x-no-archive:yes
Chuck Bollinger wrote:
otnmbrd wrote:

Chuck Bollinger wrote:

Steve wrote:

Were is vertical clearance measured from (what datum)?

The vertical clearance is measured from 'Mean High Water' in places
with a
single diurnal tide. In the Pacific Northwest, it is measured from
'Mean Higher
High Water' (MHHW).

That's what I remember being taught, and then at some subsequent time
being told that all the charts were going to go to MHHW (or maybe it
was MHW that all of them were going to go to). I don't remember why,
nor have I been able to find a reference on the internet.

Interesting ..... Can you show me where this information comes from? In
looking at CP 7, it list all heights as above MHW, unless otherwise
stated, and the only major change to this I can find is for the Columbia
River, which uses MLLW below Harrington Point, and "Columbia River
Datum", between there and Bonneville Dam.

We're just going out and tomorrow going to Port Ludlow. It will be Monday
evening before I'll be on the internet again. Working on it.


The tide tables provide 'Mean Tide' for each subordinate station, and
'Mean
Range'. You can figure out what MHHW is for a place (within a foot or
so) by
taking 'Mean Tide level' and adding one-half the Mean Tidal Range.
When I do that, I come within a foot of the MHHW given in my 'Tides
and Currents Pro' program, and the error results in a number less
(more safe) than the listed one.


On the right track, but be careful that the meaning of "mean Tide" and
"mean range", given in the program and tables you are using, mean G
what you want. Many define "Mean tide" as "the level half way between
mean high water and mean low water" and "mean range" as "the difference
in height between MHW and MLW" .... see the problem?

Frankly, no. One is a tide level and the other is a range.

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.


My particular tide program, gives me MHHW and "Mean Tide". In this case,
I would take the "mean Tide" X 2 and apply it to Zero tide (MLLW) and
use this as MHW .... It should, normally, give a built in safety factor.
At any rate, as I said before, be careful. There are many variables
which can come into play, and you should NEVER push the envelope too
closely.

Hmm. More later on that. Literally have to go.

BTW, I think the program you are using gives MHHW

Yes.


otn


grandma Rosalie

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.

otn

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)
__________________________________________________

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)
__________________________________________________

First off, I must confess, that out on the West Coast, I've just used
the existing tide tables and bridge clearance numbers and compared the
two to find my clearance and MLW/MLLW be damned, since most of the
bridges that I've passed under, had enough clearance, that it normally
wasn't a concern.
However, a couple things on Jack's post:
1. MHW is used to discuss bridge clearances in the US, unless otherwise
noted (we noted some differences).
2. In dealing with charted depths and tide tables, MLW is the datum for
the East (and I believe Gulf - correction any one?) Coast (with
exceptions), while MLLW is the datum for the West Coast.
3. This causes the problem (and it may or may not be). If your tide
tables are based on MLLW, how do you apply those readings to MLW/MHW
(used for bridge clearance) to get the closest possible reading?
(realizing that all of these readings are subject to error due to many
factors of weather, etc.. )

otn

Jack Dale wrote:

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.

Understood, however, see above

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)

Again, understood

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.

Disagree with using MLLW for all US Charts as the datum, the rest
understood.

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)

Understood


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)
__________________________________________________

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)
________________________________________________ __

LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which
relates it as MLW (with an exception for Boston) (also the reason I
asked for confirmation).
My particular tide program does not have the East Coast, but again some
older stuff I was looking at did, and it used MLW for tide datum.
At any rate, you need to be sure which datum (as I think I've been
stressing, or at least should have) you are using, but it still doesn't
clear up the situation of how you change MLLW to MLW or MHW, from the
data given in the particular tide tables, which I think is the jist of
the discussion, when looking at bridge clearance, and I think Chuck was
having the same problem (although he came up with a workable solution),
that I do (although I have basically said "the hell with it" use what
you've got and be sure you have a built in safety factor).

otn

Jeff Morris wrote:

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)
______________________________________________ ____

Here's a thought, Jeff. Check your chart and latest Coast Pilot for
Boston Harbor.
What is the "datum" being used for bridge clearance? MHW? MHHW?
I'm trying to get us all on the same reference plain.

otn

otnmbrd wrote:
LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which
relates it as MLW (with an exception for Boston) (also the reason I
asked for confirmation).
My particular tide program does not have the East Coast, but again some
older stuff I was looking at did, and it used MLW for tide datum.
At any rate, you need to be sure which datum (as I think I've been
stressing, or at least should have) you are using, but it still doesn't
clear up the situation of how you change MLLW to MLW or MHW, from the
data given in the particular tide tables, which I think is the jist of
the discussion, when looking at bridge clearance, and I think Chuck was
having the same problem (although he came up with a workable solution),
that I do (although I have basically said "the hell with it" use what
you've got and be sure you have a built in safety factor).

otn

Jeff Morris wrote:

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)
__________________________________________________

Answering the question in your number 3, on US domestic charts* there
is a box labeled "Tidal Information", which shows tide heights at
several important points one the chart. On a random East Coast chart,
it shows MHW, Mean Tide Level, MLW, and Extreme Low Water for each
point, which allows you to figure bridge clearance =
stated clearance above MHW
plus difference between MHW and MLW
less present height of tide referenced to MLW

I put an asterisk on my generalization in the first sentence --
although we own about 600 charts (did a circumnav a while ago), we
don't have any for the US West Coast, so it's just an informed guess
that on West Coast charts the box shows both the depth datum and the
height datum.

Remember, too, that this kind of calculation has a lot of room for
error, particularly with local wind conditions, which can change the
water height by several feet, and with local error -- clearance
numbers aren't always right.

If I were going through a bridge for the first time and was within
three feet of the calculated clearance, and didn't have good local
knowledge available, I'd absolutely send someone up the mast to watch.
This assumes conditions under which you have complete control of the
boat, preferably with a small current against you, as the worst
possible outcome would be to be forced under the bridge and lose the
stick with a person at the top.

An alternative might be to put someone ashore and have him or her
watch from the bridge.

Jim Woodward
www.mvfintry.com




otnmbrd wrote in message ink.net...
First off, I must confess, that out on the West Coast, I've just used
the existing tide tables and bridge clearance numbers and compared the
two to find my clearance and MLW/MLLW be damned, since most of the
bridges that I've passed under, had enough clearance, that it normally
wasn't a concern.
However, a couple things on Jack's post:
1. MHW is used to discuss bridge clearances in the US, unless otherwise
noted (we noted some differences).
2. In dealing with charted depths and tide tables, MLW is the datum for
the East (and I believe Gulf - correction any one?) Coast (with
exceptions), while MLLW is the datum for the West Coast.
3. This causes the problem (and it may or may not be). If your tide
tables are based on MLLW, how do you apply those readings to MLW/MHW
(used for bridge clearance) to get the closest possible reading?
(realizing that all of these readings are subject to error due to many
factors of weather, etc.. )

otn

Jack Dale wrote:

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.

Understood, however, see above

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)

Again, understood

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.

Disagree with using MLLW for all US Charts as the datum, the rest
understood.

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)

Understood


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)
__________________________________________________

OK, I don't have the latest stuff at home - its on the boat. However, from a 8 year old
chart 13270 of Boston harbor there is a table that lists various heights for "Boston
Light":

Height referred to datum of soundings (MLLW)
Mean Higher High Water 9.7 feet
Mean High Water 9.3 feet
Mean Low Water 0.3 feet
Extreme Low Water -3.0 feet

elsewhere it says:

HEIGHT
Heights in feet above Mean High Water

The tables that convert between the various heights is on a number of charts in my BBS
ChartKit, but the comment on bridge heights I couldn't find without going to an actual
chart.

I also have the same chart from 1867. It lists the minimum and maximum observed tides
from the "reference plane," plus the mean spring and neap low tides from the reference
plane, plus the mean range of the spring and neap tides. It doesn't list what the
"reference plane" is, nor does it have any bridge heights. It does have the "Corrected
Establishment" for determining the state of the tide relative to the full moon, and lists
longitude relative to the State House on Beacon Hill.


"otnmbrd" wrote in message
k.net...
Here's a thought, Jeff. Check your chart and latest Coast Pilot for
Boston Harbor.
What is the "datum" being used for bridge clearance? MHW? MHHW?
I'm trying to get us all on the same reference plain.

otn

otnmbrd wrote:
LOL, relax, Jeff, I'm not Neal. I'm reading some older stuff which
relates it as MLW (with an exception for Boston) (also the reason I
asked for confirmation).
My particular tide program does not have the East Coast, but again some
older stuff I was looking at did, and it used MLW for tide datum.
At any rate, you need to be sure which datum (as I think I've been
stressing, or at least should have) you are using, but it still doesn't
clear up the situation of how you change MLLW to MLW or MHW, from the
data given in the particular tide tables, which I think is the jist of
the discussion, when looking at bridge clearance, and I think Chuck was
having the same problem (although he came up with a workable solution),
that I do (although I have basically said "the hell with it" use what
you've got and be sure you have a built in safety factor).

otn

Jeff Morris wrote:

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)
__________________________________________________