Last year a contractor asked me to check my topo because the layout surveyor said it was wrong by several feet. It didn't take long to figure out they had run from the BM on the East side of the bridge, but used the elevation from the BM on the West side of the bridge, a difference of several feet. Dumb error and a failure to check to a second BM. Really dumb because a check was one setup away.
Completely different site miles from the above: Today I compared the elevations on my topo to the development plan and noticed everything was off by several feet. Once again it didn't take long to figure out they had run from the BM on the East side of the bridge, but used the elevation from the BM on the West side of the bridge, a difference of several feet. Dumb error and a failure to check to a second BM. Really dumb because a check was one setup away.
I'm starting to see a common theme here, namely cutting corners.
> I'm starting to see a common theme here, namely cutting corners.
No, no, the common theme is mixing up the east side bench mark with the west side bench mark. Clearly, the solution is to set the bench marks on the north and south sides of the bridge, avoiding the troublesome east and west sides.
I've seen this on highway improvement jobs too. Typically just a few tenths between BM's, but enough to cause issues on flat sewer lines. We'll run a level loop between them all, and hold the BM the design was based on. Our DOT requires Level Loops on all control before topo, with limits on the error between runs. I think many are cheating these runs, otherwise these errors wouldn't exist. I could get better results hitting each with a GPS observation.
You just need to buy some Cracker Jacks compasses for your contractors.
> > I'm starting to see a common theme here, namely cutting corners.
>
> No, no, the common theme is mixing up the east side bench mark with the west side bench mark. Clearly, the solution is to set the bench marks on the north and south sides of the bridge, avoiding the troublesome east and west sides.
Jim, [sarcasm]You forgot your sarcasm font!![/sarcasm]
More in the interest of playing devils advocate than anything else I would like to point out that spatial awareness is not something that can be counted on. Certainly common sense is not something than can be counted on either.
It is always good practice to distinguish set or described bench marks from others on the same site or in the same vicinity. Set brass disk at S.E. corner, set chisel square at N.E. etc.
Of course sometimes there is no choice, but when there is choose wisely.
My personal favorite is when surveyors describe a bench as "Found BM Set in concrete..."