[WWW Tide Site Selection]

WWW Tide Predictor FAQ

Contents
  1. How can I find site xxx, for which I need a prediction?
  2. Can you please add site xxx to the list?
  3. A site that used to be on your page is gone - what happened?
  4. To set my tide wristwatch, how can I get the highest tide of the year?
  5. Can I make a link to your website or to predictions for particular tide sites?
  6. Why are there two entries for a site, such as xxx and xxx(2)?
  7. Why did I get a "Usage Limit Exceeded" message?
  8. What is the distinction between the XTide program and this WWW site?
  9. What do "datum", "mean lower low water", "slack tide", etc. mean?
  10. Why are there two tides each day? How do tides work?
  11. Why doesn't the link I made to my graph work any more?
  12. Who maintains this website and how much money do you get from it?
  13. What updates have happened to the site?

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 neighboring section.

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 or site disappeared.

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 http://harmonics.unh.edu/xtide/files.html , and an email address: rmk@unh.edu .

A site that used to be on your page is gone - what happened?
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 and Walt Bilofsky's remarks on the subject.

To set my tide wristwatch, how can I get the highest tide of the year?
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 selections:
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 Exceeded" message?
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 (dave@flaterco.com). 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, tidecomment@gmail.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 (rmk@unh.edu) at http://harmonics.unh.edu/xtide/files.html . 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", etc. mean?
The following publication has definitions of many tide- and current-related terms: Tide 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 especially their list of publications).

Why are there two tides each day? How do tides work?
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: http://co-ops.nos.noaa.gov/restles1.html

Why doesn't the link I made to my graph work any more?
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 (tidecomment@gmail.com). It runs on a surplus DX2/66 in the laboratory of David Wethey. (Correction: as 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?


WWW Tide/Current Predictor: http://tbone.biol.sc.edu/tide
Dean Pentcheff <tidecomment@gmail.com>
Biological Sciences, University of South Carolina, Columbia SC 29208 USA