Chart Datum for Tides

[jackdale]

What chart datum is used by the tide prediction algorithms?

[HappySeagull]

Good question!
"Mean Lower Low Water or it's equivalent" is what I found.If you find wild anomalies with the your local tide book,please report em!

added: or consistent ones!beyond what you'd expect from calculated tides being indicators and nothing more,that is.

[sailorF54]

No chart datum as such

The tides at a station is computed from
- a time reference re UTC (= GMT = Zulu)
- a mean sea level (in m, at least in Europe) (which obviously implicitely depends on some datum chosen by the surveyors)
- a series of harmonics (with 'classical' names) characterized by an amplitude value (m) and a phase angle (°).
e.g.'M2' = 'Principal lunar, semidiurnal'; (i.e. main lunar component, period twice a day)
The more harmonics are known, better the prediction is...

Wikipedia will tell you a lot more...
http://en.wikipedia.org/wiki/Tide

Exemple

BREST, France
01:00 :Europe/Paris
4.09 meters
J1 0.0053 89.00
K1 0.0650 89.00
K2 0.2016 178.00
L2 0.0583 138.00
M1 0.0048 89.00
M2 2.0400 138.00
M3 0.0000 0.00
M4 0.0540 164.00
M6 0.0000 0.00
M8 0.0000 0.00
N2 0.4150 119.00
2N2 0.0546 119.00
O1 0.0670 342.00
OO1 0.0029 89.00
P1 0.0217 89.00
Q1 0.0200 294.00
2Q1 0.0017 89.00
R2 0.0060 178.00
S1 0.0000 0.00
S2 0.7460 178.00
S4 0.0000 0.00
S6 0.0000 0.00
T2 0.0439 178.00
LDA2 0.0143 178.00
MU2 0.0659 119.00
NU2 0.0783 119.00
RHO1 0.0025 89.00
MK3 0.0000 0.00
2MK3 0.0000 0.00
MN4 0.0190 117.00
MS4 0.0340 239.00
2SM2 0.0000 0.00
MF 0.0000 0.00
MSF 0.0000 0.00
MM 0.0000 0.00
SA 0.0470 243.00
SSA 0.0000 0.00

[jackdale]

I did some quick comparison of Canadian Tides and Currents Vol 5 and the tides shown for that area. They are close enough for planning purposes, but I would still use the official publications.

[NAV]

It's simple: if the chart datum is not known (like in the digital tidal data of Open CPN, but also in some CM93 charts) the given tidal heights or waterdepths are useless. Certainly in areas with large height differences between HW and LW, or when the keel clearance of your vessel is expected to be marginal. Using a chart with water depths based on a known Chart Datum X, and tidal information based on a known Chart Datum Y is also asking for problems, unless you know the difference in height between the 2 reference levels at your location.

Many countries use nowadays LAT (Lowest Astronomical Tide) as Chart Datum. This is a result of international standardization. In the US still MLLW (Mean Lower Low Water) is used.

Mean Sea Level is a reference level too, but is never used as a Chart Datum.

For the situation in the North Sea, see attached document.

[sailorF54]

Originally Posted by NAV

It's simple: if the chart datum is not known (like in the digital tidal data of Open CPN, but also in some CM93 charts) the given tidal heights or waterdepths are useless. Certainly in areas with large height differences between HW and LW, or when the keel clearance of your vessel is expected to be marginal.

Many countries use nowadays LAT (Lowest Astronomical Tide) as Chart Datum. This is a result of international standardization. In the US still MLLW (Mean Lower Low Water) is used.

Mean Sea Level is a reference level too, but is never used as a Chart Datum.

I may not have been clear on the way the official authorities compute tides.
The mean level is (obviously) established re the chart datum that applies in their country.
e.g. it will be LAT in France. In my example, the mean level in Brest in 4.09 m above the LAT chart datum zero value, because it is the official datum in France.
The tide values are not useless as the (vector) chart themselves are based on official data using the same datum. e.g. The CM93 charts for france, based on official SHOM charts, report depth based on LAT zero.
Now the tide values in opencpn (or any application) are based on official tide coefficients (if they have not been tinkered with): anyway they are the only one available.
So both sets of data (depth and tide levels) match and can be used. The only thing to remember is that some backward countries still use MLLW (depth and tide level) as you have mentioned...

[NAV]

Originally Posted by sailorF54

So both sets of data (depth and tide levels) match and can be used.

If the properties box of a CM93 chart states "unknown" for the Chart Datum, and the OpenCPN digital tidal data doesn't give an indication of the Chart Datum used, I have to conclude that it's only an assumption that both use the same Chart Datum, not a fact.

In above situation, only if you exactly know the local details from other means, and can deduct that your CM93 chart depth and OpenCPN tidal data comply, you can safely navigate using this information. This seems to be the case for you in the Brest area.

I wouldn't advise to act this way when sailing in unknown areas.

[sailorF54]

Originally Posted by NAV

If the properties box of a CM93 chart states "unknown" for the Chart Datum, and the OpenCPN digital tidal data doesn't give an indication of the Chart Datum used, I have to conclude that it's only an assumption that both use the same Chart Datum, not a fact.

In above situation, only if you exactly know the local details from other means, and can deduct that your CM93 chart depth and OpenCPN tidal data comply, you can safely navigate using this information. This seems to be the case for you in the Brest area.

I wouldn't advise to act this way when sailing in unknown areas.

I quite agree with you in the situation described by your IF clause.

----
For those who may not be familiar with the more esoteric features of OpenCpn, you get the chart datum by
- ticking 'META objects' in the Toolbox / vector charts panel
- deselecting 'allow chart quilting' in Toolbox/ settings
- right-cliking on some part of the chart / object query / coverage
The VERDAT (vertical datum) feature gives the information
It may be in full words and code, or code only
For codes see e.g.
Definition of VERDAT

But, on CM93, I have come across code (59) e.g. in Portugal and many Spanish charts have no VERDAT value.

Edited
Here is another list with code 59 for CM93
http://www.coaps.fsu.edu/pub/eric/RS...R/cm93attr.des

[NAV]

Getting more familiar ;-), I noticed that my CM93 charts metadata shows "59" as vertical datum (translated in the second list you sent as "LAT"). But when I put the cursor on the chart bar at the bottom of the screen, it show in the chart properties window: "Chart Datum: Unknown". So "59" seems not to be correctly translated in OpenCPN.

[sailorF54]

Originally Posted by NAV

Getting more familiar ;-), I noticed that my CM93 charts metadata shows "59" as vertical datum (translated in the second list you sent as "LAT"). But when I put the cursor on the chart bar at the bottom of the screen, it show in the chart properties window: "Chart Datum: Unknown". So "59" seems not to be correctly translated in OpenCPN.

You are right
The only VERDAT values I could find in OpenCpn are the following ones in attdecode.csv (s57data folder)

verdat,12;Mean lower low water;
31;Local low water reference level;
32;Local high water reference level;
33;Local mean water reference level;
34;Equivalent height of water (German GlW);
35;Highest Shipping Height of Water (German HSW);
36;Reference low water level according to Danube Commission;
37;Highest shipping height of water according to Danube Commission;
38;Dutch river low water reference level (OLR);
39;Russian project water level;40;Russian normal backwater level;
41;Ohio River Datum

Some (e.g. 10 LAT) might be hardcoded in the source, but I am too lazy to go into that.

Edit
No, that is not correct.
On French chart with known VERDAT (10), the pop up also reports "Soundings: unknown"

[P_Dub]

Originally Posted by jackdale

I did some quick comparison of Canadian Tides and Currents Vol 5 and the tides shown for that area. They are close enough for planning purposes, but I would still use the official publications.

In Canada, the Chart Datum is Lowest Normal Tides, which is an average of 19 years of observations, using the lowest of each year, whenever available for the reference stations. Secondary stations are compare to the main stations for a comparable tide. The analysis is harmonics and some adjustment has to be made for certain stations. Also, other methods are used for comparaison. For the area you are looking for you will have to contact the West Coast Office of CHS.
Regards.
P_Dub

[sailorF54]

Originally Posted by P_Dub

In Canada, the Chart Datum is Lowest Normal Tides, which is an average of 19 years of observations, using the lowest of each year, whenever available for the reference stations.
P_Dub

That would translate as VERDAT52 in CM93...
That is what is reported for the one point chek I've made (Baie de Gaspe)

[jackdale]

Originally Posted by P_Dub

In Canada, the Chart Datum is Lowest Normal Tides, which is an average of 19 years of observations, using the lowest of each year, whenever available for the reference stations. Secondary stations are compare to the main stations for a comparable tide. The analysis is harmonics and some adjustment has to be made for certain stations. Also, other methods are used for comparaison. For the area you are looking for you will have to contact the West Coast Office of CHS.
Regards.
P_Dub

I am aware of this. I teach coastal navigation, including the calculation of tides at secondary ports.

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

Jack

[HappySeagull]

doesn't "MLLW" =LMT? I think so.No way the neap tides would be included...Lower Low Water referring to what we call "Spring Tides" a "mean" referring to the cycle of moon/sun returning very nearly to the same place at the same time.Someone told me to save my tide books and re-use them but I never did save enough!
.In Canada,wet coast,we do get a very few minus tides...but the charts themselves have info of Local means which can vary 3 or 4 inches on a chart.
..Anyways,per the OP,I quoted the xtide text from which the OpenCPN's tides are somewhat related to...For me, the big thing is knowing how MUCH the tide's going to rise or fall so I can float off

You can read the harmonic file in a text editor,by the way.

[jackdale]

Originally Posted by HappySeagull

Good question!
"Mean Lower Low Water or it's equivalent" is what I found.If you find wild anomalies with the your local tide book,please report em!

added: or consistent ones!beyond what you'd expect from calculated tides being indicators and nothing more,that is.

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)