[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
>    
(whoops, pressed the wrong button then - sorry for the empty message)
I meant to say: Also see this article: 
http://en.wikipedia.org/wiki/Prime_Meridian
It includes a photo of a GPS on the meridian line at Greenwich, which 
shows that it doesn't read zero.

Basically, where the zero meridian is depends on what reference system 
you are using, and how it is defined.
The Greenwich meridian (as defined by Sir George Airy) is different from 
the WGS84 zero meridian. WGS84 is used by GPS, and OpenStreetMap, and 
most other online maps.

Craig