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  1. #1
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    Nov 2007
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    Default Chart correction NZ North Island . . .

    . . . has moved 4cm as a result of a month long 'slow earthquake'!

    https://www.mirror.co.uk/news/world-...otion-14974912

  2. #2
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    Sep 2010
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    Alness / Loch Ness Northern Scottish Highlands.
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    Default Re: Chart correction NZ North Island . . .

    These events have been possible precursors for giant, "megathrust" quakes and appear to have occurred on the undersea plate boundary off Japan two months before the magnitude 9.0 quake in March 2011, which generated a deadly and destructive tsunami.
    I would be just a little worried if I was in NZ just now.

  3. #3
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    Default Re: Chart correction NZ North Island . . .

    Quote Originally Posted by ProDave View Post
    I would be just a little worried if I was in NZ just now.
    Why-do you think Brent Swain is planning another visit?

  4. #4
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    Default Re: Chart correction NZ North Island . . .

    IIRC most places on the earth are moving somewhere else at about 2 cm a year. What a wonderful thing WGS84 is.

  5. #5
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    Default Re: Chart correction NZ North Island . . .

    Quote Originally Posted by RAI View Post
    IIRC most places on the earth are moving somewhere else at about 2 cm a year. What a wonderful thing WGS84 is.
    They're going a lot faster than that (but WGS84 doesn't show it).

    Rotation of Earth c 1,000 mph at equator, say 500mph for UK.
    Earth's orbit of sun c 66,000 mph.
    Earth and sun through Milky Way c 43,000 mph.
    Rotation of Milky Way c 483,000 mph.
    Milky Way path across universe c 1,300,000 mph.

    Make sure you've got your chart corrections up to date!

  6. #6
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    Default Re: Chart correction NZ North Island . . .

    Well, everything's relative.
    http://itrf.ensg.ign.fr/

  7. #7
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    Cambridge, UK
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    Default Re: Chart correction NZ North Island . . .

    Quote Originally Posted by RAI View Post
    Well, everything's relative.
    http://itrf.ensg.ign.fr/
    +1. WGS84 ignores plate tectonics and the motion of continents; the systems given by this link are required when doing that sort of work. I haven't personally used them, but some of my geophysical colleages have.
    +

  8. #8
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    Default Re: Chart correction NZ North Island . . .

    Quote Originally Posted by AntarcticPilot View Post
    +1. WGS84 ignores plate tectonics and the motion of continents; the systems given by this link are required when doing that sort of work. I haven't personally used them, but some of my geophysical colleages have.
    +
    I think WGS84 gets updated regularly following the ITRF updates. The difference is that WGS84 is based on the GPS ground stations which are a subset of the ITRF array.

  9. #9
    Join Date
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    Default Re: Chart correction NZ North Island . . .

    Quote Originally Posted by RAI View Post
    I think WGS84 gets updated regularly following the ITRF updates. The difference is that WGS84 is based on the GPS ground stations which are a subset of the ITRF array.
    No, WGS84 doesn't get updated. In fact, was supposed to be replaced by now, but that hasn't happened and (given the universal use of WGS84) would cause substantial difficulties. I've just looked at Wikipedia and it has some nonsense in the last paragraph - WGS84 is an ellipsoidal datum, so changing geoid models does not affect it. Changing datums is a major thing; it caused substantial confusion in the UK over a period of 10 years roughly between 1995 and 2005, and no doubt still causes problems if old civil engineering projects that were planned on OSGB1936 are reactivated. When the EU started to map things like land-cover etc. on a continent wide basis, they had terrible trouble matching across borders because of the use of different datums (and also different ways of defining land-cover types).

  10. #10
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    Default Re: Chart correction NZ North Island . . .

    Quote Originally Posted by AntarcticPilot View Post
    No, WGS84 doesn't get updated. In fact, was supposed to be replaced by now, but that hasn't happened and (given the universal use of WGS84) would cause substantial difficulties. I've just looked at Wikipedia and it has some nonsense in the last paragraph - WGS84 is an ellipsoidal datum, so changing geoid models does not affect it. Changing datums is a major thing; it caused substantial confusion in the UK over a period of 10 years roughly between 1995 and 2005, and no doubt still causes problems if old civil engineering projects that were planned on OSGB1936 are reactivated. When the EU started to map things like land-cover etc. on a continent wide basis, they had terrible trouble matching across borders because of the use of different datums (and also different ways of defining land-cover types).
    How about this:-
    https://en.wikipedia.org/wiki/World_Geodetic_System
    "The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).

    Many of the original authors of WGS 84 contributed to a new higher-fidelity model, called EGM2008.[13] This new model will have a geoid with accuracy approaching 10 cm, requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84). "
    With higher accuracies from the GPS/Galileo signals, both systems have to improve their reference frame accuracy.

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