Bridge Deformation Study

Can't be done. Ridiculous Spec if that is the accracy requested

There is a major league difference between reporting to the nearest 0.001' and reporting accuracies to the nearest 0.001', and from what I see a majority of clients (and surveyors, which is admittedly pathetic...) don't understand the difference. I've seen a lot of clients say something along the lines of well XYZ Company says they can detect movement within 0.01' or 0.001'! In reality, they're usually just reporting decimal precision to 0.01' or 0.001'... The actual accuracies achieved are usually vastly different.

I agree with John, though. Ridiculous spec.

I agree with John. The bridge will move a lot more than that while my Labrador Retriever walks across it. Or when the sun shines on it. But for any monitoring job a higher spec'd gun is always better. I reiterate my comment in your other thread about tribrachs, tripods, etc.

I've done many monitoring jobs with a 3" gun and, even at that level, have been sufficiently convinced that structures move by a hundreth, or much more, in the normal course of a day. Bridges in particular are often quite flexible and move enough by the passage of vehicles to be visible to the naked eye.

A wooden bridge will bounce around as you stand on it with cars crossing.

I believe you're a public employee, so there's not much liability for your survey department to publish numbers without any meaning. However, a private firm should never agree specs like that. It's better to simply walk away from those jobs if they won't tone down the requirements.

Any one asking for that accuracy or promising that accuracy has no business being involved in the bridge project.

@ john-hamilton

Thanks for the reply. Reading all the replies makes me want to talk with whoever is in charge of the project.

John is spot on. Unfortunately, every monitoring spec I see now days is for 0.001'. Every time I see it, I have to educate the engineers that you might be able to those results in a climate-controlled environment but even that is a big 'might'.

@ Steinhoff

Thanks for pointing out the difference between reporting a number and its actual accuracy or should that be precision? Do precision requirements for this kind of job call for 1 sigma, 2 sigma, 3 sigma, etc. levels?

You can report 50 sigmas but that doesn't mean the instrument can actually measure that. I think that's what the guys are getting at, you gotta tell the client our instruments aren't good enough for what they're asking.

a few ago there was a fire under a major bridge here in Pittsburgh (City of Bridges). The newspaper quoted the surveyor monitoring the bridge that it hadn't moved more than 0.001 inch. Even if it was 0.001 feet, or 0.001 m, I call bull, especially since it was just a regular survey firm, not one that does deformation surveys

@ field-dog

With monitoring we are generally more interested in accuracy (i.e. the determining the "true location" of something right now versus your baseline position... or at least the best you can statically get in terms of "true location" relative to the monetary cost & error budget).

Obviously precision/repeatability is very important, and a combination of accurate & precise work is pretty much always necessary. BUT... We need to remember the underlying question, which is: where is it now, and what's the difference between where's it at now relative to where it was previously? And most importantly: is that difference significant?

EDIT: to clarify, my post above is me basically saying "a lot of people don't know the difference between decimal precision and positional accuracy, even though they are wildly different." There's a reason why when I publish project control (which I publish to 3 decimal places usually), I have a boilerplate statement stating "COORDINATES ARE REPORTED TO 3 DECIMAL PLACES, WITH THE UNDERSTANDING THAT DECIMAL PRECISION DOES NOT NECESSARILY REFLECT ACCURACY."

Thanks for the reply. Reading all the replies makes me want to talk with whoever is in charge of the project.

Had a guy ask for control...he wanted to have a control network...I asked how accurate?

"Perfect" (I said not possible.)

"0.001" (I said give me $200k.)

Do something fun: run the geometry of the set up through Star*Net's pre-analysis routine. It will tell you the geometric strength, and given the parameters of the instrument, you can make some analyses of the expected accuracies.

That said, if you have a strong baseline, and you are sitting on the same place and not changing setups (say a pedestal) and just shooting the same location over and over as something moves across the bridge, I would expect a good station within 500' to have relative precision between shots of 0.005' for distance, and under 0.01' overall could be achievable, depending on the stability, sun, heat changes, target, etc. I assume we are talking about no load shot, then drive a dump truck on it, and park it, and then measure it with sets of angles.

If you are looking for vertical deformation, measured dynamically, I would just use a scale attached on the bridge with very good level, and make a series of observations as the load went back and forth.

The <b style="">newspaper <b style="">quoted the surveyor...

I think I found the source of the error.

No one despises journalists more than those "quoted" by them.

Idaho Code 55-1705. STATED OR IMPLIED ACCURACY. The accuracy of coordinates shall be as stated in the document containing the coordinates. The expression of coordinates to decimals of the units used may not be construed as a statement of expected accuracy or reliability, unless so stated in the document containing the coordinates. Statements of accuracy must be defined as relative, absolute, or both.