Total Station Precision
For setting property corners a 5″ gun is more than sufficient. Also, remember that to come close to the guns performance you will need top quality tripod, tribrach, target rod, and glass in top condition.
You will rarely see a setup where the setup from the ground to through the tribrach, out to the prism and back + the training of the crew + the needs of the job is sufficient to be useful for a 1" instrument.
For instance, look through the Leica catalog. There is a big difference in price between the tribrachs, legs, prisms, etc. at different quality levels.
Are you shooting to a rod with a circular bubble?
What are your state's accuracy standards? The 0.01 ft is most likely a requirement for resoluttion expressed on a plat, but the accuracy spec will be more like 0.05 or 0.10 ft for city lots and some ppm for large parcels, or else a 95% confidence least squares limit value.
Put some distance numbers on the angle value. At a quarter mile, 5 seconds is 0.03 ft. And repetition can tighten that. You have to have careful adjustment of your plummet, level, centering, and sighting to get that good even with a 1 second instrument.
One guy is concerned about the possibility of going to court over setting a property corner that wasn’t set right because we only use a 5” instrument. This guy wants to buy a 2” or a 1” instrument.
(Chuckles)
I want to see that court case. Many of the corners I recover were set with 5' instruments, much less 5". If you are using GPS most of the time, your 1:XXXX on a 300' line exceeds a 5" instrument by an order of magnitude or two. If I agreed with my own property corner within 0.01' I would be surprised.
Unless doing high end work on control surveys, 1" guns are overrated. In today's world of GNNS and constraining control networks with a variety of tools, 3" guns or 5" guns are fine. People seem to forget the results that we got with interpolating the verniers on 30" optical instruments, using proper procedures.
My survey instrument is rated at 2 seconds and estimates to 1 second. I think it helps me get repeatable results in both horizontal and vertical measurements quickly. While I agree that turning muliple sets of angles may improve the precision of the result, I rarely get differences of over 1 second with 1 set of angles (2 direct and 2 inverted).
Other factors are needed to improve precision. 1.) A rotating tribrach adapter with optical plumment rather than an optical plummet in the tribrach. 2.) A heads-up detachable bubble rather than the circular bubble that is integrated in the pole. 3.) A bipod for use with the pole. 4.) Making sure the tripods are of good quality and that all the nuts and bolts are tight.
@ bill93
Put some distance numbers on the angle value.
At a distance of 200' a 5" angular error would only produce a linear error of 0.005'.
Please explain how total stations are rated in terms of arc seconds.
I would not hesitate to use a 5” gun. Most states regulations are still written in a way that a 20” theodolite or transit could achieve the angular requirements and then throw in a chain or steel tape one can still meet 1:20000 precision if correct care and procedures were used. Now I like .5” and 1” angular and 1mm +1ppm edm but that’s my geodetic side . Of course like others have stated that tight instrument is useless on bad tripods and glass tribrach and glass and rod.
I was handed a small traverse to adjust a few days ago. The loop did not meet requirements. I was lucky as they discovered the tribrachs were out of adjustment. As they traversed throughout the site to do the Topo. They made enough cross ties and such that I was able to save a complete re run of the loop all of that was done after they fixed the bad tribrach.
Redundancy always helps. For a 3” gun 4 rounds gets you down to like 2.4” depending on how the gun is specked though.
@ Norman_Oklahoma
IMO the sweet spot between precision and cost is the 3″ model.
That sounds reasonable.
@ dmyhill
Are you shooting to a rod with a circular bubble?
We use a rod with a built-in circular bubble.
@ dmyhill
I want to see that court case.
There was no court case. My coworker is concerned about a possible future court case where a judge might rule against us for not using a precise enough total station to set the property corner in question. I know that sounds ridiculous, but it's true. He's using this reasoning to say we need a 2" or a 1" second total station to set a corner that won't be disputed because we'll set it with 2" or 1" precision. Oh boy. The knowledgeable will prevail. I'll write a paper with a list of our typical jobs, the state requirements for precision, and the equipment we should have. Florida's minimum technical standards have changed, so if no precision is indicated I'll just show the math. You can't argue with math.
