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    Re: Navigating Around Hills and Dips in the Ocean
    From: Fred Hebard
    Date: 2003 Aug 18, 11:32 -0400

    Just how big are the largest of these dips and rises?  We have
    estimates ranging from 20 to 200 meters.  A 400-meter change in
    elevation of the sea surface is hard to imagine.  I might note,
    however, that work would be done moving across a gradient in the
    gravitational field even if there were no change in sea-surface
    It would seem that accounting for a 400-meter-equivalent change in
    gravity might be a worthwhile consideration in ship routing.  How many
    tons of fuel would that be equivalent to, assuming some standard ship
    displacement, such as your average oil tanker?
    On Sunday, Aug 17, 2003, at 08:06 US/Eastern, Trevor J. Kenchington
    > Dan Allen asked:
    >> Hmmm.  How could these dips be known about almost 30 years ago before
    >> GPS?
    > Satellite radar altimetry. I am not sure when that started but it was
    > certainly long before GPS and is far more accurate, with measurements
    > of
    > sea-surface elevations to a precision of centimeters, I think, and
    > certainly a metre or so. The longitudinal and latitudinal precision of
    > the measurements won't be nearly as precise as with GPS but we are not
    > discussing here small elevations with steep slopes (e.g. wind waves)
    > but
    > rather large, low hills and large, shallow dips.
    > I have never understood how the altitude of the satellite above the
    > centre of the Earth is known with sufficient precision for this kind of
    > work, since its orbit must be distorted by the same gravitational
    > anomalies as affect the ocean surface. However, either NASA has
    > gathered
    > the necessary data for corrections or else the speed of the satellite
    > combines with its distance from the nearest large mass of anything such
    > that the error is negligible.
    > Trevor Kenchington
    > --
    > Trevor J. Kenchington PhD                         Gadus@iStar.ca
    > Gadus Associates,                                 Office(902) 889-9250
    > R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    > Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    >                     Science Serving the Fisheries
    >                      http://home.istar.ca/~gadus
    Frederick V. Hebard, PhD                      Email: mailto:Fred@acf.org
    Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
    American Chestnut Foundation                  Phone: (276) 944-4631
    14005 Glenbrook Ave.                          Fax: (276) 944-0934
    Meadowview, VA 24361

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