
A91. Interpolation of spectra
Spectra are interpolated by Swan during the generation of boundary conditions,
and in the output of spectra.
The interpolation procedure is not a linear interpolation binbybin, but
such that the shape of the spectra is preserved as well as possible.
This tested by interpolating two Jonswap spectra with different parameters.
2 cases are considered:
 A91a
 a stationary with two spectra given by means of parameters at two ends of
one side of the computational grid
 A91b
 two spectra are given by means of TPAR files at two ends of
a time interval on one side of the computational grid
It is noted that the wave height does not vary linearly; the energy varies
linearly, so the wave height varies as the square root of distance or time.
The wave period and direction also do not vary linearly; this is mainly
because Swan computes a weighted average, so the larger energy has the
largest influence on the outcome.

