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    Re: True North and Precession
    From: Ian Vaughn
    Date: 2021 Feb 15, 18:34 -0800

    I'd leave an authoritative answer on precession to others, but might I suggest a more modern reason for the discrepency?  2.4 degrees sounds plausible for a projection / datum shift.  I calculated the difference between UTM grid north and true north as about 1.5 degrees west at Bejing, but could have made a math mistake-- and Google Earth is infamous for using goofy datums/projections.  If it is a projection issue, I'd expect different errors on east/west vs. north/south lines.

    Either way, I'd bet on a Chinese survey team from 600 years ago over Google Earth's datum management.  This issue comes up often enough ESRI posted an article about it.

    https://support.esri.com/en/technical-article/000009982#:~:text=Google%20Maps%20and%20Microsoft%20Virtual,geographic%20coordinate%20system%20(datum).

    Disclaimer: I'm not really a GIS person, I just sometimes play one in real life.

    --Ian

       
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