Can someone explain the numbers on charts

[audi321]

My understanding is that the numbers on a sea chart (or tidal river) relate to the worst case scenario of the lowest possible tide (LAT?). So am I right in thinking that if (for example) the high tide at 12pm is 5m and the low tide is 0.5m, if the chart shows 3m at a particular point, then at high tide the water depth would be 8m and at low tide the water depth would be 3.5m?

Or have I made something far more complex, far to simple and dumbed down lol.

[Equus]

Tides don't rise and fall by the same amount every day.

[audi321]

Equus said:

Tides don't rise and fall by the same amount every day.

Yeah, I get that, but that's not what I'm asking and the examples I've given were just at an example at a certain point in time.

[Equus]

audi321 said:

Yeah, I get that....

So you need to relate the tidal range to the mean sea level, not the lowest astronomical tide.

You can't take your tidal range on any given tide and relate it to the LAT.

Or to put it another way: your tide fluctuates either side of a mean, not up and down from the LAT.

[audi321]

Equus said:

So you need to relate the tidal range to the mean sea level, not the lowest astronomical tide.

You can't take your tidal range on any given tide and relate it to the LAT.

Or to put it another way: your tide fluctuates either side of a mean, not up and down from the LAT.

So are the numbers on the charts the mean sea levels then?

Maybe I need to be more clear with what I’m asking. Here is a chart of my area and I’ve ringed a number 9.4m.



Here’s also the tides for that particular location. High tide is at 15:45 and is 6m.



What I’m asking is, at 15:45 this afternoon. Is the depth at that point ringed 15.4m? Then at low tide (22:46) is the depth 10.6m?

[Phud]

Yes and the depth is taken from a chart datum, for me in the south west my datum is Newlyn on your chart you will be told chart datum and the tidal ranges are above or below that datum

[audi321]

Phud said:

Yes and the depth is taken from a chart datum, for me in the south west my datum is Newlyn on your chart you will be told chart datum and the tidal ranges are above or below that datum

Thanks that’s what I needed to know. So chart datum and LAT are the same thing, and the tide heights are +\- these chart numbers

[Badda]

audi321 said:

Thanks that’s what I needed to know. So chart datum and LAT are the same thing, and the tide heights are +\- these chart numbers

Pretty much yes.

[Equus]

audi321 said:

Phud said:

Yes and the depth is taken from a chart datum, for me in the south west my datum is Newlyn on your chart you will be told chart datum and the tidal ranges are above or below that datum

Thanks that’s what I needed to know. So chart datum and LAT are the same thing, and the tide heights are plus these chart numbers

EFA.

If your chart datum is LAT (lowest astronomical tide) then your tide heights from tide tables will always be positive.

The only caveat is that tide tables don't take account of weather (obviously), only gravitational effect, so you can get negative figures in reality, if a really low tide is combined with weather effects.