[OSM-newbies] Greenwich and the origin of longitude

[Andy Robinson]

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?
>
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http://en.wikipedia.org/wiki/WGS84 should answer your question

Cheers

Andy