Many thanks to all who, as typical of the list, sent relevant and interesting posts. I would like to comment on some as follows:
IM: the confluence website - crazy but yet more fun. RM: the old civil 'degradation' of accuracy of GPS was removed many years ago. Dedicated GPS receivers are more accurate than smart phones or tablets who use the identity of their routers to give a location, although better than just a Postcode. See more below. JT: Google Earth matches the precision in that I found the WGS84 zero line to be exactly where Frank and I were making our observations. The Google Earth cursor shows incredible precision - down to hundredths of a second of arc. Centimetre accuracy? PP: chasing after ephemera (the lat/long lines are moving over long periods, even a year). Never mind, we simply move our GPS receivers to suit! FE: the 'confluence dial' in Austria. I remember it as being rather massive and to have moved it would have strained our party... TT: GPS averaging apps. Following this I bought two, more below. KK: HERESY because the WGS84 meridian has nothing to do with the sun. The satellites have large solar panels - just think of WGS84 as being solar powered! Spin-off: the dials on the zero meridian UK, France and Spain. RM: precision with free apps. All very good, and I tried an experiment with my Axxera GPS receiver and two averaging apps (GPS Averaging and Perfect Mark) on my iPad. The Axxera quotes a CEP of ±2.5 metres, but the experiment was to see if I could determine a change in longitude by placing a 1 metre rule aligned with the latitude in a open area on my lawn. As it happens, for my latitude (51º 22' N) 1 metre corresponds to 0.001 minute of arc in longitude. The systems update every second so it was no trouble to average 100 samples for 3 sets of readings for each averaging app. For the GPS Averaging I got 0º 48.1357' and 0º 48.1367' at either end of the rule, and for the Perfect Mark 0º 48.1362' and 0º 48.1377. In other words definitely resolving a metre on the ground. I expect that repeating for another 600 readings half an hour later (all different satellites and bearings) I may have obtained higher precision still by combining both sets of data. Amazing. Thanks again for the fascinating insights. Regards, Doug PS An image of Frank King and BSS member Ian Butson recording the event of 0º 0' 0". Note the Greenwich landmarks in the background. The Greenwich meridian passes to the left of the power station chimneys. > 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)
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