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