HARTID(1)

NAME
hartid - predicts tides from harmonic constants

SYNOPSIS
hartid year day hr min sec nterms samp

DESCRIPTION

Given the ``harmonic constants'' for a tidal series, hartid computes the predicted tides for a specified time. The harmonic constants (in the format specified below) are read in from the standard input, and the predicted tides written to the standard output. The computation (described in more detail below) infers the value of small constituents from those of larger ones, so only a few are needed to give a good result. The arguments on the command line are:

year day hr min sec
    The time of the first output value, in Greenwich time. Note that the time must be after 1900 AD.

nterms
    The number of values to be written out

samp
    The sample interval, in seconds.

HARMONIC CONSTANTS

The harmonic constants are often prepared by harprp. The format of this file is:

1.  The first line should contain either the letter ``l'' if the phases will be given relative to the local potential (as with the $kappa$ of traditional tidal analysis) or a ``g'' if the phases will be given relative to the Greenwich potential (as with the $G$ of traditional tidal analysis). NOTE, however, that phases are to be given with lags regarded as this is the reverse of the traditional usage.

2.  If the first line contains an ``l'', the second should contain the East longitude in degrees of the locality; for Greenwich phases this entry is omitted.

3.  The following lines (starting with the third for local phase, the second for Greenwich phase) contain the specification of the tidal constituent (its Cartwright code—see below), followed by the amplitude (in whatever units are appropriate) and phase (of whatever type, in degrees). These lines are read with a formatted read statement, the format being (6i2,2f9.0). The first character of each line is therefore a blank (since the first number is always a single-digit integer). The only exception is the last line, which should simply contain a ``-1'' at the beginning; this signals the end-of-list to the program. A typical (though short) file might look like:

    l
    -116.455
     1-1 0 0 0 0 7.41000 -78.0
     1 1 0 0 0 0 11.7900 -94.0
     2 0 0 0 0 0 16.0600 -287.0
     2 2-2 0 0 0 5.01000 -282.0
    -1

The program expects to be provided with at least one harmonic constant for both the diurnal and semidiurnal bands. Long-period tides (excluding semiannual and longer periods) will be included if any harmonic constants are provided for long-period tides; similarly, if the constants are given for $roman M sub 3$, terdiurnal tides will be predicted.

THEORY
In computing a series, hartid uses a larger number of tidal constituents than are input; the amplitudes of these additional constituents are found by spline interpolation of the admittance (the ratio of the potential amplitude to that observed, or in this case found from the loading computation). This automatically allows for the various nodal modulations. The fits are different for each species; note that if there are 3 or fewer harmonics for a species, a piecewise straight-line fit is used. A total of 9 long-period, 30 diurnal, 24 semidiurnal, and 4 terdiurnal constituents are used in the prediction (but no overtides). The cutoff was a level of 4% of the largest tide ($roman M sub f$) for the long-period tides, 0.5% of the largest tide ($roman K sub 1$) for the diurnals, and 0.3% of the largest tide ($roman M sub 2$) for the semidiurnals. (The rms accuracy should be about this, but at some times smaller missing constituents could combine to give a larger error). It should be noted that 3 of the long-period, 7 of the diurnal, and 5 of the semidiurnal constituents are in fact nodal ``satellites'' to larger ones.

TIDAL CODES

The Cartwright codes and Darwin symbols for the largest tides are:

0 2 0 0 0 0 $roman M sub f$	0 1 0-1 0 0 $roman M sub m$

1 1 0 0 0 0 $roman K sub 1$	1-1 0 0 0 0 $roman O sub 1$	1 1-2 0 0 0 $roman P sub 1$	1-2 0 1 0 0 $roman Q sub 1$	1 2 0-1 0 0 $roman J sub 1$	1 0 0 1 0 0 $roman M sub 1$
1-2 2-1 0 0 $rho sub 1$	1 3 0 0 0 0 $roman OO sub 1$	1-3 0 2 0 0 $2 roman Q sub 1$

2 0 0 0 0 0 $roman M sub 2$	2 2-2 0 0 0 $roman S sub 2$	2-1 0 1 0 0 $roman N sub 2$	2 2 0 0 0 0 $roman K sub 2$	2-1 2-1 0 0 $nu sub 2$	2-2 2 0 0 0 $mu sub 2$
2 1 0-1 0 0 $roman L sub 2$	2 2-3 0 0 1 $roman T sub 2$	2-2-0 2 0 0 $2 roman N sub 2$	2-3 2 1 0 0 $epsilon sub 2$	2 3 0-1 0 0 $nu sub 2$

3 0 0 0 0 0 $roman M sub 3$

AUTHOR
D. C. Agnew