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29-08-2020, 13:04
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#1
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Moderator
Join Date: May 2012
Location: At sea somewhere in the Pacific
Boat: Jeanneau Sun Fast 40.3
Posts: 6,380
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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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Our books have gotten 5 star reviews on Amazon. Several readers have written "I never thought I would go on a circumnavigation, but when I read these books, I was right there in the cockpit with Vinni and Carsten"
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29-08-2020, 13:25
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#2
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Registered User
Join Date: Jun 2010
Location: St. Petersburg, Florida
Boat: Gemini 3200
Posts: 982
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Re: Why Mean Low Tide?
What's MTD?
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29-08-2020, 13:29
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#3
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Moderator
Join Date: May 2012
Location: At sea somewhere in the Pacific
Boat: Jeanneau Sun Fast 40.3
Posts: 6,380
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Re: Why Mean Low Tide?
Quote:
Originally Posted by fgd3
What's MTD?
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MTD was typo - I've gone back and edited it so now it reads MLD
sorry about that
__________________
https://www.amazon.co.uk/s?k=carsten...ref=nb_sb_noss
Our books have gotten 5 star reviews on Amazon. Several readers have written "I never thought I would go on a circumnavigation, but when I read these books, I was right there in the cockpit with Vinni and Carsten"
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29-08-2020, 17:47
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#4
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Registered User
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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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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29-08-2020, 17:57
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#5
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Registered User
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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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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29-08-2020, 18:03
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#6
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Marine Service Provider
Join Date: Dec 2010
Location: Little Compton, RI
Boat: Cape George 31
Posts: 3,010
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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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Ben
zartmancruising.com
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29-08-2020, 18:04
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#7
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Registered User
Join Date: Jul 2005
Location: Bellingham
Boat: Outbound 44
Posts: 9,319
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Re: Why Mean Low Tide?
Quote:
Originally Posted by carstenb
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?
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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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29-08-2020, 18:21
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#8
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Registered User
Join Date: Sep 2011
Location: Good question
Boat: Rafiki 37
Posts: 14,206
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Re: Why Mean Low Tide?
Hmm, from Canada's officialdom:
Quote:
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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29-08-2020, 18:38
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#9
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Registered User
Join Date: Jul 2018
Location: SF Bay Area
Boat: Other people's boats
Posts: 1,108
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Re: Why Mean Low Tide?
Quote:
Originally Posted by carstenb
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.
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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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29-08-2020, 19:09
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#10
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Registered User
Join Date: Mar 2008
Location: Calgary, AB, Canada
Posts: 6,252
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Re: Why Mean Low Tide?
Quote:
Originally Posted by Benz
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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LAT is observed once every 19 years.
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29-08-2020, 19:30
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#11
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Registered User
Join Date: Mar 2011
Location: Southport CT
Boat: Sabre 402
Posts: 2,728
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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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29-08-2020, 20:19
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#12
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Registered User
Join Date: Aug 2010
Location: Central California
Boat: M/V Carquinez Coot
Posts: 3,782
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Re: Why Mean Low Tide?
Unfortunately, most measured depths are decades old.
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Kar-KEEN-ez Koot
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30-08-2020, 03:34
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#13
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Marine Service Provider
Join Date: Dec 2010
Location: Little Compton, RI
Boat: Cape George 31
Posts: 3,010
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Re: Why Mean Low Tide?
Quote:
Originally Posted by jackdale
LAT is observed once every 19 years.
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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.
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Ben
zartmancruising.com
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30-08-2020, 03:37
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#14
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Registered User
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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Re: Why Mean Low Tide?
Quote:
Originally Posted by jackdale
LAT is observed once every 19 years.
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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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30-08-2020, 03:43
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#15
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Registered User
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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Re: Why Mean Low Tide?
Quote:
Originally Posted by Benz
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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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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