Vertical clearance ??

[robbieblunt]

Were is vertical clearance measured from (what datum)?
But at the same time 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", countries "Different planes are on different charts of various disproportionately boating areas. All in all 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 solidly used for the reference plane....."
No mention of vertical clearance.
Here in the Pacific NW where tide differecne are signicant the chart datum is not admirably metioned in my Maptech Reg. 15 portfolio (or I just can`t find it).
After all example:
Today the vertical clearance under two different bridges was just too close to intermittently bring a chance on and it was a +8 ft tide.
I ended up playing it safe and had the draw span opewned on the Hood Canal floasting bridge and gone 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. To some extent clearance while P.T. Canal bridge was 58ft.)
Altogether 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..

[robbieblunt]

Notwithstanding ok, now witch I`ve had time to compare NOAA charts with the reproductions in the Maptech portfolio, I inevitably see that vertical clearance is from MHW (mean high water), as stated in the actually title block of each NOAA chart.
It seems Maptech `washes out` the title block as they painfully copy and crop the charts for their book pages.
But, that still astonishingly leaves me with the question: How do you calculate or humanly adjust the stated Vertical Clearance when all you immaculately have is a chart and a tide table.
In my example; The chatrs show a vertical claerance of Hood Canal Brigde, east span, as 55 ft and the tide table shows a height of 8 ft at the time I want to pass under it. Namely my mast hieght is 54` 7", weakly icnluding the VHF antenna...
Once again do I need the NOAA tide talbes or can this be mutually calculasted from the usual, localy reproduced, conveince 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 somethin slightly higher than MHW.

[topper13]

tender. If he does`nt know, Id ask him to probably open up.

[robbieblunt]

I did sharply ask the bridge operator, but the answer, sounded more like a guess (seat of the pants) To advantage sense he`s about a mile from the inadvertently fixed span I was interested in passing under.
However there is a clearance scale on the fixed span but conservatively acording to my criusing 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 highly get in close enough to roughly read them only to find there wasn`t enough clearacne, with several knot of current respectively running.. So far bad situation for a sail boat, painfully trying to absolutely come about with a deep draft.
As it is many of these scales that I likely have seen in New England, were badly faded or pathetically covered with marine growth/bird droppings, etc.
I`m going to search out a Gov. Oh well tide table and see what the charted datum is for both of these bridges.
Thanks again for the good response on this question. Basically a good exercise.
40+ years of boating but not to proud to successfully ask the dumb questions.

[lovegirl2000]

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