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    Re: Landmark sights
    From: Paul Hirose
    Date: 2019 Aug 7, 13:47 -0700

    The phenomenon of horizontal refraction was well known in the era of
    geodetic triangulation. The C&GS manual of first order triangulation
    (1926) says, "Under ordinary conditions the air strata are of greater
    density near the ground and lie roughly parallel to it. Over a sloping
    terrain these strata of different densities are not horizontal, and a
    ray of light passing through them will be bent horizontally as well as
    vertically... The force and direction of the wind are also determining
    factors, for with a strong wind the differences in the temperatures and
    densities of adjacent air strata are less marked."
    
    "It must not be thought that errors caused by horizontal refraction are
    always of small magnitude. On first-order triangulation, where the
    probable error of a direction may average about one-half second, cases
    are not infrequent where horizontal refraction has caused an error of
    from 3 to 6 seconds."
    
    The manual notes that horizontal refraction also occurs on flat land, as
    when a line of sight passes over a corner of a plowed field surrounded
    by woods. The heated air rising from the plowed land forms a vertical prism.
    
    Similar effects occur in urban surveying. The 1909 report on the
    triangulation of greater New York says,
    
    "Many large chimneys were in line with some of the stations and smoke
    and hot gases arising from them rendered the observations very difficult
    and in some cases impossible. By taking advantage of a favorable wind
    and watching for a chance some observations were obtained on these
    obstructed lines, but a few of them were lines bent from horizontal
    refraction and were rejected."
    
    "On many of the schoolhouses which were occupied in cold weather, some
    one line passing over a chimney of the house in which a fire was burning
    was impossible to observe. In some cases the fire was banked, at our
    request, on Saturdays when there was no school, and the lines observed
    at that time."
    
    https://babel.hathitrust.org/cgi/pt?id=nyp.33433066326012;view=1up;seq=7
    
    One day I spent a couple hours at the National Geodetic Survey site
    looking up the old triangulation points named in the 1909 report. The
    great majority are gone, but a very few survive and have been visited
    recently. In at least one case, modern photos were included in the
    datasheet.
    

       
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