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PostPosted: Wed Feb 08, 2006 7:23 pm 
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Joined: Sat Jul 24, 2004 6:06 pm
Posts: 920
Location: Walmer, Kent
Here's a fun one. The Register website notes that Google Earth shows the Prime Meridian running about 100 metres to the East of its correct position.
A page at the Royal Observatory's website suggests this is not Google's fault but a general problem with the GPS system because the Earth is not a perfect sphere.


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PostPosted: Wed Feb 08, 2006 9:38 pm 

Joined: Sat Apr 30, 2005 3:10 pm
Posts: 336
Location: London/Kendal (51.5157N,-0.1215E)
Yes, the wonderful 'Geod' problem. What shape is the Earth, a 'geod' ie an Earth shape. This creates havoc across all kinds of comuting needs. The International Geomagnetic Reference Field, used to calculate various properties of the Earth's magnetic field at various altitudes around the globe has one big committee to decide on the parameters of the field and another to decide on whether their current Earth shape is Earth-shaped enough, given modern needs and data availability. Thinks its just a very slightly deformed sphere...

Perhaps google could borrow their spheroid?

Given all the relatavistic corrections included in the GPS system, I'd expect it'd be relatively easy to put the spheroid in?


They do use the same spheroid. Worrying for my magnetic field calculations...

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PostPosted: Wed Apr 19, 2006 8:45 pm 

Joined: Fri Dec 10, 2004 10:34 pm
Posts: 373
Location: Surrey
I have been told there is another problem with the Prime Meridian at Greenwich that they only discovered when they surveyed the location of Herstmonceux relative to Greenwich. I believe that the survey had an offset of 10's of yards which they could not account for. I believe they eventually identified that the meridian used by the Ordnance Survey is offset from the meridian as shown on the ground in Greenwich. I believe the OS meridian runs through one of the older transit telescopes.

Can anyone confirm or deny this ?

Engineer @ Work - Astronomer @ Play

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