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Hi John & All, Mar 9, 2007
Several of your comments are not clear to me.
For example, is the subsidence at Truro of 30 cm over a period of 50
years by any chance ? Or over some different period ?
Is the mean sea level for mapping purposes the mean of all tide
turning points (high and low) over the 19 year period, a mean of high
tides only or something else ?
What reference point would be used to establish that mean sea level
in Maine is 38 mm higher than in Florida, departure from the surface of
some idealized oblate spheroid or something else ? Also I am curious
about measurement techniques. I suppose if a large number of
measurements are taken by satellite then confidence limits can be made
small but, I would think, that one could get a wide range of results
depending upon methods used to eliminate other factors. For example, the
diurnal range in atmospheric pressure (Florida; July) is about 1.6
millibars or roughly 16 mm of water. Average pressure systems in
Janurary range over about 30 millibars (300 mm water) and pressure over
time can vary by 80 millibars (800 mm water). Seawater will tend to
reach hydrostatic equilibrium with atmospheric pressure, so with all
this sloshing around due to atmospheric pressure alone, a difference of
38 mm looks quite small.
According to isobar maps in my old Meterology book, atmospheric
pressure in Maine is about 3 millibars lower than in Florida for the
months of Janurary and July. This pressure difference would cause mean
sea level to be about 30 mm higher in Maine during these months so I
wonder if the observed 38 mm difference is due entirely to differences
in atmospheric pressure.
And (I don't know offhand by how much) even those rock-solid
benchmarks in Maine also rise and fall with tidal effects. Does some of
the the 38 mm difference between Maine and Florida then tell us that
Florida sand is more responsive to tidal forces than Maine bedrock ?
Also I wonder if the 10 cm rise of sea level per century is a long
term average as opposed to a current value. The schematic curve for
eustatic sea level rise in Grant (1975 Fig. 9) shows a 5 metre rise in
5,000 years (i.e.10 cm /100 years), but the curve is asymptotic, with a
current slope much less than 1 mm/ year.
Yours truly, Dave Webster, Kentville
john belbin wrote:
>
>Geodetic levelling has shown that the Truro region has subsided
>about 30 Centimetres and the rest of the region by about 20 centimetres.
>
>The notion of sea level is a locally determined convenient point from which
>to measure other things, such as depths, heights, relative elevations and
>the form of the terrain. It is found by measuring the rise and fall of the
>ocean at tide gauge stations over a period of 19 years and averaging the
>fluctuations recorded. It is an average figure, there is nothing absolute
>about it and it will vary from place to place with the shape of the Earth.
>It is constantly fluctuating and considerably varied. Sea level in Maine is
>38 mm higher than in Florida. If you want to use a GPS for elevations you
>have to take and adjust for local variations otherwise your figures will not
>bear much resemblance to reality. When our maps say things like "elevations
>are based on mean sea level" we are not all using the same standard, merely
>a convenient approximation. Sea level has now been shown to contain hills
>and valleys, smaller but similar to those found on land, due to
>gravitational variations and other factors. Some advanced radar satellites
>now use the "bumpy" surface of the sea to tell us the shape of the ocean
>bottom hidden far below in a manner we have never seen before.
>
>Hopefully this is a little clearer than Fundy mud!
>
>
>
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