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    Re: Landmark sights
    From: Paul Hirose
    Date: 2019 Aug 9, 21:15 -0700

    On 2019-08-09 8:31, Peter Monta wrote:
    > For example, how stable were the ground marks at transit telescopes?  As I
    > understand it, they put a target, collimator, and light source some
    > distance from the telescope to the south, to serve as an azimuth
    > reference.  If it habitually moved around multiple seconds, I would think
    > that would render it useless, and they would have gone with something
    > internal instead, such as a rotary encoder on the telescope's vertical axis
    > (microscope-reading scales).  Did they maintain statistics on
    > reference-mark performance at places like Greenwich, Pulkovo, or USNO?  Of
    > course they dealt with only a few marks directly to the south, not spread
    > out over the entire horizon, but at least it's a data point.
    Bear in mind that when an astronomical transit instrument observed its
    collimators, the distance was extremely short (by surveying standards)
    and presumably pains were taken to minimize temperature gradients around
    the building. This 1920s description of the Greenwich instrument
    mentions collimators north and south of the transit. Furthermore, when
    the latter was set vertical, openings in the tube give a clear line of
    sight so one collimator could observe the other.
    I don't see how the high stability attained in such a controlled
    environment can be relevant to geodetic triangulation, where the sights
    were many kilometers long over varying terrain.
    Determination of the collimation error:
    At sextant accuracy I'm not worried about horizontal refraction. Anyway,
    it wouldn't be detectable since you don't have the internal checks of
    triangulation, where the angles must sum to 180 plus the "spherical excess".

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