[time-nuts] IEEE Spectrum - Dec 2017 - article on chip-scale atomic frequency reference

jimlux jimlux at earthlink.net
Sat Dec 9 19:39:31 UTC 2017


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









> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> 




More information about the Time-nuts_lists.febo.com mailing list