Tide Units

[jackdale]

Originally Posted by HappySeagull

...can somebody find the detail where that 3 feet is obvious? Overlapping Canada/US charts that differ in a drying rock for instance? I haven't found it yet....

Attachment 38262
Attachment 38263

Canadian charts like 3462 use Lowest normal tide in Canadian waters and Mean Lower low water in the US. That is they use the chart datum employed by the respective countries.

Check the title block for that information.

[HappySeagull]

aah. You are right- Silly me, hoping for an easy clue. Of course they are sharing the drawings + data....Imagine the CHS survey boats tooling around Puget Sound, eh?

[HappySeagull]

The mixture means that the best way to set these up is to have the US in feet and the "Taller" Canadian sourced(?) in metres...that would make it as obvious it could be.... but I really don't like that 3 feet!

[HappySeagull]

...and I have found a solution that agrees with the tidebooks of both countries. ..but the harmonics files need editing.

[P_Dub]

The only possible solution I see is :- if you use Canadan charts, use Canadian tide predictions; if you use American charts use American tide prdictions.
The US tide mean level must be increase of approximately 3 feet or the Canadian reduce by 3 feet. What a way to end up!
Bye.

[jackdale]

Originally Posted by P_Dub

The only possible solution I see is :- if you use Canadan charts, use Canadian tide predictions; if you use American charts use American tide prdictions.
The US tide mean level must be increase of approximately 3 feet or the Canadian reduce by 3 feet. What a way to end up!
Bye.

That is what I tell my navigation students.

To make matters worse, when using the official tables the corrections for secondary ports are given in feet. When the CHS tried to move to metric corrections there was so much outcry that they retained the corrections in feet.

Of course we could all use lowest astronomical tide as chart datum, but the expense of changing all those charts would be prohibitive.

[HappySeagull]

The solution is to fix these files so they agree with an official publication somewhat and can be looked up in those with the same name. Leave just a couple of stations that are in the Canadian tide books,alongside the U.S. ones...say, Port Townsend,and Seatlle leaving them metric for Canadians with their overlapping charts. Maybe I'll post it somewhere, if anybody wants it.
These checked so far agree with US tidebook near enough today :Cherry Point, FridayHrbr, Bellingham, Anacortes, Port Townsend(...and Canadian metric- no 3 foot difference)... there are more to check.

There will be fewer tides in the area altogether.

Unfortunately, once one starts, it's hard to stop. To include metric and feet on the Canadian side is cumbersome. They are in fact both there, but in the same position so you can't see them. The metric tides are generally more accurate. The foot tides are saltier. If they are both visible, it's kind of cumbersome -looking.
Some have likely noticed that several currents are duplicated as well, being American Datasets + Canadian ones. A bit odd. A little confusing? Like having the British and the Italians writing separate versions of tidebooks for Denmark. ....

[HappySeagull]

Yikes.
A Quick fix looks like this for Puget sound and Juan de Fuca. Cape Alava,Neah Bay, also looked pretty funky, so it's out too...

if anybody cares to see, rename your original Harmonic.idx to "origHarmonic.idx" or whatever and slip this revised one into the tcdata folder...


HARMONIC.zip.doc

[HappySeagull]

...in case anybody else cares, here's something different...
It actually restores some tides in Washington (and Alaska too!) from a different file. Hopefully I've retained most of the sensible changes from OpenCPN's idx. I made BC metric in this version and suited myself per currents.

Using it with OpenCPN's original HARMONIC (and many others)
Someone might notice that many worldwide "illegal" stations are included but they don't work. I haven't bothered to delete these.... AND this breaks some Scandinavian and Quebec stations too. I haven't bothered to fix these...

HARMONICedits.zip.doc

Weirdly, throughout the tides where there's a foot/metre option, times and heights are not just simple conversions..Times of turn do not "Agree". If one was converted from the other, I'd like to know which! I suppose it has to do with the harmonic sums. It applies to the WXtides too.