- How can I find site xxx, for which I need a prediction?
- First, notice that there are categories on the
top-level site selection page.
Make sure you're looking in the right section. Sites
near the edges of a geographic category might actually be in the
Check for sites that might be geographically close enough (but be
cautious about extrapolating tide information across any distance).
Some sites might simply be geographically misplaced: check the
list of all sites (long though it is).
Still can't find it? See:
adding a site
- Can you please add site xxx to the list?
- The short answer is no. But the longer answer is maybe. The sites
for which we generate predictions are primarily sites for which long-term
observations have been collected (at the site with real instruments) by
governmental bodies. Those observations have then been distilled into
a set of mathematical constants ("constituents") for a tidal prediction
equation. So these are not purely mathematically derived (as are, for
example, lunar phase or sunrise/sunset calculations), but must be
based on actual observations at the site.
The newest version of the software upon which this WWW predictor is based
(the XTide program by
David Flater) can also use known time and height offsets from those
base sites to calculate predictions for subsidiary sites. If that
information is known, it can be used to generate predictions.
Getting tidal information may not be easy. Governments are increasingly
reluctant to release it, both because they can sell the information
and for "security" reasons (i.e. they think that an
invading army could use that information to optimize when they send
troops ashore on the beach). See David Flater's information on
how to get hold of such data.
If and when you get new tidal constituents that you'd like to see on
this site, send them to Bob Kenney - he maintains the database of
tidal information. I get it from him. He has a web site at
and an email address:
- A site that used to be on your page is gone -
- There are several possible reasons:
Simplest is that we may have updated the tidal site database and
your site name may have been changed. Even a slight change will cause
old links to fail. Check the directions on
finding a site to see if you can locate it under
a new (presumably improved!) name.
If we receive information that predictions for a site are incorrect
we will remove the site from the database. Since we have no means to
collect or verify information on our own, we prefer to err on the side
of caution and delete even potentially dubious sites.
Particularly if the site of interest is outside of the U.S., then it
may have fallen prey to the United Kingdom Hydrographic Office's
copyright claims. In 2001, the UKHO asserted copyright over all
tidal constituents that they maintain (some of which are outside
the U.K.). This forced us to remove all sites for which we do not
have documented rights to use the data. We certainly think this
assertion of copyright is unfortunate, but we lack the means to
productively fight it. For more information, philosophy, and possible
ways to rectify the situation, see:
David Flater's remarks
on the subject.
- To set my tide wristwatch, how can I get the highest tide of
- First, find your desired location in the list of prediction sites
(see the instructions on finding a site). If you cannot
find your site (or one that you think is close enough), then we can't help you.
Assuming you did find a site that's close enough, get a basic prediction
page for that site. Then scan down on the web page and make the following
Select display type
* Extreme Highest and Lowest Tides Only
Select presentation options
1 year Length of time to display (ignored by One-Month Calendars)
Starting time and time display options
Start at: proper year Jan 01 at 00 : 00
Then click the "Make Prediction Using Options" button, and you should get the
information you need.
- Can I make a link to your website or to predictions
for particular tide sites?
- Yes, easily (you can also preselect some of the options).
Instructions are here. You may make a
link to our site without asking our permission. We do not participate
in link exchange programs, nor do we put up icons for "web awards" which
we may "win", so please don't ask us to put a link to your
site on our site.
Of course, if I find that a link from your site is causing such a heavy
load that our operation is being compromised, I may contact you about
the problem. So far, this has seldom occurred.
- Why are there two entries for a site, such as
xxx and xxx(2)?
- In some cases we have more than one dataset for a given location.
They may be based on data collected at different times, from slightly
different specific sites, or simply have come to us from two different
data sources. To find out what we know about a given data source,
generate a prediction, then select the Show site information from
database option on the form, and resubmit the prediction.
The next page will show you the text from the database, which may
help you determine which version is more useful for you.
- Why did I get a "Usage Limit
- To protect the website from computer programs that hammer it with
requests several times per second, we've had to limit the number of
requests per second from any single client. The limit can be
reached by doing several tide calculations in succession. If that
happens, just wait about a minute, then do a Reload
in your browser. Your sequence be "reset" by then
and you'll be able to continue.
- What is the distinction between the XTide program
and this WWW site?
- All the prediction output that you get here (text, tables, graphs, etc.)
is generated by the XTide program
written by David Flater
(firstname.lastname@example.org). What is
provided here at this web site
is a software "wrapper" around that program that allows you
to select a site and click off options, then view the results over the WWW
on your browser. I (Dean Pentcheff,
email@example.com) wrote and maintain
that wrapper around XTide; David Flater wrote and maintains XTide.
The XTide program runs on Unix systems either from the command line or
under the X Window graphics system. It is available under the terms
of the GNU licence from David Flater's web site (see above). It
does not run on Windows, Mac, or Palm Pilot systems (though
there are a few spottily-supported attempts to port it to those
systems, links to which are at David Flater's website).
The database of tidal constituents is maintained by Bob Kenney
If you have a source of tidal constituents for sites not on our list,
please contact him.
- What do "datum", "mean lower low water", "slack tide",
- The following publication has definitions of many tide- and
and Current Glossary [NOTE: This is a 600KB PDF file].
The site hosting the glossary
(NOAA's National Ocean Service)
also has a great deal of other interesting and useful information (see
list of publications).
- Why are there two tides each day? How do tides
- I am not an oceanographer, so I'm not a good person to ask
about how tides really work. The best reference online that I've seen
is at the National Ocean Service at:
- Why doesn't the link I made to my graph work
- If you generate a graph at the tide site, it only exists for a
few minutes (or we'd quickly run out of disk space). If you'd like
to be able to look at the graph again, copy it to your local disk.
Details of doing this differ between browsers, but may be something
like clicking the right mouse button over the image (for
the Mac this might be option-button, ctrl-button, or something similar)
to get a property menu, then select "Save image as..." (or something
similar). Now you've got a copy of it on your own hard drive after
our copy evaporates.
- Who maintains this web site and how much
money do you get from it?
- The WWW Tide Prediction site is maintained by Dean Pentcheff
It runs on a surplus DX2/66 in the laboratory of David Wethey.
of January 2000 it runs on a surplus P90.
as of May 2009 it runs on an HP Athlon 5000+ based box.
If you'd like, you can see some details about it).
We receive no money for it and provide it as a public service.
- What updates have happened to the site?
- May 2009
- Updated constituent database and XTide program to David
Flater's most recent updates (required some minor code tweaks).
- January 2008
- Major server hard disk crash. Reconstituted everything.
Daily remote-site backups are a good thing.
- August 2005
- Added "Distant date" capability by using a second, differently
compiled xtide executable. Involved extensive
reworking of time/date handling within the web interface
(not user-visible), to use the Perl Date::Manip module
instead of the standard Perl time routines.
- Minor changes to email addresses and page formats.
- June 2005
- Reworked to use David Flater's "new" XTide (new as of a couple
of years ago). This allowed us to use the new database format
in which tidal constituents are being distributed.
- Integrated the new cleaner tidal constituent database entries
with the older "legacy" sites. Retaining the "legacy" sites,
since they offer much broader coverage than the new ones do
- Minor change to put "[map]" links first on the site selection
pages, thus tidying those a bit.
- Changed mapping to bounce through a redirect rather than going
directly to MapQuest. This makes it possible to shorten the
site URLs in the site selection pages, saving text length.
- Prior to June 2005 No records of changes to the site were