Why Mean Low Tide?

[cal40john]

Originally Posted by requiem

Quick, the depth on the chart is 10 feet, the bridge clearance on the chart is 40 feet, and the current height of tide is 5 feet: what's the tallest vessel that will fit?

You don't know. Somewhere on the chart is the MHHW above datum (for our area anyway). If MHHW is 10', then 45' is the tallest.

[boatman61]

Originally Posted by cal40john

You don't know. Somewhere on the chart is the MHHW above datum (for our area anyway). If MHHW is 10', then 45' is the tallest.

My experience is the depths shown on charts are MLW..
The height shown for bridges is at HW..
Yes, most charts also carry a range table for the area.

[StuM]

Originally Posted by requiem

Quick, the depth on the chart is 10 feet, the bridge clearance on the chart is 40 feet, and the current height of tide is 5 feet: what's the tallest vessel that will fit?

Impossible to say without further information. By how much is the current tide above or below MHW?

https://vdatum.noaa.gov/docs/datums.html
"Note: to support harbor and river navigation, bridge clearances are typically referenced to mean high water (MHW);"

Quick, the bridge clearance on the chart is 40 feet at MHW, I have an airdraft of 39 feet. Does that mean that I can get under the bridge regardless of tide?

No unless there is a VERY small tidal range, both diurnally and neaps to springs.

If the datum is HAT, Yes except in adverse weather conditions.

[requiem]

Originally Posted by boatman61

So.. MLW is 10ft, you have 5ft of tide.. thats kinda like silly unless one also knows the tide range.. it could be anything from 5ft to 30ft.
If its 5ft you have 40ft minus your mast top gadgets.. if its 30ft you have 65ft minus your gadgets.

Exactly so, and whilst tidal predictions, like chart depths, are easy to find, the values used for HAT or MHHW are often tucked away in the margin.

For me it would be more logical to index everything to the same height rather than have discontinuities in the numbers, where one uses MHHW for clearance, MSL for towers and lights, and MLLW for depths.

Edit: but, I can see the greater risk of accidents that might cause.

[Adelie]

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

Anyone know why?

I've never seen datum to MLT. I've only ever seen MLLW or occasionally MLLWS (Mean Lower Low Water (19yr average of daily lower low; Mean Lower Low Water Springs, The 5yr or 19yr average of the deepest lower low for each month (actually 28.45d))

Apparently the USCG in TX is moving from MLT to MLLW which according to the following PDF is higher, so MLT is more conservative than MLLW.
https://www.westerndredging.org/phoc...-Low-Water.pdf
The first or second slide indicates MLLW is a more internationally recognized datum and is cited in legislation, so the are getting line with that.

UKHydrographic office started work to transition form MLLWS to LATin 2007:
https://www.hydro-international.com/...0circumstances.

Apparently MLT is also not a normal NOAA datum, a legacy datum maybe.
https://tidesandcurrents.noaa.gov/datum_options.html
https://vdatum.noaa.gov/docs/datums.html
https://www.worldtides.info/datums

Here you can look up the all the various datums and their values for your locale.
https://tidesandcurrents.noaa.gov/st...ml?type=Datums


LAT is used in UK, Australia, France, Canada(?),...
MLLW - USA
Mean Low Water Springs (Germany)
Mean Lower Low Water Springs (Netherlands)
Mean Lower Low Water Springs occurring every six months (Indonesia)
Indian Spring Low Water (India, Japan ...): mean level minus the sum of the amplitudes of the four main tide components.



http://charts.gc.ca/documents/public...rt1-Carte1.pdf
https://www.tides.gc.ca/eng/info/verticaldatums

Page 18 of http://www.euref.eu/symposia/2016San...05-Alberts.pdf
shows that altlantic Europe is mostly, LAT
Baltic is mostly MSL
and the Med is a mishmash.

The difference between LAT & MLLWS is mostly on the order of 6-12" in large areas with large tidal ranges (10-15'), and up to 18" in areas with very large tidal ranges.
For most of the world it probably will be on the order of a couple inches.
https://wiki.openstreetmap.org/wiki/...ronomical_Tide

[Adelie]

Originally Posted by StuM

...

Quick, the bridge clearance on the chart is 40 feet at MHW, I have an airdraft of 39 feet. Does that mean that I can get under the bridge regardless of tide?

....

NO...

[StuM]

A quiz question:


What datum is used for the heights of lights etc?


A. In the US
b. In your location if not US:

[jackdale]

Originally Posted by StuM

A quiz question:


What datum is used for the heights of lights etc?


A. In the US
b. In your location if not US:

In Canada - Higher High Water Large Tides.

US - Mean High Water

Some of my Canadian charts include both countries.

[StuM]

Originally Posted by requiem

Exactly so, and whilst tidal predictions, like chart depths, are easy to find, the values used for HAT or MHHW are often tucked away in the margin.

You can't have a HAT value or any other datum value for a whole chart unless it's very large scale. Like depths, they are always related to the reference datum of the nearest tidal station.

Queensland semi-diurnal station HATs for example can be anything from 1.5 to 7.5 metres above LAT.
(Heck, it varies by 2 metres just over the 40NM between the north and south ends of the Great Sandy Strait)

(The "datum plane" against which depths are measured is not a reasonably smooth surface like mean sea level, it dips and bulges.)

[requiem]

Originally Posted by StuM

You can't have a HAT value or any other datum value for a whole chart unless it's very large scale. Like depths, they are always related to the reference datum of the nearest tidal station.

That's a good point. It'd be convenient to have the reference stations charted, along with the values. On my paper charts it's just a small table listing a few stations and the offsets for MHW, MLW, etc, but the current ENCs don't have that.

I'm told that the S-100 standard will have a subcategory for it (S-104: Water Level Information for Surface Navigation). I'm hoping that means one could eventually "right-click" an area and see both the station and the offsets.

Oh, and you got me on the quiz, MHW would indeed be the one for the US. Not sure why I have MSL in my head.

[jdmuys]

Two observations:

1- Several people in this thread have been using feet for the unit of depth. This is an even worse idea than yesteryear as with current computer-based charting systems able to mix raster (scanned) and vector (fully digital) charts, you can now have on the same screen soundings in the modern meters and the antiquated feet (and even - oh my - fathoms). This is not an obscure theoretical possibility. I experience this very often.

Switching to the metric system for water heights (soundings, tides, drafts...) would perhaps be one of the less painful first steps as they are very rarely mixed with other length measurements.

2- The international Hydrographic Organisation recommends that Lowest Astronomical Tide (LAT) be adopted as the international Chart Datum. Was it not for our American friends, we might eventually achieve this.

[Benz]

Originally Posted by jdmuys

Two observations:

1- Several people in this thread have been using feet for the unit of depth. This is an even worse idea than yesteryear as with current computer-based charting systems able to mix raster (scanned) and vector (fully digital) charts, you can now have on the same screen soundings in the modern meters and the antiquated feet (and even - oh my - fathoms). This is not an obscure theoretical possibility. I experience this very often.

Switching to the metric system for water heights (soundings, tides, drafts...) would perhaps be one of the less painful first steps as they are very rarely mixed with other length measurements.

2- The international Hydrographic Organisation recommends that Lowest Astronomical Tide (LAT) be adopted as the international Chart Datum. Was it not for our American friends, we might eventually achieve this.

This is an artificial problem since scanned charts--and all charts everywhere--could easily use fathoms and feet. It was only with the introduction of meters into a system that already worked perfectly (miles, fathoms, feet, etc) that the current confusion in depth units began. It's not the fault of Americans for continuing to use a system that has been working perfectly since Captain Cook and before--its the fault of the nerds who keep trying to force a new and imbecile system on us.

[jdmuys]

Originally Posted by Benz

It was only with the introduction of meters into a system that already worked perfectly (miles, fathoms, feet, etc) that the current confusion in depth units began. It's not the fault of Americans for continuing to use a system that has been working perfectly since Captain Cook and before--its the fault of the nerds who keep trying to force a new and imbecile system on us.

You are right. All the scientists everywhere in the world, including in the USA are imbeciles for using the metric system everywhere. They claim it's simpler and avoids mistakes because you only use power of tens for scaling. As if multiplying by combinations of powers of 20, 12 or other numbers was more difficult. And NASA scientists and engineers are all imbecile for losing the $100 millions+ Mars Climate Orbiter because they tried using the metric system to compute its trajectories.

Sometimes, the arrogance and hubris of some people is surprising, from a country that didn't even have a continental map of their own country until over 20 years after the metric system was adopted.

[StuM]

Originally Posted by Benz

This is an artificial problem since scanned charts--and all charts everywhere--could easily use fathoms and feet. It was only with the introduction of meters into a system that already worked perfectly (miles, fathoms, feet, etc) that the current confusion in depth units began. It's not the fault of Americans for continuing to use a system that has been working perfectly since Captain Cook and before--its the fault of the nerds who keep trying to force a new and imbecile system on us.

So did the Dutch, Spanish, Portugese explorers use fathoms and feet?
How about the Arabic, Chinese, Polynesia seafarers?

The rest of the world have all agreed on one logical, easy to use set of "International Units".

Even the British Empire/Commonwealth finally saw the light.

It's time for the US to join in and use consistent units both internally and internationally. (maybe it will prevent another Mars Climate Orbiter disaster).

[boatman61]

Originally Posted by requiem

Exactly so, and whilst tidal predictions, like chart depths, are easy to find, the values used for HAT or MHHW are often tucked away in the margin.

For me it would be more logical to index everything to the same height rather than have discontinuities in the numbers, where one uses MHHW for clearance, MSL for towers and lights, and MLLW for depths.

Edit: but, I can see the greater risk of accidents that might cause.

Off course it would lead to more accidents.. if folks have difficulty comprehending that all depths shown on charts are MLW and bridge heights are logically the clearance at MHW then chucking in extra even more baffling info would lead to chaos..