Understanding tide tables

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Ok I know I should be able to figure this out but looking for some help on understanding the tide table. What i am trying to understand is the +/- ft markings on the tide table and what the time listed actually specifies. Some plain language explanations would be great. I think I get them but not quite sure.
Which Tide Table are you looking at?
Those in the US are referenced to Mean Lower Low Water (MLLW), "The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch."
 
Why are simple things so complex when attempts are made to explain them? I'm no expert, but I've been using tide charts for a long while. Inlet diving requires entry just before mean high tide, and exiting shortly after the tide turns outward. OTOH, seining is best accomplished just before low tide, during low, and just after the tide begins to come back in. The little ft. markings on generalized charts usually indicate how much higher or lower the tide will be compared to average (mean) sea level. You may notice that these differences are most extreme during a neap tide and during a spring tide, based on the Moon's position relative to that body of water. The tide charts I use are extremely precise, giving different times, sometimes by only a few minutes, for high and low tide for points only a few hundred meters away from each other. Low tide in an oceanic inlet will sometimes occur close to an hour before low tide in the river or bay immediately behind it. It takes that long to drain in certain topographical structures. The obvious reverse is also true. Tides are extremely variable within a relatively small area, and are affected by numerous factors. It should also be pointed out that tides are more extreme the closer the body of water is to the poles, and less extreme the closer they are to the equator, hardly noticeable at the equator itself. Latitude is an important element.

Always use an area specific detailed chart for any significant purpose.
 
Yeah tides interacting with inlets or canals can make timing your passage in a boat (or your dive if you're diving in the "interaction zone".
I grew up around Mass Bay, cruising or delivering sailboats through the Cape Cod Canal, which connects Mass and Buzzard's Bays. Tidal range in Mass Bay was 9 feet, but only 4 feet in Buzzard's Bay. So the Canal current stopped and reversed itself at half-tide, not at high or low tide as one might see in "open" water.
In New Oleans where I am now, to get from Lake Pontchartrain to Mississippi Sound (the "ocean"), you pass through an inlet or "gut" called the Rigolets, which is scenic but the tide runs hard, about four knots, and current changes on the half-tide--the Lake (actually a bay obviously), never "catches up" with the Bay tide, so you have to time it right or that four miles of gut will take forever. Get it right and you zoom through in your 5-knot auxiiary saioboat making a scorching nine knots ;-)

In diving at Phil Foster Park in Palm Beach, you have the same phenomenon, but to a much lesser effect., in order to dive during slack high waterl But it's a big deal to us divers without an auxiliary engine....
 
@Storker

An education.

I guess its because all the tidal date I have seen must have been published by the UKHO, or at least the UKHO are the source of the data. On all of my electronic tide tables the chart datum is fixed to LAT. My apple apps adjust for BST, but the chart datums are still fixed to LAT. One of the apple apps clearly states the tide date is under license from UKHO.

I am now aware that chart datum can move if the data is not from the UKHO, thanks.
I know the Imray stuff follows the UKHO convention and uses LAT for chart datum, but they are a British Chart company.


Does this mean that charts published by other organisations outside the UK move the chart datum? Is there a particular reason / need for this?

I suppose there might be an advantage to break the copyright complications with UKHO, but I wouldn't have thought that would have been needed. I would have thought that these days there is some sort of reciprocal agreement between the different Hydrographic Offices of different countries. It's not cheap if you are doing the Hydrographical mapping world wide.
 
Does this mean that charts published by other organisations outside the UK move the chart datum? Is there a particular reason / need for this?
I'm not very well read on this stuff.

But IME, our national tide tables refer to a certain, specified chart datum. As I assume other national tide tables do. However, that datum may (well) differ between certifying bodies, and as I've mentioned, when I use my tide table app I can choose between different zeros. For me, where the zero is doesn't matter much. I look at the tide level (mid, a little high, a little low, very high or very low) and follow the marine charts (and my echo sounder) for depths and free-sailing heights when I'm piloting a boat.
 
I'm not very well read on this stuff.

But IME, our national tide tables refer to a certain, specified chart datum. As I assume other national tide tables do. However, that datum may (well) differ between certifying bodies, and as I've mentioned, when I use my tide table app I can choose between different zeros. For me, where the zero is doesn't matter much. I look at the tide level (mid, a little high, a little low, very high or very low) and follow the marine charts (and my echo sounder) for depths and free-sailing heights when I'm piloting a boat.

This issue about the chart datum interests me. I was always taught that LAT was always used, to minimise the risk to shipping. From a depth point of view if you used the depth shown, you would always have more water under you than that stated on the chart (except on very rare occasions).
By changing the chart datum to something else, this expectations is compromised.
Granted, we have GPS these days, which means we have a much more accurate idea of where we are physically. But this doesn't negate the issue of depth under the boat.
Your local Charts, what chart datum do they normally use if not LAT?
How does that equate to LAT?

Thanks
Gareth
 
The astronomical tides are not necessarily the lowest water.....prevailing winds, for example, can change the water levels.
LAT only considers astronomical effects.
The US tide charts use Mean Lower Low water, which is from observations. If there were only astronomical tides, LAT and MLLW I believe would be the same. Here are the actual definitions from NOAA Tides & Currents.
MLLW
Mean Lower Low Water
The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.
LAT
Lowest Astronomical Tide
The elevation of the lowest astronomical predicted tide expected to occur at a specific tide station over the National Tidal Datum Epoch.
 
Your local Charts, what chart datum do they normally use if not LAT?
IIRC, they use average low tide or perhaps lowest low tide for depths and average high tide for sailing height. I might well be wrong, though, this is taken from memory. If you're really interested in my local conditions, google "den norske los", those books are freely available as PDFs. I've never bothered with sailing height since it hasn't been relevant for my boating, and no matter the charts I pay attention to my depth sounder if I'm in gnarly waters. I'd rather trust real-time data than chart data.
 
@tursiops , Yes I am aware that there are occasions when the tide can be lower that LAT, but these are expected to be exceptional cases.
In addition, as you correctly state, air pressure, and prevailing winds can also change the actual depth on the day. But irrespective of the datum and tide table used, these are factors that you would not normally be able to account for.
From a diving point of view, if wind and tide are having that much of an effect, I seriously doubt I would be diving!

@Storker. Thank you for your original comment. It has sent me on a search for knowledge.
As far as I have been able to find.

Chart Datum does change.
Charts for Tidal Waters use LAT (Lowest Astronomical Tide).
LAT is used by the UK and Australia, t
The USA uses MLLW (Mean Low Low Water), which is based on an average calculated over 19 years. What I don't understand about MLLW, this seems to be a moving figure, as it is a 19 year average.


Charts for non tidal waters (e.g. Baltic Sea) use MSL (Mean Sea Level).

Lowest Astronomical Tide is used because it is defined as the lowest tide level that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. This ensures water is always deeper than specified on the chart other than under exceptional conditions.

Mean Sea Level. The average depth of water. However, there appear to be several MSL definitions.
The simplest, the midpoint between the mean low and mean high tide.
However, the global MSL, is used for chart datums and to calibrate altitude (aviation).

I will continue my education.

My thanks @Storker
 
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