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There pretty simple to use Tom. The bob hangs from center underside to center the level over a given point on the ground, as in a corner stake. To level the sighting head: set the base level bubble, swing head 180 degrees then level bubble again, swing 180 and see where it hits on the marks, split the difference and swing 180 until level bubble is split the same on both 180 degree swings. Your not looking to center the bubble each time.
Example, the first bubble shows on the 1st mark on the right hand side, you then swing 180 and now the bubble shows 2nd mark on left hand side, then center level would be between the 1st and 2nd marks when the head is turned 180 degrees from starting position and would land on the 1.5 mark when the head is turned 180 degrees in "level". Another words the level bubble has to be on oppisite marks when you swing 180 degrees. Accuracy is 1/4" per 100ft on the average but if setup correct will do 1/8" easy. To check head level to optic level:
Setup as I just described and shoot a level mark on a object in front, swing 180 and shoot a mark on another object. Move the transit 10ft or so and resetup. Now shoot the marks on both objects and the difference is how much the head level to optic level is off. There is an adjustment but alot of time to set correct! As you can see orperator error can be great so it's up to you how close your transit will be! You might want to check out a book in the library on transit use and trig functions.
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