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    Re: Tidal constants
    From: Frank Reed CT
    Date: 2006 Sep 1, 16:06 -0500

    Geoff you wrote:
    "I see that US port  constants are available free on the internet, but it
    wouldn't be a surprise  in our 'Thatcherised' society that such information
    for the UK has to be paid  for:  the same philosophy as for access to  weather

    "The" US constants are only examples of possible  constants. There's no
    perfect set. For example, the constants included with  implementations of XTide
    (which is the engine underlying most Internet-based  tide calculators) are
    distinctly different from the constants you will find on  the NOAA/NOS site.

    Generally, the 'thatcherization' you refer to seeks  to get a piece of the
    pie from commercial ventures, which is not unreasonable.  It is, however, a
    nuisance for hobbyists. But you will usually find that you can  get this
    information for private non-commercial use if you ask around... 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  (note that this
    sort of reverse-engineering does violate the UK copyrights but  as long as you
    don't publish the results it's not a problem). This is similar to  generating
    tidal constants from observed tide gauge data but since there's no  noise in
    the inputs, the calculation will converge very rapidly on the correct 
    constants. About one year of hourly data is sufficient to get values for the  major
    tidal harmonics and most of the minor ones, too.

    "I have  written, for interest really, a programme to calculate tide heights
    from the  Meeus(?) algorithms which works accurately enough for most
    practical  purposes."

    Meeus tidal algorithms? Where did you get  them?

    "Times of high and low water are pretty good - who  can
    spot the precise moments of these anyway? "

    How many tidal  harmonics have you included?

    "but tide heights (for the  nicer
    places to visit!) depend on the topographical shifts which are  reflected in
    the most recent tidal constants."

    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.

    42.0N 87.7W,  or 41.4N 72.1W.

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