Total Station accuracy question - 1" what?

[Augusto 3D]

I am just curious as to what most of your guys here are using. I keep ready posts where folks are recommending 1" or better for 3d scanning workflows !!! Every robot I see on the sites we work on is 3" or 5" on the low end. Our surveyors (subs) use manual 5" guns on our jobs and we seems to be getting good results (unless our scanning is just that bad).

The Topcon GTS-1200 is not a 1" gun and it's apparently designed for BIM and scanning workflows. It comes in a 2" and 3" variants.
https://www.topconpositioning.com/total ... l-stations

What am I missing here - why are folks recommending 1" guns even for a large warehouse, whats the obsession?
School me a little I guess.

[Leandre Robitaille]

For 3D scanning 2'' is more than enough.
But. A total station accuracy is just the very very tip of the iceberg. having extremelly accurate (talking mm here) control points surveyed by a total station involves so many factors, so many things can go wrong, first off keeping your total station leveled, sun, wind, curved poles, 8min bubbles, uncalibrated bubbles, that little 1mm offset in the magnail notch, etc (dont worry so many more things can go wrong). The places for errors are very high and you need a very experienced surveyor with well calibrated nodal traverse prism and with those good 45'' bubble on a tripod.
https://www.waypointtech.com/geospatial ... l-stations
in example ^^
Even using those requires a lot of understanding in the calibration process.
These prism are amazing for the XY accuracy in your traverse (add to this the accuracy of your total station). Then the next thing is Z accuracy, everything is down to the tape witch is not ideal, you have to compensate those Z values with a digital level to get those mm accuracy on your whole traverse. And yet again, a traverse is maybe no ideal, sometimes a network adjustment might be better and understanding how those work is also essential.

In the end it all depends on the project requirements. Understanding of how surveying and laser scanning works it very essential in order to provide the required accuracy, much more than 1'' vs 3''. Also it is important to have profound discussions with your clients, not everything needs to be down to the mm accuracy.

[landmeterbeuckx]

I've worked for 11 years wih a 5" robotic and have done tons of construction staking and others.

More important then a 1" is the ability to use the instrument properly. For bim and general laserscanning 5" is more then enough.

I have a 1" bit it's needed in specific niches over here. It's more having the correct paperwork with your gear

[Martin_au]

A good surveyor with good equipment and a 5" instrument will be give much better results than an average surveyor with average equipment and a 1" total station.

To gain the benefits of a 1" total station requires excellent technique and excellent equipment. The sweet spot for most engineering survey work is 3", mostly because the cost to go from a 5" instrument to a 3" instrument is very reasonable, and it is quite possible to run into the limits of a 5" instrument.

[smacl]

As Leandre has said, there are very many other factors affecting accuracy other than accuracy of the instrument. For accurate control work, designing a network with strong geometry and sufficient redundancy, taking multiple rounds of angles and avoiding sources of errors such as short network legs are very important. High angular accuracy is very important where you are physically constrained in your network design. The most obvious case here is in tunnels, where you have very limited options to bring in external fixed control and are also limited in terms of creating strong network geometry. I do a lot of work in rail which also demands good quality and accurate control. Other cases include dimensional control, where you're trying to place a rigid item, such a steel beam, in a tight location. If the base model for design is derived from scanning in this case, it may need to have very accurate control which in turn demands rigorous method and accurate equipment.

Bottom line is that the accuracy of any scan point is limited by the accuracy of the accompanying control. The accuracy needed is entirely dependent on the job in hand.

[Augusto 3D]

Smacl, so am I understanding correctly that if you are out in the open (easy site, lots of line of sight) as opposed to traversing inside a building, a 5" Total station may be okay (long range error needs being ignored here).

Reason being, you are turning bigger angles and measuring farther from the instrument

[f1982]

It doesn't matter if you've got a 1" instrument, if it's being run by a 10" surveyor.

[smacl]

Smacl, so am I understanding correctly that if you are out in the open (easy site, lots of line of sight) as opposed to traversing inside a building, a 5" Total station may be okay (long range error needs being ignored here).

Reason being, you are turning bigger angles and measuring farther from the instrument

5 seconds equates to about 2.5mm at 100m, which at first glance seems very small. The problem here is it is one of a number of sources of error for a measurement from a total station setup (distance error, collimation, pointing, centering, atmospherics, calibration etc...), and you accumulate error for each setup you make from a known fixed control point. I'd agree strongly with martin_au above that for the relatively small price difference, a 3 second instrument makes a lot of sense (1.5mm at 100m).

How you establish control also makes sense, as modern EDMs are very good so even the 5" instrument is likely to have an EDM good to about 2mm. If you resect your position based on as many targets as you can see, you're likely to get a much better result than directly sighting your position from another station using a more traditional traversing approach. The other thing is to take multiple rounds of measurements on both instrument faces so you also know how good your observations are. As Leiven mentioned, a competent surveyor will get a better result with a poor instrument than a poor surveyor with a good instrument. That said, why not be a good surveyor with a good instrument and get a great result?

[Augusto 3D]

It doesn't matter if you've got a 1" instrument, if it's being run by a 10" surveyor.

Very true, and all 1" surveyors started as 10"

[Augusto 3D]

Smacl, so am I understanding correctly that if you are out in the open (easy site, lots of line of sight) as opposed to traversing inside a building, a 5" Total station may be okay (long range error needs being ignored here).

Reason being, you are turning bigger angles and measuring farther from the instrument

5 seconds equates to about 2.5mm at 100m, which at first glance seems very small. The problem here is it is one of a number of sources of error for a measurement from a total station setup (distance error, collimation, pointing, centering, atmospherics, calibration etc...), and you accumulate error for each setup you make from a known fixed control point. I'd agree strongly with martin_au above that for the relatively small price difference, a 3 second instrument makes a lot of sense (1.5mm at 100m).

How you establish control also makes sense, as modern EDMs are very good so even the 5" instrument is likely to have an EDM good to about 2mm. If you resect your position based on as many targets as you can see, you're likely to get a much better result than directly sighting your position from another station using a more traditional traversing approach. The other thing is to take multiple rounds of measurements on both instrument faces so you also know how good your observations are. As Leiven mentioned, a competent surveyor will get a better result with a poor instrument than a poor surveyor with a good instrument. That said, why not be a good surveyor with a good instrument and get a great result?

So, based on what you guys input, I am seeing that while 1" is definitely overkill, if you were to bring on a total station on a job, you want the higher accuracy you can get with a competent operator. (From what I see it's much much more involved and technical that running a scanner, but someone with attention to detail can learn and succeed at providing good results with good equipment, be a 3" operator with a 3" gun over some time) or am I over simplifying it?

I like you last sentence, and I agree .
I think that setting control for scanning specially mobile like the VLX is a skill every competent scanning company needs to have regardless of it being done by a surveyor or not or in-house by an engineer or even higher skilled scan tech?

Even if you are hiring someone to shoot targets, knowing how things work can help you qualify consultants IMO. So I appreciate the input.

Seems like setting accurate control on a site for the purpose of scanning can certainly take some time, possibly hours compared to the actual scanning ? What is the ratio of any? What are you guys experiences?