Re: accuracy of ISS / Venus transit track

From: Thomas Fly (
Date: Thu Jun 03 2004 - 20:18:05 EDT

  • Next message: Matson, Robert: "RE: accuracy of ISS / Venus transit track"

    Thanks Ed- those are details I hadn't considered!  I'm passing this on to my
    accuracy thread, for anyone else that may be interested.
    Of course, this raises the question as to what, exactly (if not the height of
    the Earth's surface above the WGS84 ellipsoid) the data in the GTOPO30 DEM
    In fact, GTOPO30 represents the oceans with the number -9999 (meters)- which is
    just as well for most amateur & professional astronomers & satellite observers,
    who aren't likely to try to gyro-stabilize a telescope out on the ocean.
    ----- Original Message ----- 
    From: "Ed Davies"
    To: "Thomas Fly"
    Sent: Thursday, June 03, 2004 7:18 PM
    Subject: Re: accuracy of ISS / Venus transit track
    Thomas Fly wrote:
    > ... though the 3 meter elevation for the Altantic Ocean, here, is a little
    suspect ;-).
    Not necessarily.  Actually, that's pretty good accuracy.
      Ellipsoid != geoid != sea level != ellipsoid.
    (They can be equal, of course, it's just that they're usually not).
    The ellipsoid is just that - a mathematical figure.
    A geoid is a surface of equal potential energy.  A geoid is the not the same as
    the ellipsoid or even parallel to it to any high degree of accuracy.  The geoid
    doesn't match the ellipsoid due to effects like the gravitational pull of
    mountains and other variations in the thickness of the Earth's crust.
    There's the Geoid (capital 'G') which is the geoid which most closely
    approximates sea level over the whole of the Earth.
    The Geoid can vary from the ellipsoid by tens or even hundreds of metres.  (I've
    a vague memory that the biggest geoid separation from WGS-84 is about 130
    metres, but don't quote me on that).
    Actual mean sea level at any given point differs from the geoid for a varietly
    of reasons like variations in salinity, areas with typical high or low air
    pressure, prevailing winds pushing the water up against land masses and so on.
    I imagine the Gulf stream and the Azores high are significant for sea levels in
    mid Atlantic but don't really know.
    The vertical datum for the Ordnance Survey of Great Britain is at Newlyn in
    Cornwall (or is it Devon?, somewhere is SW England, anyway) where the mean sea
    level is, if I recall correctly, 80cm below the Geoid.
    Therefore, it wouldn't surprise me to see a mean sea level which is up to a
    hundred or so metres above or below the WGS-84 ellipsoid and at least a few
    metres from the Geoid.
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