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Description of the photograph
Title: Standing on prime meridian in Greenwich
Taken from:

At the Royal Greenwich Observatory, London, England.

Description:

British astronomers have long used the Royal Observatory as a basis for measurement: four separate meridians have been drawn through the building. The basis of longitude, the Prime Meridian, established in 1851 and adopted at an international conference in 1884, passes through the Airy transit circle of the observatory. It was long marked by a brass strip in the courtyard, now upgraded to stainless steel, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky.

This old astronomical prime meridian has been replaced by a more modern prime meridian. When Greenwich was an active observatory, geographical coordinates were referred to a local oblate spheroid called a datum, whose surface closely matched local mean sea level, called the geoid. Several datums were in use around the world, all using different spheroids, because mean sea level undulates by as much as 100 metres worldwide. Modern geodetic reference systems, such as the World Geodetic System and the International Terrestrial Reference Frame, use a single Earth-centered oblate spheroid. The shift from several spheroids to one worldwide spheroid caused all geographical coordinates to shift by many metres, sometimes as much as several hundred metres. The Prime Meridian of these modern reference systems is 102.5 metres east of the Greenwich astronomical meridian represented by the stainless steel strip. Thus the strip is now 5.31 arcseconds West.

Keywords: wall, brick, house
Image data
Image sizes: 1044*1500px, 638kb
Origin: Originally a digital image, with postprocessing
Date/Time: 19970508-170500
Camera:
Position coordinates:WGS84 LONG -0.00150471925735°, LAT 51.4779450909°
Destination coordinates;WGS84 LONG -0.00150203704834°, LAT 51.4778682468°
Air-line distance:
Bearing:
Author: Alexandra Medwedeff
Photo copyright: This photograph is copyrighted (©) by André M. Winter and others. A free permissions for re-use may be given for non-commercial purposes. Commercial use requires a license. Contact André M. Winter for any kind of use. This extended copyright notice applies in all cases. Infringements always will be persecuted worldwide. Legal court: Innsbruck, Austria, E.U.
Text copyright: The description of this photograph (or parts of) is based on this article of the free Wikipedia encyclopedia and are covered by the GNU Free Documentation License (GFDL). Authors are named on the Wikipedia page.
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