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    Re: True North and Precession
    From: Bill Lionheart
    Date: 2021 Feb 16, 09:59 +0000
    I know the shift of the Earth's rotation axis is different from the continents shifting relative to each other but I wondered how much tectonic plates  rotated on a geological time scale and this animation shows quite a bit of rotation https://youtu.be/gQqQhZp4uG8

    Archaeologists from the distant future, evolved from cockroaches or whatever,  will puzzle over any remains of things that were intended to be aligned. 

    Bill

    On Tue, 16 Feb 2021, 03:49 Ian Vaughn, <NoReply_Vaughn@fer3.com> wrote:

    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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