[OSM-newbies] Greenwich and the origin of longitude

[Ian Wills]

Thank you. I am enlightened.

I notice that even the United Kingdom Ordnance Survey Zero Meridian is
not at the Greenwich prime Meridian
(http://en.wikipedia.org/wiki/Prime_Meridian).

There is a worthwhile history of the subject by Galison (Galison,
Peter Louis. 2003. Einstein's clocks and Poincaré's maps: Empires of
time. New York: W.W. Norton.) which details the battle between the
French and English about the location of the prime meridian (the
French wanted it through Paris).

It seems like the French got he last (if hollow) laugh since the WGS
84 meridian was defined by the defined by the Bureau International de
l'Heure.


2009/3/20 Craig Wallace <craigw84 at fastmail.fm>:
> On 19/03/2009 22:33, Andy Robinson wrote:
>> 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?
>>>
>>>
>
>> http://en.wikipedia.org/wiki/WGS84 should answer your question
>>
>> Cheers
>>
>> Andy
>>
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