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LTL - Low Tide Level
21 April 2020
Dear Colleagues,

Last week I saw a discussion initiated by Daniyar at Schlumberger asking what LTL means.  I have not encountered this terminology before, either in offshore operations or as a part-time yachtsman.

Nevertheless, I would bring to your attention an alternative datum, Lowest Astronomical Tide (LAT).  It is defined as 'the lowest level to which the tide is ever predicted to fall, without allowing for possible meteorological effects.'   The Chart Datum (CD) for navigational charts for UK and other waters in North West Europe corresponds to LAT.

I attach a cartoon from UKHO Chart 5011 Ed 2 1998 which depicts CD, LAT and other tidal levels in everyday global use for marine operations.  Trust this helps.  

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Note from moderator:  Thanks, Tim.  I had actually deleted the original post, and. few others, because it felt like some of the authors are using SPREAD instead of Google and/or turning to it before making any effort themselves to find the answer. Over the years that is the feedback we have had from some of our respected members.

Whilst we appreciate that at this time of COVID-19, it may not so easy to turn to a colleague or walk down the corridor to ask a more learned person, we do ask the members to respect that this forum is designed to test out ideas and seek alternative opinions rather than as a source of (very basic) knowledge.
1 answer(s)
Scott_McNeil
Consultant
SPREAD Associates
Total Posts: 141
Join Date: 05/03/08
Hi Dave / Tim,

We often think of MSL as being a constant reference point around the world, but it's not.

For various reasons, I looked into it a while back.

It turns out there isn’t an internationally agreed standard as to what MSL is.

Indeed, just within Europe there are over a dozen different standards, and not with small differences between them either.

The difference in MSL standards between Ostend and Antwerp - about 160km apart – is a massive 2.31m.

The difference between Germany and Switzerland is 27cm, which lead to one of the more embarrassing engineering failures about 15 years ago when, during building a bridge between the two, the difference was accidentally added instead of subtracted.

So when the two ends met in the middle, they actually didn’t – one end was 54cm higher than the other.

Even within a Country, it may be different – Denmark has 10 different standards dotted around the Country.

The UK’s is based on measurements taken at Newlyn in Cornwall during WW1, but it took until 1952 before this was harmonized across the UK.

However, since then the MSL at Newlyn has risen by 20cm!
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