I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.

Paul L

"Steve" wrote in message
...
Ok, now that I have had time to compare NOAA charts with the reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW (mean
high water), as stated in the title block of each NOAA chart.

It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.

But, that still leaves me with the question: How do you calculate or
adjust
the stated Vertical Clearance when all you have is a chart and a tide
table.

In my example; The charts show a vertical clearance of Hood Canal Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the time
I
want to pass under it. My mast height is 54' 7", including the VHF
antenna...

Do I need the NOAA tide tables or can this be calculated from the usual,
locally reproduced, convience tables??

I realize, after the fact, that there would have been sufficient clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.

Steve
s/v Good Intentions

I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.

Paul L

"Steve" wrote in message
...
Ok, now that I have had time to compare NOAA charts with the reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW (mean
high water), as stated in the title block of each NOAA chart.

It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.

But, that still leaves me with the question: How do you calculate or
adjust
the stated Vertical Clearance when all you have is a chart and a tide
table.

In my example; The charts show a vertical clearance of Hood Canal Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the time
I
want to pass under it. My mast height is 54' 7", including the VHF
antenna...

Do I need the NOAA tide tables or can this be calculated from the usual,
locally reproduced, convience tables??

I realize, after the fact, that there would have been sufficient clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.

Steve
s/v Good Intentions

I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.

Paul L

"Steve" wrote in message
...
Ok, now that I have had time to compare NOAA charts with the reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW (mean
high water), as stated in the title block of each NOAA chart.

It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.

But, that still leaves me with the question: How do you calculate or
adjust
the stated Vertical Clearance when all you have is a chart and a tide
table.

In my example; The charts show a vertical clearance of Hood Canal Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the time
I
want to pass under it. My mast height is 54' 7", including the VHF
antenna...

Do I need the NOAA tide tables or can this be calculated from the usual,
locally reproduced, convience tables??

I realize, after the fact, that there would have been sufficient clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.

Steve
s/v Good Intentions

Coast Pilot 7, says MHW, unless otherwise stated.
You need to check your chart or Particular Coast Pilot, etc., if you
have any doubts ..... For instance, I believe the Mississippi river,
above a point will use "river stage".
If in doubt, G use the one that gives you the least clearance.

otn

Paul L wrote:
I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not MHW.

Paul L

"Steve" wrote in message
...

Ok, now that I have had time to compare NOAA charts with the reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW (mean
high water), as stated in the title block of each NOAA chart.

It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.

But, that still leaves me with the question: How do you calculate or

adjust

the stated Vertical Clearance when all you have is a chart and a tide
table.

In my example; The charts show a vertical clearance of Hood Canal Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the time

I

want to pass under it. My mast height is 54' 7", including the VHF
antenna...

Do I need the NOAA tide tables or can this be calculated from the usual,
locally reproduced, convience tables??

I realize, after the fact, that there would have been sufficient clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.

Steve
s/v Good Intentions

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

Yea, you are right. I always 'assumed' it was MHHW, but the Pilot clearly
say MHW. It seems like MHHW would make more sense out here.
(sorry for the multiple posts earlier - Outrage acting up).

Paul
"otnmbrd" wrote in message
.net...
Coast Pilot 7, says MHW, unless otherwise stated.
You need to check your chart or Particular Coast Pilot, etc., if you
have any doubts ..... For instance, I believe the Mississippi river,
above a point will use "river stage".
If in doubt, G use the one that gives you the least clearance.

otn

Paul L wrote:
I think it's mean MHHW (Mean Higher High Water) on the Wesy coast, not
MHW.

Paul L

"Steve" wrote in message
...

Ok, now that I have had time to compare NOAA charts with the
reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW
(mean
high water), as stated in the title block of each NOAA chart.

It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.

But, that still leaves me with the question: How do you calculate or

adjust

the stated Vertical Clearance when all you have is a chart and a tide
table.

In my example; The charts show a vertical clearance of Hood Canal
Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the
time

I

want to pass under it. My mast height is 54' 7", including the VHF
antenna...

Do I need the NOAA tide tables or can this be calculated from the
usual,
locally reproduced, convience tables??

I realize, after the fact, that there would have been sufficient
clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.

Steve
s/v Good Intentions

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