4. Optical plummet check

Because of the short sight involved, an optical plummet generally doesn't need an adjustment. However the surveyor can't be sure of that without periodically checking it. Fortunately, the check is quick and easy so it should be done as part of the TSI set up. More detail is included in Topic XV. Chapter G. Theodolite and Total Station.

The optical plummet can be checked by TSI rotation or using a plumb bob.

a. TSI Rotation

This method, described in Figure B-12,  only works with TSIs whose optical plummets rotate with the instrument.

After the TSI is precisely leveled, sight through the optical plummet and mark sure its mark is aligned with the ground mark.	(a)
Rotate the TSI 180° to reverse the plummet's position. Check to see if the marks are aligned. If they are, the plummet is in adjustment and the next step can be skipped.	(b)
If the marks do not align, the plummet is out of adjustment. The TSI is not set up directly over the ground mark. It is vertically above a point halfway between the ground mark and the current plummet mark position.	(c)
Loosen the instrument mount screw and slide the TSI halfway back to the ground mark. Tighten the mount screw.	(d)
Rotate the TSI 180° reversing the optical plummet and sight through it. The plummet mark should still be off the ground mark but on the opposite side.	(e)
Figure B-12 Optical Plummet Check

b. Pumb bob

This method can be used with a fixed or rotating optical plummet and requires a tripod with a plumb bob hanger.

With the TSI attached to the tripod, use the plumb bob to center the instrument. After accurately leveling the TSI make sure the plumb bob is still centered over the ground mark. Remove the plumb bob and sight through the optical plummet. If the plummet is in adjustment it will be centered on the mark. If not centered it shows how much the plummet is out of adjustment.

Because a plumb bob was used for set up the TSI is correctly centered over the ground point so it does not have to be moved.