Discussion  - 
Hi, so my name is Rob Walker, I'm a journalist in Savannah, GA, and I'm nervous that Mark has given me a reason to pay attention to Google Plus. (Sorry my profile is basically a half-hearted prank.)

But anyway here's something: I was in San Francisco recently and walking from Market/8th to the SF CalTrain station I kept noticing these odd little things, mounted about 10 feet off street level, a few-inches-wide tire-like object often (but not always) accompanied by small target symbols. What is it, is it functional, or some sort of street art thing. Anybody know?
Will Skelton's profile photoRob Walker's profile photoJoey Hurst's profile photoPaul Leader's profile photo
Calibration targets for auto-automobile nav array sensors. 
I frequently used to see these in London Underground stations. I'd still like to know what they are used for
Wow, this group gets results! Ask a mob of mutants, and the answers are prompt. Thanks y'all!
Rupert Applin's answer seems highly probable in light of the new Muni underground station that is currently being dug out just across the street from there.  Specifically, the above/under ground portal at 4th st. under the 80.  See: http://en.wikipedia.org/wiki/Central_Subway
As others have said these are for surveying and measuring the movement of buildings etc.

I've seen similar reflectors and targets on the downstream side of a small water storage dam in Switzerland. While we were there the lake had been drained, I presume for maintenance, and surveyors were taking measurements from concrete fixings on one side of the narrow valley by bouncing lasers off reflectors and targets on the other side. I assume they were measuring the flexing of rock due to the weight or the water and its absence.

There are also small targets on a building near me where a bridge was installed over a road last year, although I think those were used to provide consistent fixed points for the surveyors placing the bridge.
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