Optical and laser vs true plumb bob

In windy conditions (all the time in Chicagoland!) OP is the way to go. Bobs work ok, without wind

When I worked on the construction of the New City Hall in Toronto with a 'four screw' gun we had a piece of eave trough pipe to provide a 'wrap around' for the plumb bob string to prevent 'wind adjustment'.

Scary that we were setting control for precast concrete sections in an arc !

Cheers,

Derek

The optical or laser plummet has the advantages of speed and wind immunity. The price you pay is a need to frequently check that the plummet is in adjustment. There are procedures for that.

> The plumb bob, while over a point, will not move when the theodolite is leveled; while the plummets will.

With a 4-screw instrument, the plumb bob *does* move as the instrument is leveled, just not very much when the instrument is nearly plumb. This is because the axis of the footscrew fulcrums isn't coincident with the plumb bob attachment point. As the footscrews are turned, the instrument vertical axis rotates on the footscrew axis, which tips the plumb bob attachment point to the side a bit.

With a 3-screw (5/8x11) gun, the plumb bob attachment point is at the bottom of the tripod screw, which is only approximately aligned with the instrument vertical axis. The more out-of-level the tripod head, the greater the difference between instrument vertical axis and plumb bob attachment.

With a properly adjusted optical or laser plummet, you're seeing the actual projection of the instrument vertical axis. Ideally the plummet will be in the alidade, so the adjustment can easily be checked and accurate allowance made even if it's out a bit.

I use a plumb bob to mark to "true" center for adjusting my optical. A few years back, the Univ. of Fairbanks did a study on large land mass gravitational pull. Turns out Mt. McKinley is not the height (can't recall which way!) because the original survey in the 50's used a plumb bob to set up the theodolite to measure the height and these fellas used an optical. The mountain actually "pulled" the bob off the mark

-JD-

I guess I don't understand. My immediate reaction would be that the same gravitational pull would pull the tribrach bubble off the same amount that it might pull the plumb-bob. It seems like leveling the level bubble puts the tribrach perpendicular to the plumbline.

? (not arguing, just expressing my confusion)

I agree with Tom. A plumb bob and an optical plummet in perfect adjustment should give identical results over the short vertical distance involved here.

There would be a theoretical difference, which I doubt anybody could set up to demonstrate, over large height differences due to the possible slight curvature of the theoretical plumb line.

I suspect the problem was that a trig leveling calculation didn't use a good enough geoid model - perhaps only a nominal earth radius.