[OSM-talk] military vs consumer GPS and the equator

[Steve Doerr]

On 25/01/2011 22:02, Joe Richards wrote:

> The problem is my consumer GPSes (a Garmin GPSMap 60Csx and an HTC Magic
> running Android) thought that the equator was about 30-40m away from
> where a 'military GPS' had supposedly measured it and where these
> equatorial tricks were being performed.

There's a similar anomaly at the Greenwich Meridian in Greenwich, Kent, 
UK (or London, if you prefer). By international agreement, zero 
longitude is supposed to be the meridian passing through the center of 
the Airy transit circle (a telescope) at Greenwich, and a brass strip 
marks this line in the courtyard immediately adjacent to the transit 
instrument, yet a GPS receiver, set to display WGS84 co-ordinates, will 
not show this position as zero longitude. 
<http://en.wikipedia.org/wiki/Prime_Meridian#IERS_Reference_Meridian>

Whether a similar error regarding the Equator has crept into WGS84, I'm 
not sure. Lines of latitude are not arbitrary in the way lines of 
longitude are.

-- 
Steve