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    Re: Historical charts and mapping
    From: Bruce J. Pennino
    Date: 2016 Aug 19, 07:48 -0600
    I have a number of  American surveying books dating from the early 1900s.  One series of Basic, Intermediate and Advanced books is by an MIT professor.....forget his name. But a major portion of the advanced book is devoted to establishing datums, triangulation, extending known grids, curvature of earth, timing errors, lunars.......all of these matters, especially  important in laying out 1 st order control points in "New" states. Mason and Dixon did an incredible job which stood the test of time. There is a fine short book on the skills and techniques used by Mason and Dixon.  The British chart  topographers were especially talented. The book about Darwin's Captain gives a lot of insight. Many of the early engineers from West Point went into coastal surveying and harbor works. Their charts are works -of- art. Drawing was an important skill developed at West Point.  That is where Whistler "the son" further developed his natural ability from his father who was a talented surveyor and civil engineer.

    Regards to all

    From: "Frank Reed"
    Sent: 17 Aug 2016 15:33:35 -0700
    Subject: [NavList] Re: Historical charts and mapping

    Don Seltzer, you wrote:
    "But which reference meridian would have been in use in 1847?  Would surveys be referenced to the Washington meridian of that time, providing a different offset to WGS84 coordinates?"

    Well, I don't think anyone was really talking about a Washington meridian in 1835, but by the time of the re-printing in 1847 someone may have worried about it. There's only one longitude referenced in this particular chart: the Stonington Lighthouse. Let's suppose for now that they performed actual astronomical observations there. They would have used terrestrial instruments, but we can imagine them using a high-quality nautical sextant and a Mercury-filled artificial horizon. An altitude when the Sun bears more or less east or west gives local time, and the chronometer provides Greenwich time. Subtract and you get your longitude. But how, exactly, do we know that the chronometer reads correct Greenwich time? It has to have its error and rate checked at some location with a known longitude. So really our measured longitudes are referenced to that known longitude. I suspect, but don't know, that the Coast Survey in this era would have checked their chronometer errors at the nearest major port, probably New York in this case. Tricky, huh?

    Frank Reed

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