NavList:

In addition to what Frank has said, don’t forget about ‘datums’ or ‘Why a place can have more than one lat and long.’  It was the subject of first lecture on my MSc course, and I almost gave up and went home right there and then.  The brass(bronze?) line, installed into the ground, which you can clearly see on Google Maps Aerial comes out on Goole Maps as -000.00147. or 0.00147 x cos51 x 6080 = 6yds from where Google Maps puts the 0 E/W meridian.  UK readers needn’t scoff.  Using UK’s Ordnance Survey Online app the line pins at 5”W.

 Google Maps is based upon a modified WGS84 datum.  UK Ordnance Survey probably still use OSGB36 based upon the Airy Spheroid 1830.  I’m not sure which datum the people who marked out the Greenwich line used.  Suffice to say a place can have several lat & longs depending upon the datum used.  I believe there are apps available which will convert lat & longs between datums and grid.  I remember being a little surprised when GPS (in those days) put TIKI, which was floating in Linkoping Marina, Sweden high and dry a couple of streets away on the local hiking map.  DaveP