On 12/9/17 11:14 AM, Mark Sims wrote:
In the standards definitions that include "at sea level", the question these days is "which
sea level?". As ocean temperature changes sea level will change (except maybe in Washington DC). Will
the standards be amended to include something like "at sea level in 1990" or will the value being
defined drift around with the changing sea level?
Sea Level is arbitrary anyway - what is usually meant is "zero elevation
relative to some specified geoid".
The Pacific and Atlantic oceans have different mean heights relative
to the geoid
In the United States, for a long time it was the North American Datum
(NAD) that was the reference, but now, it's probably WGS84 (I'm too lazy
to go look it up).
WGS84 has a very precise definition in terms of the semiaxes lengths,
and their orientation relative to stellar references. WGS 84 uses the
IERS reference meridian for longitude.
Flattening of 1/298.257,223,563
equatorial radius of 6,378,137 m
so polar radius of 6,356,752.3142 m
The Earth Grav Model (EGM96) defines the geoid, last revised in 2004.
*that* model defines the nominal sea surface with spherical harmonics.
There's something like 100,000 specific terms in that gravity model.
Sourceforge has a program that will tell you geoid height for a given
lat lon.
https://geographiclib.sourceforge.io/cgi-bin/GeoidEval
Near my house (34N, 119W), it appears that the EGM96 height is -37.17 m,
relative to the ellipsoid defined above.
It so happens that due to crustal movement, my house is gradually rising
about 1 cm/year, but I don't know if the local sea level also rises to
match, or if the beach is getting farther away.
One can measure this, in theory
https://www.unavco.org/education/resources/modules-and-activities/gps-california-plate-motion/gps-california-plate-motion.html
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