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    Re: Measuring (and Calculating) Dip
    From: Paul Hirose
    Date: 2013 Mar 11, 20:21 -0700

    Bruce J. Pennino wrote:
    > I honestly don't remember  how my internal optical  angle scale functions in 
    the "reversed" mode ie, whether the sign of the  angles will be greater or 
    less than the previous mean value.
    Vertical circle graduations have so many variants, the Coast & Geodetic
    Survey used to require the circle be sketched on the observation log
    sheet so the staff in Washington could properly reduce the numbers. The
    Wild T3 with its "degrees" which are actually 2° is the most unusual
    I've seen.
    A common scheme is 0° to 360°, with 0° at the zenith. A near horizontal
    shot will read about 90° face left, 270° face right. Subtract the little
    number from the big one, subtract 180°, then divide by 2 to get
    elevation above the horizontal.
    The amount by which face left and right fail to sum to 360° is twice the
    index error, which should remain constant regardless of the angle
    observed. This can be used to detect blunders when reducing your data.
    If your instrument has extra hairlines for stadia measurements, you can
    take a reading with each hairline, reverse face, and repeat the process.
    The spacing of the hairlines is immaterial since the top hairline face
    left is on the bottom face right. In effect, you have a big index error,
    which is eliminated by combining readings on both faces. The Wild
    company recommends that method over multiple measurements with the same
    hairline. But it is a little more complicated, since the observing and
    recording routine must pair up the readings correctly (though I'm sure a
    goof is not hard to detect and fix).
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