There we go ...... My error ( BG My wife's loving this) was in dealing
with older publications and not having any charts here, which gave me
that info..... LOL shoulda known better.
At any rate, now we have a way to get MHW and can go back to an earlier
post for determining clearance, based on using the tide tables.
I haven't got one here .... anyone check a "Light List" for the info
Jeff has given? (I'm asking, to see if it's there, also).

otn

Jeff Morris wrote:
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
thlink.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
news:fc08jv0v8rr0u209jo82bch0ok1lm1hnvb@4ax. com...



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)
___________________________________________ _______

Since I'm the one what caused most of the confusion and gave the most
bad info, let me try an example to hopefully clear up the mess.


Your tide datum is MLLW (also called Zero tide)
On your chart you have a box showing tidal data.
In this box, MHW is listed as +6.0'; MHHW is listed at +7.0'.
You are on a sailboat approaching a bridge. The height at your present
draft to the tip of an antenna at the masthead, is 50'. The clearance of
the bridge shown on the chart, is 50' at MHW.
AT Zero tide (MLLW), you will have 6' clearance between your antenna and
the bridge. At +6.0' tide (MHW), you will hear a slight "Ping" as the
tip of your antenna touches the bridge. At any stage of the tide which
your tables show to be greater than this (+7.0,8.0,9.0, etc.) your next
stop will be the nearest boatyard for repairs.
At any stage of the tide where the numbers show a minus reading (below
MLLW) you will gain additional clearance.

All warnings about accuracy of tables and predictions based on possible
weather, etc. conditions, still apply.

BG

otn

PS got my hands on a light list .....no tidal info in there for
particular lights, that I could see.

I hope I didn't send you astray by using "Boston Light" as a reference - the table lists
several other points that are not lights and I just used Boston Light as a well known
reference point in Boston. I could have picked the Chelsea Bridge, but few people would
have recognized it. Many of the charts in my ChartKit have similar tables, listing 3 or
4 reference points.



"otnmbrd" wrote in message
nk.net...
There we go ...... My error ( BG My wife's loving this) was in dealing
with older publications and not having any charts here, which gave me
that info..... LOL shoulda known better.
At any rate, now we have a way to get MHW and can go back to an earlier
post for determining clearance, based on using the tide tables.
I haven't got one here .... anyone check a "Light List" for the info
Jeff has given? (I'm asking, to see if it's there, also).

otn

Jeff Morris wrote:
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
thlink.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
news:fc08jv0v8rr0u209jo82bch0ok1lm1hnvb@4ax. com...



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)
___________________________________________ _______

On Sat, 09 Aug 2003 22:30:17 GMT, otnmbrd
wrote:




PS got my hands on a light list .....no tidal info in there for
particular lights, that I could see.

The height of the light is from MHW on US charts. That is about all
the tidal information in that publication.

If you use the "dipping the horizon" technique for establishing a
circle of position, the height of the tide at the time of the
observation must be taken into consideration.

Jack

Again, I don't have any charts here, but check ..... where you have
multiple listings, do they relate to a tidal reference station, in some way?

otn

Jeff Morris wrote:
I hope I didn't send you astray by using "Boston Light" as a reference - the table lists
several other points that are not lights and I just used Boston Light as a well known
reference point in Boston. I could have picked the Chelsea Bridge, but few people would
have recognized it. Many of the charts in my ChartKit have similar tables, listing 3 or
4 reference points.



"otnmbrd" wrote in message
nk.net...

There we go ...... My error ( BG My wife's loving this) was in dealing
with older publications and not having any charts here, which gave me
that info..... LOL shoulda known better.
At any rate, now we have a way to get MHW and can go back to an earlier
post for determining clearance, based on using the tide tables.
I haven't got one here .... anyone check a "Light List" for the info
Jeff has given? (I'm asking, to see if it's there, also).

otn

Jeff Morris wrote:

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
link.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
arthlink.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
news:fc08jv0v8rr0u209jo82bch0ok1lm1hnvb@4a x.com...




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)
_________________________________________ _________

BG Phew, that's pushing the envelope of what I'd do for that type of
navigation.

otn

Jack Dale wrote:

On Sat, 09 Aug 2003 22:30:17 GMT, otnmbrd
wrote:




PS got my hands on a light list .....no tidal info in there for
particular lights, that I could see.


The height of the light is from MHW on US charts. That is about all
the tidal information in that publication.

If you use the "dipping the horizon" technique for establishing a
circle of position, the height of the tide at the time of the
observation must be taken into consideration.

Jack

I got back and want to answer your note.

otnmbrd wrote:



Chuck Bollinger wrote:

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).


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.

And you are absolutely right. I cannot find anywhere that says that MHHW is
used in areas of semidiurnal tide. And yet, 4 out of 5 quite well educated
mariners swear up and down that they were told, or read, that information, but,
like me, cannot actually come up with the info.

That's great, and I appreciate your head check. Bull**** needs to be stomped,
no matter how well-intentioned.

I can't see why my tide program give me MHHW which isn't really very useful. To
use it risks being off by about a foot or so, in the wrong direction. But Mean
Tide and 1/2 the Mean Range isn't rocket science, so I'll continue in that
direction.

I see the conversation is raging away. Some pretty sharp people involved. I'll
take my discredited ass and go sit on the sidelines. {:-D