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    Re: Navigating Around Hills and Dips in the Ocean
    From: Geoffrey Kolbe
    Date: 2003 Aug 16, 18:27 +0100

    At 22:29 13/08/03 EDT,David Hoyte wrote:
    >        The joint NASA-German GRACE project has released the most
    > accurate map yet of Earth's gravity field. It shows Gravity Anomaly,
    > (mGal), on a global map at the URL:
    > http://photojournal.jpl.nasa.gov/catalog/PIA04652
    >        These gravity anomalies cause the geodic heigh of the ocean's
    > surface to vary around the world by up to 200 meters, 650 feet. Ref:
    > http://www.csr.utexas.edu/grace/publications/press/03-07-21-ggm01-nasa.html
    Sharing the skepticism of many that the mean ocean height can really vary
    by up to 200 metres world wide, I went and had a look at the cited web
    site, and a few others that I could find that actually showed maps of mean
    sea level.
    The first thing to note is that what is actually said at
    is the following:
    "....a precise definition of Earth's geoid, an imaginary surface defined
    only by Earth's gravity field, upon which Earth's ocean surfaces would lie
    if not disturbed by other forces such as ocean currents, winds and tides.
    The geoid height varies around the world by up to 200 meters (600 feet)"
    Note that it does not say that the _oceans_ surface varies by up to 200
    metres, as David Hoyte reports. The Earth's geoid is not just confined to
    the oceans. The Himalayas, for example, would raise the Earth's geoid by a
    goodly amount.
    For a map of the lows and highs of the ocean's surface, see:
    But there seems to be an anomaly in the map shown. The map of the world
    (shown on the left) is colour coded and there is a colour scale to the left
    of the map which ranges from +80 to -80 metres. This would seem to accord
    with David Hoyte's statement that the mean surface of the oceans does vary
    by up to 200 metres. But the map on the right is a detailed blow up of the
    Indian Ocean. This is also a colour coded map, but the scale to the right
    of the map only ranges from +5 to -5 metres. This map shows a range in
    height over the Indian ocean of around 10 metres, whereas the large world
    map shows a variation of at least 80 metres for the same region. There
    would appear to be an error here somewhere.
    A visit to: http://www.deos.tudelft.nl/altim/atlas/indian/mss_010.html
    shows another map for the mean sea surface for the Indian Ocean which seems
    to also show a variation of around 10 metres or less.
    Does anybody have any other links to Mean Sea Surface maps?
    Geoffrey Kolbe.

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