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    Re: On polar nav
    From: Paul Hirose
    Date: 2002 Sep 23, 16:02 -0700

    George Huxtable wrote:
    > If Amundsen's measurements were to determine his distance from the pole to
    > within a mile or so, then this levelling had to be similarly precise, to a
    > minute of arc. This is asking a lot of a spirit level, which would need to
    > be as sensitive as that found in a theodolite. The levelling process would
    > have to be very painstaking. However, having been levelled once, the
    It may not be as bad as you think, George. The level on my Wild T3
    theodolite is very sensitive, 7 seconds of arc per 2 mm division.
    Bringing the instrument accurately level is a delicate task but not
    terribly difficult. I'm sure it could be done wearing heavy gloves.
    The quickest way, for me, is to turn the footscrews rapidly until the
    bubble shoots across the vial. Then I reverse the motion, but about 3x
    slower, and again overshoot the mark. I repeat the process until the
    bubble is centered within about one division. Now the fine adjustment
    can begin.
    Carefully noting the bubble position of one end of the vial, I turn
    the theodolite 180 degrees and observe the same end of the vial to see
    where the bubble settles. The midpoint of this position and the old
    position is my target. It will not necessarily center the bubble, but
    at this stage I'm only looking at one end of the bubble. At the end of
    fine leveling I do check for centering, just to see how much the level
    is out of adjustment.
    Of course the instrument actually has to leveled in two axes, so in
    practice it's being turned back and forth 90 degrees throughout the
    It has been awhile since I played with the T3, so I have just put it
    on a kitchen table and brought it level within about 2 seconds of arc.
    >From start to stop I required seven minutes. A lot of that time was
    spent waiting on the bubble to settle; these tenth second instruments
    have slow moving bubbles. It didn't help that I was quite out of
    Robert Eno said he has 30 second levels for his artificial horizon, so
    their bubbles would be much livlier than the one on my T3. With two
    levels there would be no need to turn a single one back and forth
    through 90 degrees. And of course there would be no need to level
    within a couple arc seconds!

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