[OSM-newbies] Greenwich and the origin of longitude

[Craig Wallace]

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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