Hi Steve,

Bridge heights are measured from Mean High Water - although I can't
remember where I saw the reference...

Bill

"Steve" wrote in message ...
Were is vertical clearance measured from (what datum)?

I can't seem to find this information in any of my reference books.

What I find in Chapman's, in a section on chart "Plane of Reference", states
"Different planes are on different charts of various boating areas. For
charts along the Atlantic coast the National Ocean Survey uses mean low
water as the datum for soundings. On the Pacific coast it is the mean lower
low water that is used for the reference plane....."

No mention of vertical clearance.

Here in the Pacific NW where tide difference are signicant the chart datum
is not metioned in my Maptech Reg. 15 portfolio (or I just can't find it).

Example:

Today the vertical clearance under two different bridges was just too close
to take a chance on and it was a +8 ft tide.

I ended up playing it safe and had the draw span opened on the Hood Canal
floating bridge and went out of my way to avoid going through the the Port
Townsend Canal with an overhead bridge span.

In each case the "stated" Vertical Clearance was enough for my 54 ft 7 inch
requirement, but I had no idea what tidal state their datum was based on.
(Hood Canal Bridge, east span was 55ft vert. clearance while P.T. Canal
bridge was 58ft.)

Sure hope the 10-20 min traffic delay for some 200 cars and trucks wasn't
due to my cautions and lack of knowledge on this matter..

Please enlighten me.

Steve
s/v Good Intentions

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).

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.

The vertical clearance of the east span of Hood Canal Bridge is 55 ft.
Yesterday the highest tide was 8.8 feet (at 1500). The Mean Tide for Port
Gamble is listed as: 6.10 ft. The Mean Range is listed as: 6.70. My
computation results in a figure of 9.45 feet for the MHHW.

Therefore, at the high tide (daylight hours) there was 55.6 feet under the east
span high rise. Of course, prudence leaves 2' of wiggle room to make up for
unplanned things like wind effect and such, or some dot.com yahoo with more boat
than brains zipping through at 55. But if your boat height over water is 53' or
less, well, just don't go on rec.auto.vacationing and mention it. ;=)

This information comes from Chapman, but also from U.S. Power Squadron courses
in Piloting and Advanced Piloting. Not a bad investment in time and a few bucks.

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).

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.

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?
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.

BTW, I think the program you are using gives MHHW


otn

Steve wrote:

Were is vertical clearance measured from (what datum)?

In the European canal system it is measured from PHEN, which is a French
acronym for "the highest navigable waters"(les plus hautes eaux
navigables). When the water gets higher than that they cancel all
navigation.

So a posted vertical height is always the worst case.

Cheers, Andy

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).


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

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)
__________________________________________________