At 10:03 22/08/05 -0400, Inch wrote:
>This brings me to a question. I have a very nice opitical transit and a
>bunch of laser levels.
>
>All mount on tripods and have the common "3 dial base" and bubble level in
>it.
>
>What is the correct way to level this? I always have a problem with it.
Long ago, before the days of self levelling instruments and lasers,
surveying levels had two bubbles. A round bubble for the initial setup and
a cylindrical bubble for fine levelling at each reading.
The procedure was to get the best level you could on the round bubble
initially with the tripod legs, this was a lot easier with telescopic legs.
Then set the axis of the telescope parallel with two of the screws and
level the round bubble as best you could with the two screws. After that
rotate the telescope through 90 degrees and level again with the third
screw. Provided you were careful and the base wasn't worn you were set and
the instrument would be level throughout the 360 degrees. Otherwise you
kept on relevelling until you ran out of thread... Before taking each
reading you would fine level the instrument with the cylindrical bubble,
this was calibrated like a builder's level or the nicer instruments had an
optical split bubble viewer.
The same three screw routine holds good for lasers. Spectra Physics say the
original setup should be within 5 degrees of level. Nowadays many of them
have quickset bases in place of the three screws.
Nick Brearley
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