Sunset over the bar, Bar Harbor, Maine, 1997-06-24. © 1997 David Flater.

Frequently Asked Questions

Q: Can you please add predictions for Mumble Foo Bar?

A: Sorry, I can't. Please read the section entitled What to do if your location isn't listed.

Q: Can you tell me where I can find a web page with predictions for Mumble Foo Bar?

A: Sorry, I cannot keep track of all of such web pages. A search engine may help you.

Q: I'm having a problem with a tide-predicting web page.

A: You are probably using a web page that somebody set up with a CGI interface to XTide 1. If the predictions don't have a "100% Pure C++" logo at the bottom, then it's not my web interface and I can't fix it. Please contact the web site maintainer.

Q: No, really, I'm having a problem with your xttpd. None of the images work, and the calendars are illegible.

A: You need to get a better web browser. Netscape 4 or Internet Explorer 4 should work.

Q: Please send me today's predictions for Mumble Foo Bar before 11:30.

A: Sorry, I do not have the time to provide this level of service.

Q: Can you tell me the offsets for Mumble Foo Bar?

A: Sorry, I can't. Your best bet is to check local sources as described in the section entitled What to do if your location isn't listed.

Q: When compiling XTide, I get link errors related to the PNG library even though I'm using the latest version.

A: Some Unix vendors (SGI) have stuck crappy old versions of libpng in /usr/lib. If you can't get rid of it, then you need to add the -nostdlib switch to your link line, and add -L/usr/lib at the end.

Q: I always get a warning about "using obsolete time zone database."

A: That will happen on many platforms, but it won't impact you if you are only getting predictions for U.S. locations. Please see the System requirements section for details of what this means and what you can do to fix it, if you so choose.

Q: How do I switch from tide to current predictions or vice-versa for a given location?

A: Alas, although the two are clearly connected in the physical world, they are unrelated from the perspective of XTide. Even for the same location, tide predictions and current predictions require two completely separate data sets, and rarely will you get both. If current predictions are available for a location, they will appear in the location list with the word "Current" at the end of the name.

Q: You have multiple data sets with the same name or with just numbers to distinguish them. What's the diff?

A: It often happens that I get more than one data set for the same location. Sometimes they are from different sources; other times, one is just older than the other. When I have enough information to know which one is best, I will make it the first one (with no number). But if you are concerned about matching predictions up with those from some particular source, you should try each data set and see which one matches the best.

Q: What are bogo-knots?

A: Way back before I found out that hydraulic current stations generate results that are actually in units of knots squared, I didn't know what units they were, so I called them "bogo-knots." Since that time, I have (1) changed the units in my source data to knots squared, and (2) added functionality to XTide version 2 to convert these to regular knots. Therefore, if you are still seeing bogo-knots, then you are definitely using obsolete data and an obsolete version of XTide, or accessing a web site that is using obsolete data and an obsolete version of XTide. I am not the maintainer of any such web sites, and I recommend upgrading to XTide 2, which will barf all over any harmonics files that still contain "bogo-knots."

Q: First it says high tide is at 3:15 PM but then when I run it again it says 3:14 PM.

A: XTide's accuracy is plus or minus one minute. The behavior that you witnessed is normal.

Q: Has this been ported to Windows / OS/2 / anything but Unix?

A: Yes, to varying degrees. Please see the ports page.

Q: Why do the high and low tides have such different levels to them on any given day? Does it actually coincide with the amount of pull exerted by the phase or closeness of the moon?

A: The tides do not coincide too closely with the moon. While the moon produces most of the force that drives them, the exact tide levels result from the sloshing around of huge amounts of water, the effects of the shape of the coastline, and things like that.

Q: Is there a set time advancement each day for the next high and low tide? Does it always repeat 12 1/2 hours later, or are the calculations a little more arcane than that?

A: The 12 hour 25 minute cycle is literally only a first order approximation of semi-diurnal tides. Most tide predictions involve twenty to thirty terms, and some require over a hundred. The 12:25 cycle is just the most dominant term.

Q: If it's high tide here, is it low tide in [faraway place]?

A: It's hard to infer anything over large distances since localized effects can have a huge influence on tides.

Q: What does the zero (0) on a tide chart represent?

A: Tide heights are given relative to the "datum" which in most cases is Mean Lower Low Water. The zero therefore represents an "average" low tide. You can find official definitions of "datum" and "mean lower low water" in NOAA's tide glossary at http://www.opsd.nos.noaa.gov/tideglos.html.

Q: Why is it that the tides two miles from here are an hour different than the tides here? If the tidal bulge follows the moon at 1,000 miles per hour, how can the difference be so great?

A: When the water tries to follow the moon, it runs up against a lot of obstacles, including its own inertia, the shape of the coastline, and the resonances that are set up by the continual tidal motion. In some cases the tides are fighting a permanent current, e.g., going up a river, and this slows down the tidal crest. The result is that the tides at any one place at any given time don't have a whole lot to do with the moon any more.

Q: The tides for my location are totally wrong!

A: This seldom happens anymore with up-to-date harmonics files, but if it does, let me know and I will delete the offending data set. (A disproportionate number of Florida data sets have bit the dust this way.)

Q: The tides for Mumble Foo Bar are obviously bogus because they have four high tides on this day / only one high tide on this day / tides that are just a few minutes apart.

A:

That is not necessarily a problem. Some places really do have four high tides in one day, and other places only have one. Some will generate "extra" tides once in a while when the tidal forces align in such a way as to produce an "indecisive" high or low tide (see the Portland, England example in a previous section). These extra tides can be arbitrarily close together. Official predictions may omit them, but XTide faithfully reports all maxima and minima that it finds.

On the other hand, many subordinate station predictions are generated from "corrected" versions of the data for their reference stations, and sometimes when these "corrections" get too big, spurious maxima and minima can result. This situation can be corrected by replacing the offending data set with a proper subordinate station definition in offsets.xml.

Q: What does slack water mean?

A: This and many other terms are defined in the NOAA tide glossary at http://www.opsd.nos.noaa.gov/tideglos.html.

Q: I have five constituents and some seasonal corrections for my location. Can you get this to work?

A: Sorry, I can't. Somebody with more expertise might be able to derive a full set of constituents from that, but the results would probably still only be a rough approximation of your tides.

Q: Where's the Java?

A: Gone. I tried Java as an experiment with XTide 1, and it proved to be unstable, unreliable, and not portable at all from one web browser to the next, even between the same version of Netscape running on different platforms. So I no longer use it.

Q: I want to write my own tide predicting program. Can you provide a SIMPLE explanation of the tide-predicting function?

A:

The tide prediction function is fairly simple, requiring only a cosine function. The piles of code surrounding it in XTide are to optimize the process of finding maxima and minima. This can be done less optimally with significantly less code and effort (as early versions of XTide did).

Since it is hard to draw summation symbols in ASCII, here is the pseudocode instead:

Height = Datum;
for a = 1 to numconst
  Height = Height + amplitude * nodefactor * cos (speed * time + phase)
next a

The datum is provided at the top of the data set in the harmonics file.

The amplitudes are the first column of numbers in the data set in the harmonics file.

The node factors are tabulated for each year at the top of the harmonics file, or can be calculated from scratch using the code in the Congen program, available in http://www.universe.digex.net/~dave/files/. Most likely you will just want to tabulate them.

The speeds of the numconst constituents are listed at the top of the harmonics file in degrees per hour.

If speed is in degrees or radians per X, then time is in X since the beginning of the year. The specific time zone for the beginning of the year is chosen as described below.

Phase includes a yearly adjustment called the equilibrium argument that is tabulated at the top of the harmonics file (or calculated from scratch like the node factors), minus the location-specific phases that are the second column of numbers in the data set (given in degrees). By default, you will get phases such that the time is measured from January 1 00:00 in the time zone specified by the meridian, but it is trivial to adjust the phases for any other time zone. What XTide does is adjust them all to UTC and then use the Unix time zone functions to render the output with Daylight Savings Time and everything.