Help with tricky Control Network Setup?

Check the STAR*NET help files for the "Preanalysis" section, which is made for this exact scenario.

Basically all you do is enter approximate coordinates of the locations of where you plan on placing your control points, then you specify which measurements you are planning to observe (no values required). Based on your instrument standard error settings; STAR*NET will run a preanalysis that will estimate the expected standard deviations and error ellipses for the stations within your network.

Also, assumptions are that your instrument will perform to the accuracy you specified and you will be able to observe on site the measurements you indicate.

It sounds like you have some leeway in getting from the primary control to the new control and only the new control needs to be super-tight relative to itself.

But 2 mm in 500 meters, 1:250,000 ratio is going to take very careful work and averaging of a lot of measurements with a very good instrument. I doubt GNSS will be useful at all after you get from the old to the new network (if even for that). The preanalysis JW describes will be essential to forming a measurement plan.

Don't put too big a dimple on the rebar. And is a rebar stable enough to hold 2 mm during the project?

Is that 2 mm absolutely guaranteed to the builder and expensive if off more, 2 mm at 95% confidence (1 in 20 outside 2 mm), or "only" 2 mm at sigma?

First thing to consider is what sort of monuments you are going to install. No point in measuring to that degree of accuracy to monuments that aren't going to be stable. Or that have a big fat centering mark that take up a high percentage of your error budget.

Second, centering errors, both at the instrument and the target are going to be a major source of error. Plan on using tripod mounted targets, tribrachs adjusted daily, and high quality glass.

StarNet preanalysis is a great idea.

mrewenmacdonald, post: 409036, member: 11707 wrote:

I'd love to hear your thoughts!

Cheers,
Ewen

Hello Ewen. What total station are you using?

A well measured network of this size could just about be measured to 2mm inter-station accuracy relying on distances alone to deliver the goods. That is, well conditioned, braced figures measured with an accurate EDM, with care taken to remove constant and cyclic errors. I've done no dedicated tests to determine how much different monuments settle after being placed, or hammered in, but I suspect you'd measure them after giving them some time (days?) to settle. I think to really achieve 2mm if it's 2.5 or 3 sigma will require some real attention to detail, and may not be doable with some 'lesser' total stations. 1 sigma, not so much trouble.

After spending an inordinate amount of time testing and squeezing every last tenth of a mm out of a total station I'd enjoy such a challenge. Good measuring.

Hi just to clarify on the equipment, I am running a Leica TS15 R400 2" Robotic Reflectorless, and leica manufactured round prisms although I do have a Leica 360 degree prism I could mount as well but I have been told they may not be as accurate as I think they are. I have backsight distances ranging from 200-500m depending on what point I am trying to tie in.

When it comes to GPS Receivers, I have 2 (So I am hoping I can still produce enough redundancy without letting the timeline get too carried away (I Could rent some more if it would help greatly)). The Rovers are Leica GS14s

mrewenmacdonald, post: 409057, member: 11707 wrote: Hi just to clarify on the equipment, I am running a Leica TS15 R400 2" Robotic Reflectorless, and leica manufactured round prisms although I do have a Leica 360 degree prism I could mount as well but I have been told they may not be as accurate as I think they are. I have backsight distances ranging from 200-500m depending on what point I am trying to tie in.

first, run the instrument through a field calibration on the day to ensure current values for compensator, Hz and V collimation and ATR offset values. Tilting axis calibration not so essential unless you have decent elevation changes through the project.

2" is good. The 3" TS15 tests to about 2" angle stdev so I imagine the 2" will likely perform between 1" and 1.5" angle stdev. These specs are achievable with single face readings with an instrument in good calibration, but I'm not suggesting you only use the single face. You could. But don't. A feature/peculiarity of the leica instruments I've tested is that they will turn a near perfect right angle, so assuming yours does the same then you should include some right angles in your network and assign a low stdev for each direction involved in the right angle. provided you are taking the proper care to ensure setup stability during the measurement period though. And I do mean right angle within a few minutes. If you are manually sighting the pins for your directions then you don't have to test for side offset of the prism centre. but if you are an ATR person, or you sight the glass, then for the best angles then you might want to check if the centre of the prism as shot with ATR, or eye, agrees with the manually sighted pin. If it doesn't you can shim some of the plastic prisms so that the prism centre comes more in line with the pin.

Take the time to determine the individual prism constants and create custom prisms with the correct offset for each in the reflector list. You should be able to use a distance stdev in your network adjustment of 0.7mm + 1.5ppm after that as long as you are constantly on top of the temp & pressure values, and provided the EDM is operating as the factory intended it to. The EDM in that series of instrument is excellent. Take the proper care and it should deliver accurate measurements.

If you do the stuff I've mentioned then you'll be making the best measurements you can with that instrument. With appropriate network design you'll now give yourself the best chance of achieving the desired spec.

if you have a face of the 360 prism aligned exactly square to the instrument you may be able to make accurate readings to it but I can't say for sure. You'd have to determine the prism offset to one particular face and use only that face if you want the highest distance accuracy with it. I haven't tested the 360 prisms enough to know how much they vary face to face.

PS. Not to state the obvious but instrument distance measuring mode in standard or average. Fast or tracking will not do.

mrewenmacdonald, post: 409036, member: 11707 wrote:

Primary control but it is so far away that it is no good to me here. the thing is that with the amount of stuff in the way (Including trees and permanent structures placed after the primary control was created)

Thinking "outside the box"...
Can you see any of the primary control or any of the new points from the top of the "permanent structures"? If so are they solid enough for a setup?

I would wonder, and I honestly don't have enough knowledge of the subject, if GNSS observations would work for more than a semi-rough confirmation of the network. If you are looking to produce or confirm <2mm accuracy, how much of your time and budget would you want to invest in satellite observations.

"I have a control network to setup for a project which requires 2mm relative accuracy between points (Site Size: Approx: 300m x 500m) . There is already existing Primary control but it is so far away that it is no good to me here. the thing is that with the amount of stuff in the way (Including trees and permanent structures placed after the primary control was created) I don't have great geometry anywhere to run sets of angles to adjust the coodinates of the rebar I've Pounded with enough redundancy to give us a warm and fuzzy."

How good is the primary control?

Where is it relative to the project site?

Does it encompass the site?

Can you set on a primary control point and observe more than one project control point?

How many new site control points do you have?

How many site control points can be observed from more than two other SCPs?

Will you lose intervisibility as the project develops?

I doubt the 2" robot is your best choice. The 360å¡ prism is out for the control setup. You must be able to test all your prism setups to within 0.0005m of each other. Prism targets must have precise capability and all be properly oriented. No odd setups allowed. Use tripods for all points. Break and reset all points for reobservations.

You may have to create better control geometry. And whether or not you have great geometry you must be doing sets, for control layout, control layout checks and construction layout.

The client would be paying me a minimum days pay to pre plan this.

Paul in PA

Mark Mayer, post: 409052, member: 424 wrote: centering errors, both at the instrument and the target are going to be a major source of error.

Paul in PA, post: 409065, member: 236 wrote: Break and reset all points for reobservations.

That's very important because any centering error would be systematic for a single setup. If you rotate your tribrach and re-center, both at the instrument and prism, then the tribrach error gets randomized and your sighting error gets randomized so that they can average out.

Star*Net or other least squares tool assumes every error is independent of every other error so the pre-planning or post-analysis won't give realistic accuracy numbers if you don't do this.

And I'm still curious about how the client interprets the 2 mm spec.

Paul in PA, post: 409065, member: 236 wrote: You must be able to test all your prism setups to within 0.0005m of each other.

Thinking of the glass needed for this. You could empirically prove out any number of Leica mid-range tribrachs to that accuracy, but for the glass it looks like Leica's gph1p is rated for that. I really don't know of any other, and probably wouldn't want to mix and match between brands.

Use free stationing / resection with HI=0.00 to eliminate centering error at the instrument.
Use the reflectorless to set marks on vertical faces, eliminating bubble error and prism error.
When that control is established and proven, set the ground control from it using a short rod and a single round prism that you have tested for exact offset.

2mm is beyond what I could promise with my equipment.
That is not saying I can not do it, it is just going to be more than a simple run traverse and viola, there it is.
I have a Sokkia 330R, 3 second gun, and to achieve 2mm the closure will need to be within 2mm.
You would need to pull out an use all the bells and whistles of your modern TS or Robot inventory and keep track and enter everything from temp, pressure, repeated turning and to return and run several complete traverses thru the control points to hope you can achieve repeated accuracy.
good luck

My first Q is: 2mm/500m standard err or 95%?
How many points?

Never trust a tribrach. Plumbing of targets may require transiting from about 8-10 ft with a T1 or better. Or vertical collimators. Several permanent post monuments, as many as possible, is in order.

I can't imagine holding existing control if the key element is very high relative accuracy. One X Y, one azimuth. Relating to existing control may impart scale and distortion. Is the control known to be 1:250,000?

Triangulation is the most reliable backbone. This maybe a T3 job. With a rigid triangulation (0.6" std err) 8-16 sets, and redundant redundant redundant. The distances (2 mm std err) should not be allowed to deform the geometry. The residuals to the angles should be the same minimally constrained or with distances included. Then all of the distances will "best fit". Essentially all of the distances combined wil scale the triangulation. So calibrating mirrors individually for offset and of course using one calibrated EDM. Buy a good mirror. 360 mirrors aren't for this.

I just ran a similar project. The triangulation can be taken to a much higher reliability than distances. And, each distance may be better estimated by using calibrated eccentric bars. A mirror can be set 10 cm long, center, and 10 cm short allowing for averaging. (There could easily be 1-2 mm stagger.) I managed 2.5 mm over 180 meters. It was hard. And nothing worse than a sunny day.

2 mm over 200 m is hard. 500 m four times harder. And swearing to a confidence region that small takes sterile procedures. I don't think a total station can output the angles well enough, could be wrong.

A net like this sounds more like metrology. I hope that helps. Maybe one or two ideas for you.

Bill93, post: 409070, member: 87 wrote: That's very important because any centering error would be systematic for a single setup. If you rotate your tribrach and re-center, both at the instrument and prism, then the tribrach error gets randomized and your sighting error gets randomized so that they can average out.

Star*Net or other least squares tool assumes every error is independent of every other error so the pre-planning or post-analysis won't give realistic accuracy numbers if you don't do this.

And I'm still curious about how the client interprets the 2 mm spec.

I hadn't even finished editing that post. Thanx.

For that level of relative confidence, a few 100 ft, or 30 m baselines established, by the book, with a NIST calibrated Invar standard is cheap insurance.

R.J. Schneider, post: 409071, member: 409 wrote: You could empirically prove out any number of Leica mid-range tribrachs to that accuracy

Meant the <2mm, not the 0.0005' Paul mentioned.

Before you really invest any more energy in this project, be sure of what is going on.
It just may be a blunder... They MEANT 2cm, not 2 mm.
I have seen that happen before.
Some engineer, used a total sta. Found out that it's "least count" was 3 decimal places. So, this amounted to 1 mm, so he demanded a project accuracy way beyond what was actually needed...

Nate The Surveyor, post: 409125, member: 291 wrote: Before you really invest any more energy in this project, be sure of what is going on.
It just may be a blunder... They MEANT 2cm, not 2 mm.
I have seen that happen before.
Some engineer, used a total sta. Found out that it's "least count" was 3 decimal places. So, this amounted to 1 mm, so he demanded a project accuracy way beyond what was actually needed...

Really. 1:250,000 over a 1/2km project is the highest accuracy one could even ask for.

That kind of accuracy, over a few hundred meters, is for the likes of Fermi Lab, maybe atomic power plants.

Rebar in earth won't do. Trucks, heavy equipment, paving? compaction, pile driver? Even 100 ft away would make me nervous.

Is the cite fully developed? Or under construction?