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