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Old 29-08-2020, 13:04   #1
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Why Mean Low Tide?

As far as I know, the US is the only country to use Mean Low Tide (MLT) on their carts. All others use LAT (Lowest Astronomical Tide).

Why MLT - it really isn't as reliable as LAT and when I'm sailing or anchoring, I'd much rather see LAT than MLT. MLT tells me nothing. A big spring tide and the depth can easily be a couple of feet lower than MLD.

Around the Baltic MLT is also used - but the Baltic has no tides so it doesn't matter.

Anyone know why?
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Old 29-08-2020, 13:25   #2
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Re: Why Mean Low Tide?

What's MTD?
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Old 29-08-2020, 13:29   #3
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Re: Why Mean Low Tide?

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Originally Posted by fgd3 View Post
What's MTD?
MTD was typo - I've gone back and edited it so now it reads MLD

sorry about that
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Old 29-08-2020, 17:47   #4
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Re: Why Mean Low Tide?

> MTD was typo - I've gone back and edited it so now it reads MLD


What's MLD?
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Old 29-08-2020, 17:57   #5
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Re: Why Mean Low Tide?

Why? I guess it's the same reason the US still uses feet and inches, pounds, miles etc when the rest of the world uses logical units
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Old 29-08-2020, 18:03   #6
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Re: Why Mean Low Tide?

Because MLT is the average low. LAT is only seen once or twice a year. Tide tables give you the exact predicted feet above or below datum, so it pays to learn to read them, and it isn't hard to do.
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Old 29-08-2020, 18:04   #7
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Re: Why Mean Low Tide?

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Originally Posted by carstenb View Post
As far as I know, the US is the only country to use Mean Low Tide (MLT) on their carts. All others use LAT (Lowest Astronomical Tide).

Why MLT - it really isn't as reliable as LAT and when I'm sailing or anchoring, I'd much rather see LAT than MLT. MLT tells me nothing. A big spring tide and the depth can easily be a couple of feet lower than MLD.

Around the Baltic MLD is also used - but the Baltic has no tides so it doesn't matter.

Anyone know why?
Most places, at least on the west coast of the US, do not use MLT. The charts use MLLW. Mean Lower Low Water. This is the average of the lower low tide of the day. Daily tides tides are then given as a signed difference from MLLW. Except on extreme tides, the number is positive.
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Old 29-08-2020, 18:21   #8
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Re: Why Mean Low Tide?

Hmm, from Canada's officialdom:

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On most Canadian coastal charts the surface of lower low water, large tide, or LLWLT , has been adopted as chart datum, but the term " lowest normal tide, " or " LNT, " has been retained on the charts since it encompasses a variety of other choices for chart datum on some older charts.
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Old 29-08-2020, 18:38   #9
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Re: Why Mean Low Tide?

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Why MLT - it really isn't as reliable as LAT and when I'm sailing or anchoring, I'd much rather see LAT than MLT. MLT tells me nothing. A big spring tide and the depth can easily be a couple of feet lower than MLD.
I'm not sure that measure tells me anything on its own; it's only meaningful in relation to the current tide. For me, that means I'm used to seeing minus tides every so often, but it's always the sum of the two (tide and soundings) that provides meaning.

What does feel odd to me is the Great Lakes; the first time I saw a chart for one I was surprised at how shallow the depths were in supposedly navigable spots.
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Old 29-08-2020, 19:09   #10
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Re: Why Mean Low Tide?

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Originally Posted by Benz View Post
Because MLT is the average low. LAT is only seen once or twice a year. Tide tables give you the exact predicted feet above or below datum, so it pays to learn to read them, and it isn't hard to do.
LAT is observed once every 19 years.
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Old 29-08-2020, 19:30   #11
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Re: Why Mean Low Tide?

MLT might be more likely to provide a depth that you would find on your depth-finder, so you could use it as a navigational tool to determine where you were. If your depth sounder only matches depths that show on the chart every 19 years, it is not going to be very useful. Also, indicating LAT depths on a chart, since they actually happen less frequently and are theoretically shallower than MLT, might keep you from taking a route that would be perfectly safe to use most of the time.
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Old 29-08-2020, 20:19   #12
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Re: Why Mean Low Tide?

Unfortunately, most measured depths are decades old.
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Old 30-08-2020, 03:34   #13
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Re: Why Mean Low Tide?

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LAT is observed once every 19 years.
Which makes it even less useful as a datum. What's the lowest yearly called? There's too many acronyms to really keep it straight.
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Old 30-08-2020, 03:37   #14
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Re: Why Mean Low Tide?

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LAT is observed once every 19 years.

Depending on atmospheric pressure, surge, EL Nino/La Nina and a number of other factors.
It's a "calculated" figure usng harmonics etc but can't tae weather conditions into ccount.



It my never be observed in the 19 year period.
You may also see a low tide lower than LAT.
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Old 30-08-2020, 03:43   #15
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Re: Why Mean Low Tide?

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Which makes it even less useful as a datum. What's the lowest yearly called? There's too many acronyms to really keep it straight.

It's just as useful as any other datum. As long as tide predictions use the same datum, it has the major advantage that you don't have to mess around with plus/minus numbers.
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