Why does GPS show the Prime Meridian in the wrong place?
Date: Thursday, August 20 @ 06:49:19 UTC
Sticking with the subject of maps and datums, a frequently asked question is why GPS does not show the Prime Meridian at Greenwich as zero? This issue also occurs in Google Maps, with both claiming the Meridian is about 100m to the east. But why?
The differences occur because when the longitude was first calculated, the Earth was assumed to be perfectly round whereas it's actually an oblate spheroid, which means it's fatter at the equator.
The error that resulted from this went unnoticed until the late 1960's when the first satellites were launched. A receiver was mounted on the roof of the Greenwich Observatory and calculations showed that an offset of 5.54" existed thanks to the more accurate calculations that were possible. This offset has since been incorporated into the WGS84 datum.
It's largely elementary now unless you're a tourist in Greenwhich park, or a guide at the observatory!
Source: Royal Observatory
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