Distant Date Issues

By typing in a year in the "Distant date" section of the WWW interface for tide predictions, you have chosen a "distant date" (one that is outside the range of 1970-2025). There are some cautions that you should know.

First, if you want a prediction for a year between 1970-2025, just select that year from the pulldown menu under "Standard time range" instead of typing in a year number.

Assuming that you do want to make a prediction for a more distant date, then be aware of the following:

All times will be reported in UTC (Greenwich Mean time, or Zulu time). The code that permits the use of more distant times forces the use of UTC, since we cannot easily make local time corrections. You will have to do that manually for your location of interest, based on the reported times in UTC.

Tidal predictions for distant dates should be used extremely cautiously. That applies both to the time and the magnitude of the tidal predictions. Tide predictions depend on the local topography of the shore. If that has changed, the predictions will be wrong (in a way that is not predictable). Some tidal predictions will be wrong beyond ten years. Global sea level change will affect all tidal predictions (again, unpredictably), and that will certainly become evident within a century.

Furthermore, the mathematical underpinnings for the predictions have some assumptions built into them that are local (in a time sense) to our century (or thereabout). We don't know how bad it is to violate those assumptions by stretching the maths to cover distant dates.

Distant date predictions are available only for sitenames in boldface. This is due to limitations in the databases for the "legacy" (non-boldfaced) sites, as compared with the "new" (boldfaced) sites. If you're really desperate, it may be possible to regenerate the "legacy" database so that it will permit a distant time prediction (contact Dean Pentcheff directly (see the contact information at the end of each tide page)).

The bottom line: the further away that you go from the 20th Century, and the more the shoreline may have changed (by human-caused or natural forces), the more likely you are to get completely bogus predictions. You would be well advised to do anything you can to get independent confirmation of the reasonableness of the predictions (historical records, etc.) before you give them any degree of credence.


For a bit more about the issues, see the following information by David Flater, author of the XTide program that does the actual tidal predictions: