My Boston Harbor chart says the datum for soundings is MLLW. My Reed's (East Coast 2001)
says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum."

It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is
significantly different from the US, and is usually synonymous with Lowest Low Water,
Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of
reference. Got that? There will be a quiz on Monday.

And yes, everyone should be aware that local weather conditions can add or subtract
several feet to the height of the tide.

"otnmbrd" wrote in message
nk.net...
Check your datum. Boston Harbor may use a different datum "Boston Harbor
Low water datum"
Also, just because you are using a particular datum which states that
your high might be, say 6.0', there is nothing that says you will not
get higher highs and lower lows, than this, at varying times.

Jeff Morris wrote:

Most of this is correct. However, I would take issue with the statement


Use MLLW to deal with depths on US charts. MLLW will normally be the
shallowest that the water will be.


Since this is the average of the lowest tide for each day, roughly half the days will
have
a lower tide. In some locations this might not be significant, but in Boston, for
instance, there are several days every month that are more than a foot below MLLW.
There
are several days a year that are two feet or more below MLLW.

"Jack Dale" wrote in message
...

On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd
wrote:



Chuck Bollinger wrote:


But something bothers me about Mean Tidal Level being half way between
MHW and MLW, especially where there are two diurnal highs and lows.
Can't put my finger on it, but that seems like one of those shortcuts
that can introduce errors. Kind of like those situations where computing
from the results of a computation introduces error. Another thing to
research.

This is part of my problem with this. If we need to find the height of
MHW and our tide datum is based on MLLW, I'm not sure how we can
directly convert with any certainty from the info given.
Also:
Diurnal - Single high and low
Semi Diurnal - two high and low
Mixed - Variations/inequalities in highs and lows .... what we have on
the West Coast, with variations in local

This is one area I've always been weak on, so BG hopefully this old
dog can learn some new tricks.


There is no need to convert anything. They are different
measurements.

On US charts use MWH to deal with clearances and heights. The
clearance is normally the minimum clearance available under a bridge,
overhead lines, etc.. Use your tide tables to determine if you have
additional clearance.

Mean High Water (MHW): A tidal datum. The average of all the high
water heights observed over the National Tidal Datum Epoch. For
stations with shorter series, simultaneous observational comparisons
are made with a control tide station in order to derive the equivalent
datum of the National Tidal Datum Epoch.
(http://co-ops.nos.noaa.gov/mapfinder/mhw.html)

Use MLLW to deal with depths on US charts. MLLW will normally be the
shallowest that the water will be. Use your tide tables to determine
how much water you have under you on that day at that time. Also
this information will let know how much additional depth you have over
underwater rocks that are a danger to navigation, how much water is
over rocks awash and whether or not drying rocks are visible.

Mean Lower Low Water (MLLW): A tidal datum. The average of the lower
low water height of each tidal day observed over the National Tidal
Datum Epoch. For stations with shorter series, simultaneous
observational comparisons are made with a control tide station in
order to derive the equivalent datum of the National Tidal Datum
Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html)


Always read the title block to establish datum for clearances and
depths, and ensure that you use the appropriate tide tables. Canadian
datum is based on Lowest Normal Tide, clearances are based on Higher
High Water, Large Tides. For US charts use US tide tables, use
Canadian tide tables for Canadian charts.

BTW - the space between MWH and WLLW on US charts is the green stuff
(foreshore).

A couple of years I attempted to create an online lesson for reading
tide tables
(http://www.acs.ucalgary.ca/~jodale/e...t/content.html).


Jack

________________________________________________ __
Jack Dale
Swiftsure Sailing Academy
Director/ISPA and CYA Instructor
http://www.swiftsuresailing.com
Phone: 1 (800) 470-SAIL (toll free)
________________________________________________ __