Ian Wills wrote: > I have noticed a puzzling thing that applies to OSM, Yahoo and Google maps. > > The origin of longitude (zero longitude) should run through the Royal > Observatory at Greenwich, near London. Specifically it should run > through the optical axis of the transit telescope in the Transit > Building at Greenwich. Certainly, the Greenwich observatory people > think so. Not only is the zero meridian marked on the ground but the > gift shop sells zero meridian T shirts, mugs and all the usual kitsch. > > The Observatory's website thinks so too - see > http://www.nmm.ac.uk/places/royal-observatory/meridian-line/ > > My problem is that in OSM, the zero meridian does not run through the > Transit Building where it should. Here is a permalink: > > http://www.openstreetmap.org/?lat=51.47786&lon=-0.001528&zoom=18 > > That is, at a longitude of -0.001528. > > The Transit building is quite clear in the Yahoo image but OSM puts > the zero meridian about 100 metres to the east of the meridian line. > > OSM and Yahoo are not alone in this, Google maps does the same. Try > entering +51° 28' 40.27", +0° 0' 0.01" into Google maps and check the > image. (You will need the fraction of a second). > > The resolution of longitude at the latitude of Greenwich is about 1 > metre so OSM, Yahoo and Google should be able to get closer than 100 > metres. > > - I find it hard to believe that all mapping, or at least mapping in > the UK, is out by 100m. Can anyone explain this discrepancy? > > - Has anyone been to the meridian line at Greenwich with a GPS to > check the position? > > _______________________________________________ > newbies mailing list > [email protected] > http://lists.openstreetmap.org/listinfo/newbies >
http://en.wikipedia.org/wiki/WGS84 should answer your question Cheers Andy _______________________________________________ newbies mailing list [email protected] http://lists.openstreetmap.org/listinfo/newbies
