Bridge Deformation Study
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.
@ thebionicman
Interesting.
@ dmyhill
Do something fun: run the geometry of the set up through Star*Net’s pre-analysis routine.
Is pre-analysis available in Civil 3D? I talked with my boss today about the project. The purpose of the project is to look for a trend of horizontal movement. We have been monitoring the bridge for a little over a year now.
I've never seen or used it in anything other than Star*Net. Doesn't mean it doesn't exist in some other software program. I wouldn't perform a monitoring project without doing it first though.
Star*Net is not expensive in the scope of things, certainly not if you are talking about buying a 1" robot for kicks and giggles.
Running SN through all the included tutorials will make you a better surveyor, BTW.
