time/xtide - The NetBSD Packages Collection

XTide is a package that provides tide and current predictions in a
wide variety of formats.  Graphs, text listings, and calendars can be
generated, or a tide clock can be provided on your desktop.  XTide can
work with X-windows, plain text terminals, or the web.  This is
accomplished with three separate programs:  the interactive interface
(xtide), the non-interactive or command line interface (tide), and the
web interface (xttpd).

The algorithm that XTide uses to predict tides is the one used by the
National Ocean Service in the U.S.  It is significantly more accurate
than the simple tide clocks that can be bought in novelty stores.
However, it takes more to predict tides accurately than just a spiffy
algorithm -- you also need some special data for each and every
location for which you want to predict tides.  XTide reads this data
from harmonics files.  Ultimately, XTide's predictions can only be as
good as the available harmonics data.  Due to issues of data
availability and of compatibility with non-U.S.  tide systems, the
predictions for U.S.  locations tend to be a lot better on average
than those for locations outside of the U.S.
* Deviations of 1 minute from official predictions are typical for U.S.
  locations having the latest data.
* Deviations of 20 minutes are typical for non-U.S. locations or U.S.
  locations that are using obsolete data.
* Much longer deviations indicate a problem.