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    Re: Measuring (and Calculating) Dip
    From: Paul Hirose
    Date: 2013 Mar 09, 13:11 -0800

    Bill Morris wrote:
    > When I checked it, the theodolite turned out to have a substantial index 
    error, quite enough to account for most of the differences between the dip 
    from the NA and the dip I measured.
    >  I have negotiated another visit to the beach with the staff (wo)man and 
    have removed nearly all the index error, a none-too-easy task with a bubble 
    that has a sensitivity of 15 seconds per 2.5 mm movement. I will take 
    readings off each face (not strictly necessary with the T2) if my assistant's 
    patience will allow it.
    Bill, a mean of the readings on both faces is always good technique. It
    simultaneously reveals and eliminates index error. The T2 manual
    discourages single face vertical angle readings, except when one minute
    of error is acceptable and the theodolite is known to be in good
    adjustment (index error not more than 30 seconds).
    That is from the Wild manual for the late type T2 with automatic
    compensator. I understand yours has the manually centered vertical index
    bubble. However, I believe the recommendation above is still applicable,
    since the compensator only removes the effect of minor dislevelment, not
    index error.
    Even for their first order theodolite, the T3, Wild says the horizontal
    collimation and vertical index errors should be left alone unless they
    exceed 15 or 30 seconds, respectively. I have been able to adjust to
    that modest accuracy indoors by sighting through a window - far more
    comfortable than fiddling with the instrument outdoors.
    For your dip measurements, perhaps a reasonable compromise would be to
    do half on one face, half on the other. Or, begin by taking the vertical
    angle on both faces to some suitable target (not necessarily the
    horizon). Then do all dip measurements on one face. Finally, take
    another pair of vertical angles on both faces. The final target need not
    be the same as the first. In fact, a different target would be better -
    you would be unlikely to make the same reading blunder in the
    microscope. Before leaving the site, calculate and compare the initial
    and final index errors.
    I agree with your remark that the scale reading microscope "has its
    limits and variations." The U.S. Coast & Geodetic Survey first order
    triangulation manual says two successive readings on a Wild T3 should
    rarely differ by .3", and usually agree to .1". I'm not that good -
    "usually within .3, rarely within .1" is closer to my standard!

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