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