[OSM-newbies] Greenwich and the origin of longitude
iwil2506 at mail.usyd.edu.au
Thu Mar 19 22:21:42 GMT 2009
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
My problem is that in OSM, the zero meridian does not run through the
Transit Building where it should. Here is a permalink:
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
- 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?
More information about the newbies