slop at the end. In my minds eye I can see errors of more than 2mm being allocated to some points by this process.As others have said, reversible prism holders with plummets are necessary. At least for the forward station.
Forced centering is only as good as the stability of the tripod and stability of the ground beneath it. A tripod leg can sink into the ground or even slip in the clamp much more often than the point in the ground can move. We are in fact traversing about the points in the ground, not some point above it.
I will recenter because I have to stay above that point in the ground. Since I always set up my foresight before turning on my instrument, my policy is to do my backsight confirmation shots and foresight traverse shots before any sideshots. I may take all my sideshots then, and even stakeout to a search point or two. When I am done I recheck my backsight and then move it ahead to set the next foresight. Those that shoot the next traverse point last so they don't have to push some extra buttons in the data collector are trusting too much that things don't move given enough time.
I have sometimes left a foresight in the woods overnight and come back the next day to find it still perfectly over the ground point.
My first experience with the potential of things moving though, was as a teenager surveying with my father. We were cutting line for a road layout a few hundred feet beyond our setup. He taps me on the should and says "Don't say anything and turn around". There was a buck rubbing off velvet on the tripod leg. He then said, "Don't move or we will be chasing the transit setup through the woods".
Paul in PA
I guess I just don't understand the logic of not moving the gun over the point when you set up on it. Like someone above said, your surveying points on the ground, not some point hanging in the air. What if you have to go back and use the points later to set corners?
As Squowse wrote about the compounding errors above, moving it every time may create a larger absolute error of closure.
So which is more representative of the location of the "points on the ground", adjusting out an extra tenth or so throughout your traverse, or having each point being half the diameter of the head of the tack "off"?
Tripod Blues......
I agree with Squowse in that the tribrach should not be moved. That said however.....
If the tripod has obviously significantly moved then corrective action is required. But wait, is this due to bubble drift, stability problems with the equipment (tripod & tribrach) or sinking of the tripod due to poor terrain conditions?
Its decision time if a leg gets kicked or nudged by an animal!
Poor ground influence may be alleviated by driving wooden stakes to support each leg or the station may be established from the main traverse and isolated as a sub-station to prevent the introduction of unacceptable error on the main traverse line. Re-centering the instrument will of course introduce a random error. Depending on the length of the traverse leg a greater orientation error will be introduced in station re-centering on short legs and should be avoided where possible.
When the tripod is well planted in the ground catch the tripod support plate and try twisting to see if the head is stable and that clamps and stirrup feet engage properly. Do the same perhaps less vigorously with the tribrach on a stable tripod. Look out for rough foot-screw movement or wobble!
Don't forget that the more accurate vertical angle compensator can be used to level up the T/S. Being housed internally this is less subject to solar radiation. Ideally shade the instrument with some sort of parasol device when appropriate. Avoid setting up on blacktop surfaces during warm spells if possible - or again take corrective action.
When levelling the T/S, shade the bubble from the sun with your own shadow, then turn the unit through 90deg and allow the Theodolite Uprights (Standards/Yoke) to shade the bubble vial again as you move round the instrument. Allow the instrument to become accustomed to environmental conditions whilst keeping the instrument shielded. Differential heating of the T/S does have an impact.
The electronic dual-axis bubble simplifies the T/S levelling procedure.
If a tripod leg has sunk into the earth and the station point is at ground level the optical plummet will not necessarily register this movement - Has the bubble moved off because of solar radiation or is it indeed tripod movement!?!
Tripod movement effect may be simply demonstrated by a Kern Centering Tripod - place the ferrule (point) of the centering rod on a station punch mark and center the tripod as usual by centering the bubble and checking through 180deg. - now stand on one of the tripod feet and sink this further into the ground. The ferrule remains on the point but the bulls-eye bubble on the centering rod naturally migrates in the direction of the leg movement. This is because the point of rotation is pivoted about the station point and the arc of rotational movement is through the tripod head. The centering of the head has of course been disturbed by the relative amount of leg movement.
This scenario is analgous to that of the optical plummet. Whereas the mechanics of the movement are obvious from the physical centering rod, the same motion occurs with the optical/laser plummet. So after tripod movement simply glancing into the O/P and the comfort of seeing the graticule/laser dot fairly on the point is misleading!
With forced/constrained centering as Squowse remarks the individual eccentricities of centering do not have an accumulative effect along the course of the traverse. Accuracy of centering with the optical plummet is 1mm or better (1/24 of an inch) - however interchangeability of instrument and target can be about 1/10th or better than this in the case of forced centering. The individual mis-centering of the tribrach over an individual station mark may be regarded as an acceptable or unacceptable eccentricity depending on the task at hand but is nevertheless isolated to the particular station in question. Logically, as regards re-occupation, if you can center the instrument to 1mm then you can re-occupy this station at any other future time to such limits.
It goes without saying that the bulls-eye bubble in the tribrach should be in agreement with the alidade bubble/compensator. If this condition does not prevail after bubble calibration then the plane defined by the 3 small bearing pads within the tribrach cup are not planar or the rotational axis of the alidade is not parallel to this plane and may be in question (rare occurence with quality instruments nowadays!).
It is true that the traverse is run from tribrach center to tribrach center and this geometry is transferred to the physical station by plumbing with due care and the particular performance characteristics of the plumbing solution.
The ideal situation is to run the traverse independently of other observations so that there is minimal delay in station occupation time so movement and refraction should be minimised by short-lived occupancy. This is particularly true for reciprocal Trig. Heights which suffer greater impact than distance measurement. Tripod movement will be minimised where equipment is in good operating condition.
Stability is the goal and recovery the means - so tighten up!
But hey life's a compromise! - You may have to combine control and other observations but let the final required deliverable dictate the method.
When the station is at ground level some surveyors bring the graticule/laser dot on to the station mark using the foot-screws - then level up the tribrach bulls-eye bubble by extending/retracting the legs (similar in principal to the Kern Centering Tripod). When the mark is observed again through the plummet it will be found to be fairly well centered. This is the reverse process in effect of leg sinking.
Bubble vials and plumments should be checked by reversal. Non-reversible plummets being less ideal may be checked by outlining the trivet plate of the tribrach on the tripod head with a pencil and rotating the tribrach by physical reversal on the footprint outline. Not ideal but a quick field check!
One thing to remember - centering is particularly critical in occupation of the stations for the initial introduction of orientation and likewise for closing on the same or other known stations - ensuring empathy with overall control and internal consistency on closed traverses.
One tip, in the case of a stable but out-of-adjustment tribrach - on setting up approximately align the tripod head and the optical plumb telescope with the back station as this will set the eccentricity in the same relative position with regard to B/S each time. The contained traverse angle will be modified by a constant lateral shift with the same mathematical sign at each set up. The resultant angular accumulation of error will be distributed accordingly by the adjustment. Otherwise the eccentricity of the plummet over the station mark will have a random rather than systematic effect and adjustment more arbitrary. If lower accuracy traversing is envisaged with a bi-pod prism pole always point the bubble in the direction of the BS for the same reasons.
A similar condition applies to Targets/Prisms which may exhibit lateral eccentricity of the graticule - dedicate one target for backsights and another for foresights. Adjustment should take care of the systematic error. Ideally if the BS & FS targets are interchanged during the same T/S setup but between balanced angle sets, any target lateral eccentricity will be removed as the angle will be too small in one instance and too large in the next. This however is tiresome requiring leg work and time - unless justified by the nature of the work!
Prism pointing is important and poorly aligned prisms give rise to random pointing errors. The critical aspect is the optical center of instrument and prism and corresponding translation to the ground stations. Prism glass has a different refractive index to air.
With robotics it is important that the tribrach remains fairly central on the tripod plate after centering due to motorised rotational and gravitational forces exerted during the observation set.
Whilst GPS has replaced traversing in many instances, the advent of the multi-function Trimble SX-10 Scanning T/S will no doubt increase re-evaluation of the traverse method and without doubt others will be announcing a similar solution at a YouTube near you real soon!
A tripod bag is no bad thing as the greater loss of paintwork on the tripod probably occurs from rolling about in the back of a vehicle. Loss of paint/varnish permits the ingress of water and subsequent malfunction. Transit bags/cases for Targets and instrument protection as stated by others elsewhere also help minimise problems when used between setups.
The rule of thumb that I have always used is if its something you do every day its worth automating and spending effort, time and money on getting it right and as effortless as possible for the benighted surveyor.
Apologies for some of the foregoing as it may be regarded as trying to teach your granny to suck eggs but where the obvious is stated it is for the sake of completeness.
PS - Bottom Line *!?* - Stop fighting with the bubble as it is not an infallible indication of tripod shifting!
Happy Traversing
J, Bradley
fish eye level= bulls-eye level = pond level = box level = circular level = ?
I slide it for the same reasons the other sliders listed.
Fredh, post: 425033, member: 12570 wrote: Running standard closed traverse, anywhere along the line; double the foresight with tri-pod/brach/prism, move the instrument forward, level up and you're plummet is off the point by a tich, not much but a noticeable amount, what do you do....
I am sliding back over, but out of the 40+/- guys I've crewed with over the years (most recent being the senior surveyor at my new firm), about half (and usually the more seasoned) would leave it be; reasoning that if you move it you are no longer occupying the point as shot.
My feeling is by merely leveling the brach to the vial on the instrument vs the fish-eye, I have already changed the location, and whatever the difference that's part the adjustment process.
Interested in hearing your thoughts on the pro or cons.
Is the tich caused by your cluminess setting up the Instrument, an inept head chainman setting up the prism, or settling, movement of the tripod by environmental actions. If it's your cluminess or settling of the tripod by environmental factors, I'd recenter. If it's caused by your inept head chainman, I'd fire him and start over.
If you want better mathematical closures do not slide.
If you better results then move the gun over the point.
...and adjust your tribrachs so that they all match. 🙂
I don't think enough people adjust their tribrach plummets or even know how to. That could be part of the problem. I use one of those cylinders that allows you to put the tribrach upside-down on top of a level tribrach and then I site a little hub tack with pink ribbon on it that I stuck in the ceiling tile a few years ago.
I do think that there will almost always be a little sliding to do just because the level vial on the tribrach when you are setting the prism is not as accurate as the one on the gun when you move the instrument to that point.
Tripod Blues...
For those lucky enough to have the Trimble SX-10 the plummet in the instrument has a camera which can photograph the ground station and be used for centering. An alternative tribrach with a pass through optical plummet is also available whichpermits both digital and optical plumbing to be used at the same time. To the best of my understanding the SX-10 has a facility called Point Sure which can correct for drift for example in the instance of tripod sinking - calculation of all the observations back to the initial plumb point. So would seem like a belt & braces solution.
By the way is 1 tich = 1 tad and how many are in a foot?
Check the adjustments or your tribrachs regularly to minimize this occurrence. Also ensure that all the people on your crew know how to and habitually do make the effort to carefully level and center targets over the point. If you've done both of these things and it's still a regular issue, it may be that you're using cheap tribrachs. I used to think that one brand of tribrach was as good as most any other until I had used cheap ones with better name brand ones. The name brands seemed to hold an adjustment much longer than the cheap ones. I had at least one cheap tribrach that couldn't stay well adjusted for an entire day. Come to think of it, I've had some coworkers who also fit that description.
The greater the temperature differences throughout the day, and the more direct sun exposure your setups have, the more this problem will occur, with good equipment or poor, but worse with poor.
When you move the instrument up to the foresight and find after relevelling that your plummet is off the point by some measureable amount, don't move the tribrach. As the more seasoned surveyors you referred to have told you, by doing that, you are occupying a slightly different point than the one you just measured to. Instead, mote the distance and approximate direction of the mis-centering and take that into account in the final coordinates after verifying the overall quality of your measured network (which is likely to be marginal at best with one or more bad tribrachs in the mix - but would be further downgraded by moving them between observations), and make any necessary adjustments to the network.
By making corrections to individual points after assessing and adjusting the network, you isolate that mistake (blunder) to the location where it occurred and in the magnitude it occurred. By sliding the instrument over after you've already measured the previous foresight to where the tribrach was before sliding, you push the effects of that centering error into the remainder of your network. Depending on the direction you need to slide relative to line to your backsite, the error you introduce can look like a distance error, which is additive in subsequent control point positions, and/or an angular error, which compounds geometrically into the positions of subsequent points. It makes it just about impossible to locate the actual error when assessing the network (undermines the effectiveness of your blunder detection tools if your adjustment software has them), and causes the corrections of those errors to be spread among all of the control rather than only in the locations where they occurred.
Bottom line:
1) check that your equipment is properly adjusted regularly (at least weekly for good equipment, at least daily for cheap equipment),
2) check that your crew uses appropriate skill and care (how often depends on your confidence in your crew),
3) when a centering error occurs, don't exacerbate the problem and obscure the ability to accurately account for it by moving the target or instrument during the observation process,
4a) if the centering error is relatively small and the control is not intended for unusually precise work, note the distance and general direction (noting whether from bullseye to point or vice versa) for later adjustment to the specific point,
or
4b) if the centering error is excessive when considering what the control will be used for, stop, check and adjust your equipment, go back to last setup (or as far as necessary to remove effects of maladjusted tribrach) and redo those observations.
Could a loose tripod be the issue?
No no no.
Never slide.
Ya'all is buggin me.
All your tribrachs should remain stationary.
It's called "isolation of error components".
If your plummet is out.... Fix it.
Never move your tribrach, after it's positioned.
