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*** Presented by Martin Wass at State of the Map 2013
*** For the video of this presentation please see http://lanyrd.com/2013/sotm/scpktb/
*** Full schedule available at http://wiki.openstreetmap.org/wiki/State_Of_The_Map_2013
The satellites in Global Navigation Satellite Systems get their position data regularly updated from ground stations. But how do ground stations 'know' where they are, and relative to what? The Airy transit circle at Greenwich once defined the Prime Meridian and the spinning Earth the Equator. We now know the tectonic plate Greenwich sits on is moving and the Earth wobbles... Any defined datum causes difficulties when moving away from the vicinity, say to Mars. Using several different datums raises other problems. When everything is sliding around, how do we define and use a co-ordinate system that works?