Understanding tide tables

[edwants2dive]

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.

 

[lexvil]

How to Read Tide Tables: 13 Steps (with Pictures) - wikiHow

 

[Storker]

You have to consider what is the zero. There are many zeroes. Minimum low tide, average low tide, average mid tide, minimum high tide, maximum high tide or chart zero. To name a few.

A tide table gives the water level as plusmimus some height compared to some standard level. Often chart zero. Different tables may use different standard levels. Until you know what the chosen standard level is, you're in the dark.

 

[nolatom]

Only slightly off topic, Understanding tides or trying to, can lead to appreciation of the role of the Moon and to a lesser extent the Sun in creating them. Each has enough gravity to "lift" the ocean slightly away from Earth, hence the high tides when you have Full Moon and new Moon, because the line-up of Sun, Earth, and Moon in a "straight line", accentuates that lifting effect. More so when it's New Moon, one of those two daily tides will be extra high. Conversely, when it's Half Moon, those forces are at right angles to each other and not acting as a gravity "team" any more, so you get less range of tide, whether high or low.

For me as a city kid, one of the biggest buzzes in diving (besides being weightless) is seeing the night sky undiminished by shoreside lights. The idea that they affect our tides from so far away is fascinating to me. So too is gravity to begin with. I don't understand how gravity can be strong enough to keep planets billions of miles away in the Sun's orbit (or Earth's keeping our Moon in an orbit a quarter million miles away from us ), yet weak enough that we get to jump up and down here on Earth, instead of being glued to the ground because we're impossibly heavy.

 

[Gareth J]

DEPTH
Most chart depths are based on LAT (Lowest Astronomical Tide). This is the theoretical lowest low tide that can exist.

I said 'theoretical'. In most cases low tide will be always be a positive number, e.g.
+ ?.? and high tide + ?.?.
e.g. Low tide = +1.0m, High tide = +5.7m.
Very rarely low tide is a negative number, so the actual depth will be shallower than that shown on the chart.
Which if you are navigating a boat can be very dangerous.

Using LAT normally means that this danger is avoided.

TIME
You need to be careful with time. (At least in the UK).
UK Almanac's state time based on GMT (Greenwich Mean Time). But we have a time offset in the summer, BST (British Summer Time), of +1 hour.
So you need to ADD 1 hour to that stated in the Almanac (tide table). Some local (town) tide tables have the time quoted as BST during the summer, and GMT during the winter, to make things easier for tourists! TAKE CARE!!

 

[Rollin]

Each has enough gravity to "lift" the ocean slightly away from Earth,

For how I understand it, the water is not 'lifted' at all. The very, very small gravitational pull of the Moon is about one million times smaller than that of the Earth's. But, because of the small regular pulls, seawater is moving as a standing wave. A bit like pushing someone on a swing. You are not lifting them, but by giving small pushes at the right moment you can create large motions of relatively heavy weights.

 

[Storker]

DEPTH
Most chart depths are based on LAT (Lowest Astronomical Tide). This is the theoretical lowest low tide that can exist.

I said 'theoretical'. In most cases low tide will be always be a positive number, e.g.
+ ?.? and high tide + ?.?.
e.g. Low tide = +1.0m, High tide = +5.7m.
Very rarely low tide is a negative number, so the actual depth will be shallower than that shown on the chart.
Which if you are navigating a boat can be very dangerous.

Using LAT normally means that this danger is avoided.

TIME
You need to be careful with time. (At least in the UK).
UK Almanac's state time based on GMT (Greenwich Mean Time). But we have a time offset in the summer, BST (British Summer Time), of +1 hour.
So you need to ADD 1 hour to that stated in the Almanac (tide table). Some local (town) tide tables have the time quoted as BST during the summer, and GMT during the winter, to make things easier for tourists! TAKE CARE!!

But the tide depth depends totally on your chosen zero. If I choose average low tide, it's one. If I choose chart minimum, it's another. If I choose chart maximum, it's yet another. Not to mention minimum low or maximum high (whatever that minimum or maximum might be).

Choose your zero, relate your tide depths/heights to that. But make sure that you read the right zero.

 

[nolatom]

For how I understand it, the water is not 'lifted' at all. The very, very small gravitational pull of the Moon is about one million times smaller than that of the Earth's. But, because of the small regular pulls, seawater is moving as a standing wave. A bit like pushing someone on a swing. You are not lifting them, but by giving small pushes at the right moment you can create large motions of relatively heavy weights.

Hey, I'm an English major. Willing to learn... ;-)

 

[Gareth J]

But the tide depth depends totally on your chosen zero. If I choose average low tide, it's one. If I choose chart minimum, it's another. If I choose chart maximum, it's yet another. Not to mention minimum low or maximum high (whatever that minimum or maximum might be).

Choose your zero, relate your tide depths/heights to that. But make sure that you read the right zero.

Storker.
Admiralty Charts always use LAT as the datum.
Marine Almanac's (British), always refer the tidal data to LAT (& GMT)

I can only speak for the UK, I don't have a practical knowledge of how things are across the pond, or in your neck of the woods.

Gareth

 

[Storker]

Storker.
Admiralty Charts always use LAT as the datum.
Marine Almanac's (British), always refer the tidal data to LAT (& GMT)

I can only speak for the UK, I don't have a practical knowledge of how things are across the pond, or in your neck of the woods.

Gareth

Printed tide tables usually follow national standards (which you then have to familiarize yourself with). Apps and online tables.. well, that's quite another can of worms. My favorite tide app, I have a quarter of a dozen zero levels to choose between.