Electromagnetic Wave Distance Measurement Over a Body of Water

[ridgeline]

We often use total station instruments, which utilize electronic distance measurements (EDM), to precisely measure and monitor structures. However, I noticed when measuring and monitoring a bridge over water recently, the measured distances fluctuated significantly as compared to other measurements over dry land. I can only assume the reflections off of the water are somehow bending the waves and changing the lengths as they travel over the water. Does this hypothesis seem reasonable? Does anyone have any reference literature that can explain what is actually occurring and how? I am thinking there may be some correlation between the intensity of the reflection and the amount the waves bend.. Any input would be greatly appreciated!

 

[IRstuff]

A typical laser range measurement device has a relatively small beam, so there ought not be reflections of the beam from the water. It's possible, however, that sunlight might interfere with the detector and cause false returns, but that could be readily verified by putting a shroud over the detector. Generally, since EDMs are supposed to performed outdoors, the receiver design should tolerate sunlight effects.

However, you need to describe the fluctuations; what were the amplitudes of the variations and what were the durations and frequencies of the variations. It's more likely, in my mind, that you are seeing actual fluctuations in the actual distance, due to wind, thermal, and even wave pressure or tidal, effects.

TTFN
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[chicopee]

Why not get copy of the prints from your State DOT if you are in the bidding stage?

 

[racookpe1978]

Is it a manned facility?

Could there be physical interferences (like ships or fog or boats or ice) that are fouling the path that an automated facility would not "notice"?