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Note: "Mumble Foo Bar" is a made-up place that is meant to represent whatever place you are looking for. Whatever is said in this FAQ about Mumble Foo Bar applies to your location as well.
Q: Are these predictions compliant with the new 2007 Daylight Savings Time rules for the U.S.?
A: XTide relies on the de facto standard time zone database to handle Daylight Savings Time. XTide's results will obey the new Daylight Savings Time rules if and only if the version of zoneinfo installed is sufficiently new. See System Requirements.
Q: Your WWW Tide and Current Predictor blah blah blah...
A: I am not the maintainer of the WWW Tide and Current Predictor. I am the maintainer of XTide. Although the WWW Tide and Current Predictor uses some version of XTide behind the scenes, I have no control over the behavior of the web site or its maintenance.
Q: Can you send me predictions for Mumble Foo Bar?
A: I cannot possibly provide this level of service to everyone who wants it. Please use a commercial service and/or a web site.
Q: URGENT - DROP EVERYTHING AND READ THIS! The race starts in 5 hours so I need a tide chart for Mumble Foo Bar NOW!
A: You might not believe it, but sometimes I go two weeks without reading my e-mail. Really! And when I do get back to it, there are always lots of messages just like this one, so far past their use-by dates that green fuzz has started to grow on them. The answer is the same: I cannot possibly provide this level of service to everyone who wants it. Please use a commercial service and/or a web site.
Q: Can you tell me where I can find a web page with predictions for Mumble Foo Bar?
A:
Q: Can you please add predictions for Mumble Foo Bar?
A: Probably not. Please read the section entitled What to do if your location isn't listed.
Q: How do I make the calendar print all on one page?
A: The short answer for Windows XP users is to do the following:
)
and try different options
until the calendar fits nicely on a page.Q: Can you tell me the offsets for Mumble Foo Bar?
A: You can get them easier than I can by checking the sources described in the section entitled What to do if your location isn't listed.
Q: There are multiple listings for the same place, and they give different results. What's going on?
A: There are two different approaches to predicting the tides at a given place. One approach is to calculate them directly from a data set; when this is done it is called a "reference station." The other approach is to estimate them using adjustments to the tides at a nearby reference station; when this is done it is called a "subordinate station."
Data gathered from the NOAA web site sometimes include both a reference station and a subordinate station for the same place. For example, the subordinate station may be used for published tide tables while the reference station is still relatively new and untested. The results will differ, but they should be close (assuming that there are no problems with the data). 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.
In rare cases, data gathered from the NOAA web site include two reference stations or two subordinate stations with exactly the same name and nearly the same location. When this happens, one of them has (2) suffixed to its name. Again, if you are trying to match official predictions, you should try both to determine which is better.
If you are using old legacy data or a web site that does, you may see additional listings for the same place. These may be expired and/or have dubious traceability to authoritative sources. They cannot be expected to agree with up-to-date predictions.
Q: Can you predict the tide and/or current if I give you the latitude and longitude?
A: The short answer is no. XTide cannot predict tides unless you provide harmonic constants (see What to do if your location isn't listed).
From what I'm told, the tide models that were built from TOPEX/Poseidon data work on a global scale, but they are inaccurate on continental shelves. Some organizations have constructed models that function in coastal waters in localized regions. For example, NIWA has a model for New Zealand's coastal waters, and NOAA has a model of currents in San Francisco Bay. Although XTide could make use of harmonic constants generated from these models, XTide does not implement any such models.
Q: The coordinates you provide for Mumble Foo Bar are off by miles.
A: XTide reports coordinates in degrees only. Some sources report coordinates in degrees and minutes and run these together in a confusing way. For example, a coordinate shown as 2846.330 may actually mean 28 degrees, 46.330 minutes, which XTide would report as 28.7722 degrees. If this is not sufficient to explain the discrepancy then by all means report the problem. All coordinates in the latest harmonics data are traceable to an authoritative source.
Q: Why are there no currents in the latest database?
A: The Center for Operational Oceanographic Products and Services (CO-OPS) of NOAA's National Ocean Service (NOS) does not presently supply harmonic constants for currents on its public web site. This text from tidesandcurrents.noaa.gov/ncop.html may partially explain why not:
Approximately 70 percent of the stations in the 2001 Tidal Current Tables are over 30 years old. Many of these stations are based on analyses of less than 7 days of data (the data duration is known for 24% of all stations). Channel dredging and changes in the configuration of ports and harbors over the years have significantly altered the physical oceanography of many of the nation's estuaries. Reports from local users indicate that many of NOS's tidal current predictions may be inaccurate. NOS intends to address these deficiencies by rebuilding the program and resampling the currents at every major port and estuary within the next 20 years.
Q: I am doing some historical research and need to project what the tides at Mumble Foo Bar would have been a long long time ago.
A: This is generally ill-advised.
It is technically possible to get XTide to make projections back to 1 AD (see Appendix A for details). However, such projections are usually unverifiable and are likely to be wildly inaccurate. If you work from these projections with no means of independent verification, you deserve what you get.
The perishability of tide data for a given location varies depending on how quickly the local topography changes. Some places go rotten in less than a decade. All locations are impacted by global sea level change, which becomes significant in less than a century.
Over even longer spans, the physics start to go wrong. Some of the astronomical "constants" used in the U.S. method of tide prediction really aren't constant; they change very slowly. For example, the speeds of harmonic constituents change. We are still using constant speeds that were calibrated for the year 1900. When you change the speeds of the harmonic constituents, it changes everything. As we get too far away from 1900 in either direction, eventually the model collapses and the results are garbage. As far as I know, nobody has done an analysis to determine exactly when this occurs.
When this happens in the future, we can just update the speeds and generate fresh harmonic constants that work within the new model. But we can't do that for historical predictions because we don't have the water level observations from that period in history to derive the harmonic constants. We have no choice but to use the physics of 1900, with data derived from observations in 2000, to extrapolate back to whenever, and hope that we haven't pushed the model too far.
Needless to say, the credibility of projections for anywhere reaches zero well before you get back to 1 AD. So please don't ask for BC support.
Q: I live outside of the U.S. and my location is no longer supported. What happened?
A: After a legal threat from the U.K. Hydrographic Office (UKHO) and the subsequent discovery that country-by-country permissions are now required to use harmonic constants (the data needed to predict tides), all of the data that arrived via the International Hydrographic Office (IHO) or the Table des Marées des Grands Ports du Monde (TMGPM) were removed from the harmonics files in January 2001.
Back in the old days, the collection of hydrographic data was done almost exclusively using public funds. The resulting harmonic constants were treated as scientific results, published, and distributed on request from an international data bank. But in the late 20th century, a wave of privatizations occurred, and harmonic constants became the intellectual property of the collecting agencies.
You wouldn't think it possible to "un-publish" data that were distributed with considerable freedom at one time. Nevertheless, the international data bank was abolished, the Table des Marées des Grands Ports du Monde was withdrawn from publication, and I was coerced into removing the associated data from the harmonics files.
Was I spineless? Perhaps. Those who wrote the threatening letter clearly had not done their homework and may have overreached their jurisdiction. But even with the benefit of hindsight and experience, knowing that some unethical companies habitually make legal threats to manipulate people into doing things that they aren't necessarily obliged to do, I don't blame myself for not fighting it. Why take a risk for the sake of some tide data of questionable pedigree that were doomed to become useless from age pretty soon anyway?
For my own part, I do not consider privatization to be inherently evil. It would have been reasonable to keep newly generated data secret while leaving the old data in the public domain until their useful life expired. But in my opinion it was wrong to lay claim to the old data that were once shared in the spirit of scientific openness. It was a disservice and dishonor to all of us who accepted those data on good faith and donated our own time to maintain them and add value to them to end up accused of copyright infringement. For someone who had invested himself in writing free software as a public service, the reward was a slap in the face.
Although only the UKHO made an issue of it, the fact that they did sufficed to "poison" all of the IHO and TMGPM data for every country. We can no longer assume that we have permission to use any of them. In countries other than the U.K., if you have contact with your local marine authorities and could obtain and forward to Mr. Kenney a statement that use of the old data is not restricted in your region, they could be reinstated. (Bob Kenney is the maintainer of a database of legacy data that can be used by XTide and other programs. You can find it on his web site at harmonics.unh.edu/xtide/files.html.) However, if they have newer data, it would be better if they just sent those to me and Mr. Kenney with all necessary permissions attached. I regret that former British colonies having no independent tide authority have effectively been disenfranchised.
In 2003-12, new data for 44 stations in the U.K. became available thanks to the generosity of the British Oceanographic Data Centre (BODC) based at the Proudman Oceanographic Laboratory in Liverpool. The legal details can be found at flaterco.com/pol.html. I encourage other organizations who maintain tide gauges anywhere in the world to contact us about deriving some harmonic constants.
Q: The predictions for somewhere in the U.S.A., Mexico or Nunavut are off by one hour or six hours.
A: The referenced nations have recently made changes to their time zones or daylight savings time rules. In order for XTide to give predictions in local time correctly, a recent version of the Zoneinfo library must be installed. You can obtain this library from ftp://elsie.nci.nih.gov/pub/.
Q: For Chesapeake and Delaware Canal Current, Flood and Ebb are not informative. Which way is which?
A: Based on observations by Richard W. Reynolds and friends, it appears that "Flood" for this data set indicates that water is flowing from the Chesapeake to the Delaware bay.
Q: For Cape Cod Canal Current, Flood and Ebb are not informative. Which way is which?
A: According to Reinhard Schumann, "Flood" for this data set means "current towards the east." Woods Hole current is probably the same.
Q: When compiling XTide, I get errors like....
A: Please refer to the troubleshooting section of the installation instructions. If your error is not shown there, please email dave@flaterco.com for assistance.
Q: I want to change the end time of a tide graph but the settings that I make have no effect.
A: This is Quirk #2. In graph mode, the end time is determined by the applicable width and aspect, not the other way around. Thus, neither the –e switch nor the compiled-in defpredictinterval constant have any effect in this case. In the interactive client, you can resize the window as you would any window and change the aspect from the Options menu. The applicable command-line switches are –gw for X-windows or PNG formats, –tw for text format, and –ga. For more details, refer to "Customizing XTide."
Q: The text in XTide windows is formatted ugly.
A: For windows containing lots of text, XTide uses the default font offered by the X11 environment. If this is not a monospace font, the results could indeed be ugly. The default font can be overridden using the command line switch -fn. Give this a try:
xtide -fn "-adobe-courier-bold-r-normal-*-12-*-*-*-*-*-iso8859-1"
If that looks better, you can make the change permanent by adding this line to your ~/.Xresources file:
XTide*font: -adobe-courier-bold-r-normal-*-12-*-*-*-*-*-iso8859-1
Q: When I run xttpd, it exits immediately with no errors to tell me what went wrong.
A: When executed, xttpd immediately dissociates from your terminal and starts logging all diagnostics to syslog. So look in your system logs. You will find these someplace like /var/log or /var/adm/log.
Q: I always get a warning about "using obsolete time zone database."
A: Please see the System requirements section for details of what this means and what you can do to fix it.
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. As of 2005-01, there are no currents available in the latest data (the National Ocean Service has not made them available on their web site). If current predictions are available for a location in legacy data, they will appear in the location list with the word "Current" at the end of the name.
Q: What are bogo-knots?
A: 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 / PalmOS / anything but Unix?
A: Yes, to varying degrees. Please see the ports page.
Q: Xttpd sucks! Can't XTide work with PHP?
A: A number of people have expressed interest in getting XTide to work through PHP. Thus far I have just been introducing them to each other through e-mail and waiting for cool things to happen. There is now a WordPress plugin by Mir Rodrguez.
Q: The tides for my location are totally wrong!
A: Unfortunately, there have been some problems recently with data sets being assigned the wrong meridians upstream. The symptom is that all predictions are shifted earlier or later by the same number of hours. If you can verify that this has happened by comparison with published tide tables (available at tidesandcurrents.noaa.gov/tides07/), please report the problem for corrective action.
Q: The tides for Mumble Foo Bar are obviously bogus because they have too many 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 only one tide cycle per day. Others generate "extra" tides when the tidal forces align in such a way as to produce a "double" high or low tide or a temporary reversal near mid-tide. These extra tides can be arbitrarily close together. Official predictions might omit them, but XTide faithfully reports all maxima and minima that it finds.
The legacy data contain some data sets in which harmonic constants were generated for subordinate stations by munging the constants of a reference station. This operation was fragile and sometimes it led to spurious maxima and minima. The fix is to upgrade to the latest data, which contains no "munged" data sets.
Q: I have five constituents and some seasonal corrections for my location. Can you get this to work?
A: XTide is not presently enabled to handle seasonal corrections directly. To my knowledge, seasonal corrections are only used in publications by the British Admiralty that do not allow redistribution of data, so the value of providing better support for them in XTide would be marginal at best. However, if you have legal access to such data and are determined to use it with XTide, it may be possible to synthesize values for long-term constituents to "approximate the approximation." A spreadsheet for doing this is available from www.flaterco.com/xtide/files.html#experts.
It might also help to enable constituent inference in XTide. This can be done from the control panel or using the infer setting.
Q: I have a tide watch that only goes through the year 1999. What year could I set it to that would be the same as this year?
A: Sorry. It just doesn't work 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?
A: No. The 12 hour 25 minute cycle is literally only a first-order approximation. 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: Somebody gave me a tide clock, but the instructions say it only works on the east coast. How can this be?
A: "Dumb" tide clocks assume that the 12 hour 25 minute cycle mentioned in the previous question is a good enough approximation. For the west coast, it isn't. The following tide graphs illustrate the differences between east and west coast tides. The high and low tide times that would be indicated by a "dumb" tide clock are shown with vertical yellow lines. San Francisco shows a 2-hour discrepancy on the lower high tide.
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: 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 one of several benchmarks corresponding to low tides of varying extremeness. The preferred benchmark in the U.S. is Mean Lower Low Water (MLLW). The odds of the predicted tide getting below MLLW on any given day are about half. The preferred benchmark in the Netherlands is Mean Low Water Springs (MLWS). MLWS is lower than MLLW. The predicted tide will get below MLWS on average only about twice a month. The preferred benchmark in Germany is Lowest Astronomical Tide (LAT). LAT is the lowest tide predicted over a 19 year period. The predicted tide will not get below LAT in that 19 year period, and is unlikely to get below it by any significant amount ever.
In harmonics-dwf, some U.S. locations for which a MLLW benchmark was unavailable use an estimated value of MLLW that is derived from the predictions. These estimates tend to yield predictions that differ from National Ocean Service published tables by (0.1–0.2) ft. Older versions of harmonics-dwf used LAT for these stations, which of course yielded much larger discrepancies.
For more information on datums, read the National Ocean Service publication Tidal Datums and their Applications.
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: Why are there two high tides per day, anyway? How is this possible?
A: The standard simple answer to this question is that the water on the side of the earth opposite the moon bulges out due to decreased lunar gravity in the same way that the water on the side of the earth nearest the moon bulges out due to increased lunar gravity. This is counter-intuitive in that one might expect all of the water to just rush over to the side where the moon is. To explain this, I quote from "Our Restless Tides," a NOAA tutorial at tidesandcurrents.noaa.gov/restles1.html:
To all outward appearances, the moon revolves around the earth, but in actuality, the moon and earth revolve together around their common center of mass, or gravity. The two astronomical bodies are held together by gravitational attraction, but are simultaneously kept apart by an equal and opposite centrifugal force produced by their individual revolutions around the center-of-mass of the earth-moon system. This balance of forces in orbital revolution applies to the center-of-mass of the individual bodies only. At the earth's surface, an imbalance between these two forces results in the fact that there exists, on the hemisphere of the earth turned toward the moon, a net (or differential) tide-producing force which acts in the direction of the moon's gravitational attraction, or toward the center of the moon. On the side of the earth directly opposite the moon, the net tide-producing force is in the direction of the greater centrifugal force, or away from the moon.
Q: What does "slack water" mean?
A: This and many other terms are defined in the NOAA tide glossary.
Q: I have a theory that [random phenomenon] is related to tidal forces, but I am landlocked. Can you, like, predict the "tides" for [landlocked location]?
A: There is no support for this in XTide (ocean tides have only the vaguest connection to latitude, longitude, and the position of the moon), but you can find relevant information by searching the web for "land tide."
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[a] * nodefactor[a] * cos (speed[a] * time + phase[a])
next a
The datum is provided at the top of the data set in the harmonics.txt 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 www.flaterco.com/xtide/files.html#experts. 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. Customarily, the meridian is chosen to be the standard time of the location in question to make life easier on simple tide prediction programs that don't mess with time zones or summer time adjustments. In the harmonics.txt file, you will find the meridian of each data set right before the time zone identifier, in hours and minutes: e.g., -05:00 :America/Montreal. It is trivial to adjust the phases of the constituents for any desired meridian. 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.
Q: I want to license XTide so I can build a commercial product around it.
A: XTide is released under the terms of the GNU General Public License. This FAQ about the GPL may be applicable to you.
XTide has been used by commercial packages "at arm's length," to use the wording of the FAQ cited above, but I have never licensed it by any terms other than the GPL, nor have I ever offered any kind of warranty or service that one might expect if it were licensed commercially.
PLEASE NOTE: The question whether you can use XTide is completely separate from the question of whether you can use the tide data (harmonics files). In general, data for U.S. ports are public domain, while others are for non-commercial use only. Read the boilerplate for details.
Q: I have a lot of specific questions about the GNU General Public License and/or want a ruling that my specific plan is OK.
A: Please read the GPL FAQ, available here or here. If that does not answer your question, the people to ask are at licensing@fsf.org.
Q: We are a not-for-profit organization and we want to sell calendars with predictions from your web site. Is that OK?
A: Firstly, it's not my web site. See Question 1. Secondly, all predictions for places outside the U.S. are for non-commercial use only (i.e. you can't sell them no matter what your tax status is). Lastly, if you do want to sell calendars containing predictions for the U.S., you must include all of the "NOT FOR NAVIGATION" disclaimers and agree to accept full liability in case someone has a problem.
Legalities aside, my opinion has always been that people who are selling tide predictions have no business selling anything that is not directly certified by the National Ocean Service. Beggars can't be choosers, but when people are paying for something, they have a right to hold you to a higher standard.
Q: I have a great idea to make money selling tide predictions, but I'm not good with technical stuff, so would you just do this for me...
A: No.
Q: I already make money selling tide prediction products, but your stuff is better, so would you just do this for me...
A: No.
Q: I need to do [poorly researched brainstorm having something to do with tide prediction]—how much would you charge in consulting fees to help me do it?
A: It's moot. Your plan won't work for one or more of the following reasons:
Q: How should I cite XTide within publications?
A: The web site is the best thing you can cite. For a general reference to XTide, I suggest the following, with the current date.
[1] David Flater. XTide. www.flaterco.com/xtide/. 2005-07-04.If you are using specific predictions from XTide rather than just XTide in general, then you should cite the specific version of XTide and the specific data file that you used. In this case, it would be appropriate to use the date indicated in the changelog for that version of XTide and the revision date of the data file.
[2] David Flater. XTide version 2.8.2. www.flaterco.com/xtide/. 2005-01-06.
[3] harmonics-dwf-2005-06-05-v2. Available from www.flaterco.com/xtide/files.html, 2005-06-05.
Q: What is the difference between a reference station and a subordinate station?
A: The following information was copied from NOAA's web site on 2007-02-17.
The publication of full daily tide predictions is necessarily limited to a comparatively small number of stations. These stations are referred to as "reference stations". Tide predictions for more than 3000 other locations, referred to as "subordinate stations", can be obtained by applying specific differences to the daily tide predictions for one of the reference stations. [...]
Caution: The time differences and height ratios used to calculate predictions at subordinate stations are derived from a comparison of simultaneous tide observations at the subordinate station and its reference station. Because these adjustments are constant, they may not always provide for the daily variations in the actual tides, especially if the subordinate station is some distance from the reference station. Therefore, although the application of time differences and height ratios will generally provide reasonably accurate approximations, they cannot result in predictions as accurate as those listed for the reference stations, which are based on much larger periods of analysis.
In plain language, what you need to know is this: All subordinate station predicions are approximate. Tide predictions are always at best approximations of reality, but for subordinate stations that goes double.
Q: These predictions are nonsense—what is going on here?
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2007-02-14 12:57 PM AKST 3.46 feet High Tide 2007-02-14 3:00 PM AKST Moonset 2007-02-14 7:08 PM AKST Sunset 2007-02-14 9:06 PM AKST -0.38 feet Low Tide 2007-02-15 6:46 AM AKST 3.08 feet Low Tide 2007-02-15 7:00 AM AKST 2.62 feet High Tide 2007-02-15 9:07 AM AKST Moonrise 2007-02-15 9:22 AM AKST Sunrise 2007-02-15 1:46 PM AKST 3.46 feet High Tide |
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2004-03-31 5:01 AM PST -0.64 knots Max Ebb 2004-03-31 5:49 AM PST Sunrise 2004-03-31 10:06 AM PST 0.02 knots Max Flood 2004-03-31 10:18 AM PST 0.00 knots Slack, Flood Begins 2004-03-31 11:21 AM PST -0.00 knots Slack, Ebb Begins 2004-03-31 1:01 PM PST Moonrise 2004-03-31 4:13 PM PST -0.90 knots Max Ebb |
These are extreme examples of what can happen when the time differences and height ratios for subordinate stations don't "provide for the daily variations in the actual tides." Although in the average case the offsets might yield good results, in extreme cases they can yield nonsense results like tide events happening in an impossible order or a "low" tide actually being higher than the "high" tide right next to it. There is nothing XTide can do to rationalize these paradoxes, and the tide levels that are interpolated between paradoxical events are essentially garbage.
Q: Where can I find tons of information about tides that is both more authoritative and better written than this FAQ?
A: tidesandcurrents.noaa.gov/education.html
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