Dive planning question

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trekdiv3r
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Dive planning question

Post by trekdiv3r »

I was wondering if people still used this site https://www.planyourdive.com/

The website looks like it maybe hasn’t been touched in years, and the website states that it uses XTide for predictions. I tried to do some research on XTide. It looks like it is an algorithm that was developed and updated yearly with NOAA data (https://flaterco.com/xtide/introduction.html) but the last file of NOAA data I can see on the site is from Dec 2019 (https://flaterco.com/xtide/files.html#harmonicsfiles).

So I’m not sure if this means that the predictions would still be reliable. Per XTide, “ Deviations of 20 minutes are typical for locations that are using obsolete data.”

I wonder if it would be better to just use NOAA site (https://tidesandcurrents.noaa.gov/noaacurrents/Regions).

As an example I looked at a dive for Keystone on 2/22/21, per PYD using XTide there is a slack at 9:21 a.m. and 10:52 a.m.

Using NOAA station Admiralty Inlet runs me into a new problem, as now current predictions are listed for 3 different depths (29,68, and 193 ft)(https://tidesandcurrents.noaa.gov/noaac ... holdvalue=), and none of these depth forecasts really match up with the PYD website after applying the corrections from the Northwest Shore Dives book.

So I guess my questions would be.
1) Is PYD (using XTide) considered up to date and accurate?
2) and now that NOAA splits some current station predictions into different depths, is it even possible to use and apply corrections from older resources like the Northwest Shore Dive book? It would seem that that book’s corrections were based on NOAA previously only reporting one figure for current predictions.
3) Am I overthinking this too much and what would be your method for planning a dive (at Keystone for example)?
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YellowEye
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Re: New to the dive club

Post by YellowEye »

This part of the convo dealing with XTide should probably be moved to a new thread.

But yes, xtide can still be useful. For some sites like Skyline, xtide is more usable. For deception pass, often xtide and noaa at 18 feet differ by 25 minutes, and neither prediction is consistently better. So we kind of look at both.

There are some xtide stations that were marked obsolete and then replaced over the years (including the fishnaller admiralty inlet). Also someone mentioned that Rick from Bandito charters said the slack at narrows changed by 10 minutes when they built the new tacoma narrows bridge!

Many current apps/websites use xtide beneath the covers, not just PYD. DeepZoom and others also use it.

PYD is one of the few websites out there that let you search for better dive days, which is nice. The functionality is pretty limited though. I'm currently working on an improved tool for searching for dive days, and I support both NOAA and xtide stations, and use the best one (sometimes both) for a given dive site.
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trekdiv3r
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Re: Dive planning question

Post by trekdiv3r »

Yes, thanks, I realized after the fact I accidentally replied in the new user section. Thanks for moving the post here.

I also found DeepZoom and others matching PYD via XTide. I’ve used PYD over the years and never had too much of an issue, so I still like to use it as a reference. I like to cross check NOAA when I can and also always look at conditions when I’m physically at the site. Also calling local dive shops helps (for new sites I haven’t done), and then writing that in my log book.

It just seemed a bit strange that I couldn’t find some of the source data for XTide, so I wasn’t quite sure how much it can really be trusted. Probably best to continue an all of the above approach when planning a dive in a current sensitive area.

I would surmise that some of the corrections listed in the Northwest Shore Dive book must now be obsolete as well, since some of the NOAA stations have changed to reporting multiple depths.

Thanks.
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YellowEye
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Re: Dive planning question

Post by YellowEye »

Hi.
I continue to use xtide when using Northwest Shore Dives. It turns out that the corrections we all use are overly simplistic anyway. What may be -20 minutes on one day may be -40 on the next depending on the swing and the weather. Most sites aren't that much current anyway unless you go on a really bad day. I'm curious to try to collect slack data and build a ML model for non-linear corrections. But data collection may prove challenging.

As to your original question about keystone, I just jump in one hour before slack according to xtide, and it works out pretty well, though I know some go even earlier. I prefer max flood to about 1.5 then I can easily do a dive #2 right after and not have to worry about current (SBE seems to hit around then). Using my aforementioned tool I'm working on, finding those days for the whole year is easy.

-Eric
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