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    Re: Tidal constants
    From: Geoffrey Butt
    Date: 2006 Sep 1, 20:32 -0500


    Frank wrote:
    "Another  approach would be to generate your own tidal constants from a long
    run of  predictions from some software that has licensed the official tidal
    harmonics . . . About one year of hourly data is sufficient to get values
    for the  major tidal harmonics and most of the minor ones, too."

    An interesting thought;  but I think I would get better results using the
    published data, even if a few years old.  My interest in fiinding an
    electronic source is to avoid errors in transcribing (or scanning) published
    data.

    ... and
    "Meeus tidal algorithms? Where did you get  them?"

    No, the well known astronomical algorithms with which you can calculate the
    position (RA and distance) of the Sun and Moon - and can therefore calculate
    the tide raising forces which are then mediated by the tidal constants of
    the port.

    ... and
    "How many tidal  harmonics have you included?"

    The 14 variables specified for the UKHO tidal prediction method

    ... and
    "What sort of  topographical shifts? With rare exceptions, you could use
    hundred-year-old tidal  constants (pretending that they exist and are
    calculated correctly) and they  should yield very good present-day tide
    predictions."

    All the ports, for example, of the Irish Sea (particularly Liverpool and
    Morcambe Bays), the Bristol Channel, the North Sea coasts of UK, Holland,
    Germany and Denmark are affected by shifting sand/mud deposits and by the
    changing strategies for dredging key commercial approaches.  I ran my
    programme firstly with data now 15 years old and got poor agreement with
    published tide tables for certain of these ports:  largely corrected when I
    used data published 10 years later.  Frank's assumption is true only for
    non-silting locations.

    Geoff Butt


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