bara Wrote:Hello!
My husband and I are soon to be off on our first offshore cruise together, so I was looking for tides information for my iphone, and I was quite tempted to buy the $9.99 Navionics app instead of the $9.99 AyeTides until I read this thread at sailnet suggesting that there's a significant discrepancy in tide and current data among various products here in the Pacific Northwest...
http://www.sailnet.com/forums/pacific-no...-data.html
AyeTides is a very much closer to the original sailnet poster's preferred source, the one he considers most accurate, than is Navionics. Any idea what's up here?
There are two ways to predict the tides/currents at a location. Either you measure the tides and analyze them for harmonic inputs to the tide equation, or you use a nearby station and just measure how different the station in question is from the original station.
For a reference station it all comes down to how good the harmonics are for the station, and that depends on the data (currents are especially tough to measure, which is one reason why there are many more tide stations than current stations).
Any and all difference in the predictions can be attributed to the differences in the harmonics. For secondary stations (also called subordinate stations) the problem is (1) how good the reference station is, and (2) how good the offsets are. One bad reference station can really mess up a lot of subordinate stations.
(Please note that the currents predicted this way are
reciprocating currents (they go back and forth in a regular pattern). I don't predict
rotary currents, the ones that are based upon the time and height of high water at a nearby port. These currents, common in Europe, never have a slack nor a predominate direction. They spin around as the tide goes in and out. I did come across a neat diagram, an ellipse, that can show rotary currents, but finding the data to feed into the ellipse is not easy.)
Mr. Tides and its siblings AyeTides and AyeTides XL use harmonics from one of three sources: NOAA, derived by me from tide gauge readings, or provided by third parties. NOAA harmonics are spot on (within a minute or two, we can't get exactly the same result every time as the equation isn't that precise). Deriving our own harmonics can also give good results (meaning we are within +/- 2 minutes and +/- 0.2 feet) of the "official" value - given that the value does NOT reflect the contributions of storm surges, wind, river outflows, etc - but sometimes I vary from the official publications (so far only in France, and only some stations there). Third-party harmonics can be pretty good, but sometimes they tend to age poorly (the data they used to create the harmonics is limited and as time goes by, the results begin to differ from official sources). Any time I find a station that isn't in agreement with offical sources, I mark that station and warn you to proceed with caution.
There is one other source of harmonics that I know of, the UK Hydrology Office. I do NOT use the these harmonics for two reasons: their licensing requirements are for one year only, and the harmonics themselves are, IMHO, junk. The UKHO went and fiddled with the standard method for harmonic analysis and their published harmonics can't be used by anyone else. Why they did this is beyond me. All I know is that the results from running their constants through the equation result in bad predictions, unless you know their secret changes. Perhaps this may explain part of the difference in the results you quoted above.
