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Old 13-06-2011, 17:38   #16
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Re: Chart Datum for Tides

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Originally Posted by jackdale View Post

I was asking what chart datum was being used for the tide data in OpenCPN.

Jack
...If you care to read carefully and with an open mind the points made in the previous posts, you'll find that P_Dub has answered your query for the area he has specified...I have for another region... You'll see that there isn't a single answer...and you will understand the relationship, in fact similarity, between chart datum and what you call tide datum...in OpenCpn as in all openly available tide predicting applications...
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Old 13-06-2011, 18:17   #17
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Re: Chart Datum for Tides

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Originally Posted by jackdale View Post
Canadian charts use Lowest Normal Tide, which is approx. 1.5 feet or .5 meters lower that Mean Lower Low Water (the American chart datum)
holy moly!The Yanks must feel the bump coming over the border!
Well,that they match the tidebook so well must therefore be "...or it's equivalent".anyways,I will keep looking for where I found that quote in my stuff.
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Old 13-06-2011, 18:32   #18
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Re: Chart Datum for Tides

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holy moly!The Yanks must feel the bump coming over the border!
Well,that they match the tidebook so well must therefore be "...or it's equivalent".anyways,I will keep looking for where I found that quote in my stuff.
Well, you can also refer it to the geodesic datum, but that is generally used by the land surveyor and adjusted at a zero for the marine charts.
For the Great Lakes of North America, the reference is at Rimouski, Quebec (it use to be at Fathers Point). Different places have different datum, but now by International agreement the plane should be as low as possible, so that the tides falls very seldom below it. The charts produce over the last decade seems to have reflected this reference plane.
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Old 13-06-2011, 20:41   #19
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Re: Chart Datum for Tides

My understanding is that the International agreement at IHO is to move to Lowest Astronomical Tide which will result in no negative tide predictions. This is highly problematic as all charts will need to be revised to show that change. Instead the practical agreement is that chart datum should be:

1) so low that the water level will seldom fall below it.
2) not so low as to cause the charted depths to be unrealistically shallow
3) it should vary only gradually from area to area and from chart to adjoining chart, to avoid significant discontinuities.

LNT (the Canadian Coastal standard) does not really meet this standard. What does seldom mean?

Most Canadian coastal charts use Lowest Normal Tide which does result in negative tides. In areas in which both US and Canadian waters are shown (such as chart 3462) MLLW is used in US waters and LNT is used in Canadian waters, i.e., two different chart datum points.

The Great lakes datum is based on the International Great Lakes Datum (1955) which uses Pointe-au-Pere (at Rimouski) as a reference.

(http://www.psmsl.org/train_and_info/...l/chapter5.pdf)

My concern: Are the tide calculations in OpenCNP based on MLLW, LNT, or something else? The answer seems to be "it depends." That makes the calculations, at least in my mind, undependable.

I will be sticking to my CHS Tide and Current Table which I know are based on LNT.
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Old 14-06-2011, 07:29   #20
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Re: Chart Datum for Tides

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Originally Posted by jackdale View Post
My concern: Are the tide calculations in OpenCNP based on MLLW, LNT, or something else? The answer seems to be "it depends." That makes the calculations, at least in my mind, undependable.

I will be sticking to my CHS Tide and Current Table which I know are based on LNT.
For the record, it can be stated that the tide data in OpenCpn (as in all free applications and possibly some commercial ones) ultimately and essentially comes from the tide components (aka harmonics, a misnomer) painstakingly gathered over decades by David Flater of Xtides fame.
XTide: Harmonic tide clock and tide predictor
Before the generalization of PCs, the various hydrographic services over the world often released these components, as computing tides by individuals was a long and tedious process no one undertook. E.g. The French SHOM even released a book containing a limited number of 'harmonics' for the major harbours of the world: tides could be computed with the help of a hand-held calculator by masters of cargo ships in case they did not have official predictions.
Note that no individual or small informal group can possibly obtain and process the vast amount of data necessary to compute tidal components (tidal max and min for tens of years for each station). So ALL component data comes from official hydro services (or large public institutions)
Therefore, the components are ultimatley dependent on the legal datum in the country/-ies these services are providing data for.

With the spread of PCs, a great many programmers released and often tried to sell home-made tide calculators based on public data.
From an official point of view, several concerns interacted :
1 - The possibility of errors in the algorithms used
2 - The fact that these tide predictions were based on a limited number of components (typically 6 to 12, instead of the 20 to 100+ used in official predictions)
3 - Loss of potential revenue for hydro offices in an age where public fundings was increasingly frowned upon and cut
So the thatcherian british UKHO threatened unlicensed users of the UK component data (that had previously been openly available) in order to sell both its own application, or raw component data to commercial ventures, and tabulated data to the press, nautical guides publishing companies etc...
David Flaters therefore expunged all data for which he had no official statement that it could be legally used (what is left is what you see in OpenCpn)
(Subsequently, a public laboratory in England (National Oceanography Center see Tidal predictions ) publicly released component data they had independently gathered. This is why there are tide stations in the UK)

For Europe, unauthorized component data from official sources suitable for use in Opencpn have been put together by individuals. The number of 'harmonics' is often limited. Although checked to a certain extent, comparison should be made to official data (e.g. time and height of high/low tides) before using it.
https://docs.google.com/leaf?id=0BzW...YmIyMjJj&hl=fr
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Old 14-06-2011, 08:28   #21
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Re: Chart Datum for Tides

Quote:
Originally Posted by jackdale View Post
My understanding is that the International agreement at IHO is to move to Lowest Astronomical Tide which will result in no negative tide predictions. This is highly problematic as all charts will need to be revised to show that change. Instead the practical agreement is that chart datum should be:

1) so low that the water level will seldom fall below it.
2) not so low as to cause the charted depths to be unrealistically shallow
3) it should vary only gradually from area to area and from chart to adjoining chart, to avoid significant discontinuities.

LNT (the Canadian Coastal standard) does not really meet this standard. What does seldom mean?

Most Canadian coastal charts use Lowest Normal Tide which does result in negative tides. In areas in which both US and Canadian waters are shown (such as chart 3462) MLLW is used in US waters and LNT is used in Canadian waters, i.e., two different chart datum points.

The Great lakes datum is based on the International Great Lakes Datum (1955) which uses Pointe-au-Pere (at Rimouski) as a reference.

(http://www.psmsl.org/train_and_info/...l/chapter5.pdf)

My concern: Are the tide calculations in OpenCNP based on MLLW, LNT, or something else? The answer seems to be "it depends." That makes the calculations, at least in my mind, undependable.

I will be sticking to my CHS Tide and Current Table which I know are based on LNT.
That would be best,alright where it really counts.Even that needs some corelation to weather,rainstorms and the like in the Inlets and such.
OpenCPN doesn't pretend to replace official sources and there's a huge disclaimer that pops up when you first run it...The T+C in O are free...The Yanks are very generous with their data and this is why NOAA sources are often used Canadian data is a bit rougher but suprisingly good.I note that in newer Xtide versions Fisheries Canada has allowed their data to be used but with the rider that these are not the same as CHS..."Free" has it's problems...
As it is,it seems very useful as an approximation(tides agreeing with the fact, being the ideal) and as I said,anyone noting big discrepancies in tide and current,please pass them on to the community.
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Old 14-06-2011, 08:42   #22
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Re: Chart Datum for Tides

G'Day all,

While this has been an interesting discussion, I must say that it seems pretty academic to worry about minor differences in the datum used. To really worry about discrepancies that usually amount to less than a foot is fairly silly IMO. The only time that it matters is in very shallow waters, ie at your own draft plus or minus a foot or so. In these depths other factors can easily overwhelm the harmonic calculations... things like strong on or off shore breezes, unusually high or low barometric pressure, or runoff from rain, etc. So, to depend on charted soundings in such depths is unwise in the first place, and extreme caution in navigation is required. Finally, in many situations such shallow depths are associated with bottom types that change much more frequently than they are surveyed, so to depend on accuracy of tide tables or personal calculations is risky.

Eyeballs and caution are the final solution... and a dinghy powerful enough to help get you unstuck isn't a bad idea either!

Cheers,
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Old 14-06-2011, 08:52   #23
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Re: Chart Datum for Tides

datum is singular a- you need to use DATA, as it is more than one bit of info you seek.
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Old 14-06-2011, 09:40   #24
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Re: Chart Datum for Tides

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G'Day all,

The only time that it matters is in very shallow waters, ie at your own draft plus or minus a foot or so.
What strikes me is that you haven't mentioned the most frequent use of tidal height AFAIC : picking up an anchor spot. It matters a lot if the tide is going to fall or rise 15' to 25' as in the area I often sail in (North Brittany, France).
You need it to estimate what scope will be left at high tide, or, at low tide, if you'll end up bumping your keel or the stern of your boat on those rocks that seem far away when there is plenty of water.
So its nice to know with some precision at the time you let your anchor go what the tidal height is and will be during the night...

E.g. today (in METERS !!!)
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Old 14-06-2011, 10:49   #25
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Re: Chart Datum for Tides

Looking at the data in the latest xtide, some stations contains a lot more information regarding all data, including the datum used, compared to the present OpenCPN HARMONIC file.

An example. Present file:

Code:
#
# Merged from harmonics.
# 1      872-8690 Apalachicola, Apalachicola Bay, Florida   T.M. 75 W.     8728690
# 000920 1 3 1 1 0 0 0 0 0 0 0 0 0   0   0   0 111            gwf 9/21/98
#
# rel_pedigree: -2000000000 (Traceable to authority)
# !units: feet
# !longitude: -84.9817
# !latitude: 29.7267
Apalachicola, Apalachicola Bay, Florida
-05:00 :America/New_York
0.9200 feet
The same information in the latest xtide:

Code:
# Harmonic constants from web snapshot taken 2010-12-19
# Datum from benchmark sheet, publication date 2003-04-21
# BEGIN HOT COMMENTS
# country: U.S.A.
# source: http://tidesandcurrents.noaa.gov/
# restriction: Public domain
# station_id_context: NOS
# station_id: 8728690
# date_imported: 20101226
# datum: Mean Lower Low Water
# confidence: 10
# !units: feet
# !longitude: -84.9817
# !latitude: 29.7267
Apalachicola, Apalachicola Bay, Florida
+00:00 :America/New_York
0.9088 feet
Clearly there is a lot of valuable info added that could benefit OpenCPN in many ways.
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Old 14-06-2011, 10:58   #26
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Re: Chart Datum for Tides

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Originally Posted by sailorF54 View Post
What strikes me is that you haven't mentioned the most frequent use of tidal height AFAIC : picking up an anchor spot. It matters a lot if the tide is going to fall or rise 15' to 25' as in the area I often sail in (North Brittany, France).
You need it to estimate what scope will be left at high tide, or, at low tide, if you'll end up bumping your keel or the stern of your boat on those rocks that seem far away when there is plenty of water.
So its nice to know with some precision at the time you let your anchor go what the tidal height is and will be during the night...

E.g. today (in METERS !!!)
Well, if there is to be a 25 foot tidal swing, I would not be worrying about a one foot error in the datum. Surely you would not anchor at a depth where a small error in the tidal data would put you on the bottom!

Cheers,

Jim
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Old 14-06-2011, 11:16   #27
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Re: Chart Datum for Tides

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Well, if there is to be a 25 foot tidal swing, I would not be worrying about a one foot error in the datum. Surely you would not anchor at a depth where a small error in the tidal data would put you on the bottom!

Cheers,

Jim
You're absolutely right. I should have stated that I was more concerned with the global accuracy of non-official tide prediction packages. Datum is only a (small) part of the picture.
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