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

Steve wrote:
Were is vertical clearance measured from (what datum)?

Should be Mean High Water.

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

Could have sworn it was in Chapman's, but my copy is on my bedside table
and my wife is still asleep.

Chart 1 (http://www.carolinaglobalmaps.com/pr...cal/chart1.htm)
says: the shoreline is "usually the mean high water line" (p 4) and
heights of landmarks and structures are "referred to the shoreline plane
of reference" (p8).

regards,

dave
M/V Auspicious
"a sailor fallen from grace"

Steve wrote:
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...

You have to know MHW. If it isn't on your tide table, I'd ask the bridge
tender. If he doesn't know, I'd ask him to open up.

Steve,

Does this bridge not have a water height scale? I thought all bridges
on navigable water had them.

Ron

The answers to all your questions are somewhere in:

http://pollux.nss.nima.mil/NAV_PUBS/APN/Chapt-09.pdf

Yes, you do have to be careful. Astronomical predictions often show tides a foot or two
high than MHW in some locations. Also, a storm or even a persistent wind can do the same.
And both can occur at the same time.

And, although most bridges are actually a few inches higher than listed, some are actually
lower, or have gear hanging down.


"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

x-no-archive:yes
(Ron Thornton) wrote:

Steve,

Does this bridge not have a water height scale? I thought all bridges
on navigable water had them.

I think this is an ICW thing. I know the Chesapeake Bay Bridge does
not have any such scale. And even some of the ICW bridges don't have
them.

And on the bridges which have them, they may
a) not be accurate
b) be so covered with slime and stuff as to be unreadable
c) have shifted so they are no longer helpful (like I have seen some
of them have slipped or been knocked sideways)


grandma Rosalie

Vertical clearance is usually measured above high water. The only
significant exception is small scale charts where the datum may be
Mean Sea Level. Such charts rarely show bridge clearances, so the
difference isn't an issue.

The exact datum varies from country to country. On US domestic charts,
it's always Mean High Water (the average of all high tides). On
foreign charts, including US charts made from foreign data, it's
usually a higher datum, Mean Higher High Water (the average of the
higher of the two high tides each day), Mean High Water Springs (the
average of all spring tides), Highest Astronomical Tide (the highest
the water ever gets without help from a storm) or maybe something
else.

May I suggest that you buy a copy of Bowditch (The American Practical
Navigator -- NIMA Pub 9) which will answer this question and every
other question you ever had about navigation? Bowditch just had its
200th anniversary of being continuously in print, through about 75
editions. It's the standard source of basic information on all kinds
of navigation. There's no need to buy it new -- any edition from the
last forty years will do fine. I might add, that as a serious
navigator, I own two copies of Bowditch (1962 and 1977) and don't own
Chapman.


Jim Woodward
www.mvfintry.com

"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

The link that I posted before was to the tide chapter in Bowditch. The entire contents
are online at:

http://pollux.nss.nima.mil/pubs/pubs...s.html?rid=187

There are two versions of the latest Bowditch available in hard copy - the one from the
government includes a CD which has a wrapper program to do some of the navigation for you.
It also has a calculator and a few other features. I don't know if the "commercial"
version - roughly the same price, also has that.

Old editions of Bowditch are fun - I have several, including a first and one from 1870 - I
like to see what the practices and terminology was back then.

"Jim Woodward" wrote in message
om...
Vertical clearance is usually measured above high water. The only
significant exception is small scale charts where the datum may be
Mean Sea Level. Such charts rarely show bridge clearances, so the
difference isn't an issue.

The exact datum varies from country to country. On US domestic charts,
it's always Mean High Water (the average of all high tides). On
foreign charts, including US charts made from foreign data, it's
usually a higher datum, Mean Higher High Water (the average of the
higher of the two high tides each day), Mean High Water Springs (the
average of all spring tides), Highest Astronomical Tide (the highest
the water ever gets without help from a storm) or maybe something
else.

May I suggest that you buy a copy of Bowditch (The American Practical
Navigator -- NIMA Pub 9) which will answer this question and every
other question you ever had about navigation? Bowditch just had its
200th anniversary of being continuously in print, through about 75
editions. It's the standard source of basic information on all kinds
of navigation. There's no need to buy it new -- any edition from the
last forty years will do fine. I might add, that as a serious
navigator, I own two copies of Bowditch (1962 and 1977) and don't own
Chapman.


Jim Woodward
www.mvfintry.com

"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

I did ask the bridge operator, but the answer, sounded more like a guess
(seat of the pants) since he is about a mile from the fixed span I was
interested in passing under.

There is a clearance scale on the fixed span but according to my cruising
guide, it has some error (on the conservative side), several feet less than
the vert. clearance on the charts.

Even if I could trust these scale markers, I wouldn't hate to get in close
enough to read them only to find there wasn't enough clearance, with several
knot of current running.. Bad situation for a sail boat, trying to come
about with a deep draft.

Many of these scales that I have seen in New England, were badly faded or
covered with marine growth/bird droppings, etc.

I'm going to search out a Gov. tide table and see what the charted datum is
for both of these bridges.

Thanks again for the good response on this question. A good exercise.

40+ years of boating but not to proud to ask the dumb questions.

Steve
s/v Good Intentions

You need either a "Tide Table" or tide program, so you can figure out
the clearance based on the stage of the tide at the time you will pass
under.
The tables or a program will give you the mean tide ranges (or in the
case of a tide program it will frequently tell you MHW) from which you
can calculate YOUR height above or below MHW for the particular time or
stage of the tide, in question.
If the bridge clearance is 55', and you find that MHW for a particular
bridge ( or nearest datum point)is 5' and you will be passing under the
bridge at a time when the tide is at +2', you (technically EG) should
have 58' of clearance ..... or if you are in some extreme tides and the
tide at the particular time will be +8', you will only have 52' of
clearance.
Always approach these numbers with caution and check any local "tide
gauge" on a bridge pier when available.

Steve wrote:

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