On 30/04/2014 19:33, Brooke Clarke wrote:
Hi Richard:
Prior to GPS each country had their own datum, but if you want GPS to
truly be Global then you need a common datum. There were other datums
prior to the World Geodetic System of 1984 (WGS84,
http://en.wikipedia.org/wiki/WGS84#Longitudes_on_WGS_84) but it's the
one that's the default in most GPS receivers. In military GPS
receivers there's a choice of dozens of datums. The location of zero
degrees is based on the Earth's rotation as measured by star
observations that are more accurate now that we use atomic time rather
than the Earth's rotation as a clock.
http://www.prc68.com/I/DAGR.shtml
Because of plate tectonics, i.e. most of the ground is moving all the
time in three dimensions, including bed rock on mountains where
telescopes have their foundations, there's really no such thing as a
fixed location on Earth. When they do laser ranging to the moon one
of the corrections is the elevation of the telescope at the instant
when the laser is fired and at the instant when the return pulse is
received. Called Earth Tides, and caused by the same forces that
cause ocean tides. It's measured by a very sensitive gravity meter.
http://en.wikipedia.org/wiki/Earth_tide
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
Many thanks for that - and I thought corrections for sundials were bad
enough :-)
--
--
Richard Mallett
Eaton Bray, Dunstable
South Beds. UK
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