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    Re: "Latitude"questions
    From: Paul Hirose
    Date: 2003 Sep 26, 22:22 -0700

    "Royer, Doug" wrote:
    >
    > observer would then reverse the scope 180* on the verticle plane and repeat
    > the proceedure to cancel certain errors and allow corrections to be computed
    > for others.I cannot visualize this scope.Can anyone help?
    
    
    I'm not sure what instrument you're describing, but my guess is they
    used a meridian circle:
    
    http://oacosf.na.astro.it/MUSEO/INGLESE/01i.html
    
    The scope is only able to tilt up/down, and always stays in the plane
    of the meridian. The west and east trunnions are supported on piers
    anchored in the ground.
    
    To reverse the instrument, it's lifted off those piers and turned
    around using the "reversal undercarriage" depicted on the Web page.
    After completion of the operation, the trunnion that was resting on
    the east pier has moved to the west pier, and vice versa.
    
    With a portable meridian instrument like a Bamberg (used for years
    by the U.S. Coast & Geodetic Survey), the scope assembly could be
    lifted and reversed by hand. An instrument this small left no room
    for an observer to sit comfortably beneath the scope, so the line of
    sight was bent 90 degrees by a mirror. The eyepiece was on the end of
    the horizontal axis (the left side, in the picture).
    
    http://oacosf.na.astro.it/MUSEO/INGLESE/08i.html
    
    A surveyor's theodolite is even easier to reverse. After making an
    observation and recording the angles, turn the alidade (the part that
    rotates about a vertical axis like a carousel) 180 degrees. The
    eyepiece now points away from you. Pull it up and over, rotating the
    scope about the instrument's *horizontal* axis so the eyepiece
    momentarily points straight up then comes back around to you. Now the
    instrument has been reversed. Observe the object again in this
    position. If you average the readings from both positions, any
    out-of-square conditions among the instrument's vertical, horizontal,
    and optical axes are cancelled. Also, the index error of a vertical
    angle is cancelled.
    
    
    

       
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