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    Re: theodolite altitude measurements
    From: Paul Hirose
    Date: 2004 Apr 29, 14:33 -0700

    Kieran Kelly wrote:
    > No definitely not. After using a theodolite several times the frustrating
    > thing is that every time you touch the instrument the bubbles seem to move.
    > Also if there is an error in the bubble it is quite a complicated procedure
    > to allow for. The great advantage of a theodolite is that it gives azimuth
    > as well as altitude and it is free from the "wobbly hands" of an observer.
    First off, I didn't see the post to which Kieran is replying, perhaps
    because it was HTML. I have a filter which routes all HTML email to
    Now about theodolites. Kieran is correct, it's difficult to keep the
    level bubble dead center. (The plate level on a Wild T3 moves its
    bubble 2 mm per 7 seconds of angle.) So theodolites that read their
    circles through optical micrometers have "compensators" to eliminate
    vertical angle error due to misleveling. They work by slightly
    deflecting the microscope image of the circle graduations. Some use a
    pendulous prism suspended by Invar ribbons. Others use the reflective
    surface of a fluid (not mercury) in a chamber. Either way, the
    vertical circle reading is automatically corrected, provided the
    misleveling is within the compensator's range.
    Instead of automatic compensators, vernier theodolites have an index
    level, visible in this picture:
    You level the vernier frame with a fine adjustment knob before
    each reading.
    If a compensator or index level is out of adjustment, there's a
    systematic error in vertical angles. However, the sign of this error
    reverses when you reverse the instrument. (E.g., observe with the
    vertical circle on your right instead of left.) Since averaging the
    "face left" and "face right" observations cancels index error, it
    needn't be eliminated, just kept reasonably small. The manual for the
    Wild T2 (1-second theodolite) says the face left and face right
    altitudes should agree within 60 seconds. Adjustment merely requires
    opening a cover and turning a screw.
    Simple instruments have fixed verniers for the vertical circle. With
    these, the whole instrument must be carefully leveled before measuring
    a vertical angle. Any error due to misleveling is not reversed when
    you change face. However, index error due to a misadjusted vernier
    *is* reversed, so it's still beneficial to change face.

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