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
rmallett wrote:
On 30/04/2014 17:37, Douglas Bateman wrote:
This is a new club consisting, so far, of two members: Frank King and myself.
After the successful British Sundial Society conference, the Sunday morning was
allocated to tours of the Greenwich Observatory. Quite independently, Frank and
I had the intention of location the WGS84 meridian, some 90m east of the
Greenwich brass strip. Frank had an eTrex tracker and an app on his mobile
phone, and I had an Axxera GPS tracker linked to my iPad.
The images, if the system will let them through, show 0º 0' 0". Anyone else
willing to join this new exclusive club? Plenty of places to straddle the line
between the north pole and the south pole.
Doug (and Frank)
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
I would be all in favour of marking the zero meridian for the whole of its length on land, if that's the intention.
Why is the difference 90 metres ? Wikipedia says 200 metres ("102.5 metres east of its last position") at
http://en.wikipedia.org/wiki/Meridian_%28geography%29 - see also
http://en.wikipedia.org/wiki/Prime_meridian
http://wikimapia.org/3323604/Meridian-Building says :-
"Curiously, whether by accident or design, the location of the WGS84 0° meridian is marked in Greenwich by the
presence of a waste basket on the path leading more or less due east from the observatory containing the transit
telescope."
Is it the old thing about GPS receivers being deliberately inaccurate to preserve military secrets, or is it that
dedicated GPS receivers are more accurate than smart phones and tablets ? I know that Harriet James used a hand held
GPS receiver when measuring the location of my house.
--
--
Richard Mallett
Eaton Bray, Dunstable
South Beds. UK
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
