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

[john at jfeldredge.com]

My understanding (which may not be correct) is that civilian GPS units are supposedly now as accurate as the military units in terms of latitude and longitude, but are deliberately much less accurate at altitude readings.

-------Original Email-------
Subject :[OSM-talk] military vs consumer GPS and the equator
From  :mailto:geojoelists at gmail.com
Date  :Tue Jan 25 16:02:29 America/Chicago 2011


Not too long ago I was in Ecuador at the "Mitad del Mundo" and noticed a fairly significant discrepancy between my own GPS and an official marker.  The Mitad del Mundo is a monument setup to mark the equator, after which Ecuador is named.  Obviously the equator is a line, but this is a single monument at an arbitrary longitude, not far from the capital city all the same - don't ask me why.
 

The monument is erected where they thought the equator was, before being able to measure this accurately.  A few hundred metres away is a museum where the 'actual' equator is, supposedly measured with a 'military GPS' for extra accuracy.  There are tricks there, such as egg-balancing on watching the water go down the sink in different directions - supposedly induced by the coriolis effect. 


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.  When I walked to where they thought the equator was, it run through the middle of a nearby road and car park. 


Had they just placed the museum in a more convenient place than the middle of the nearby road (which couldn't be moved)?  Or is this sort of discrepancy known and accepted?  Didn't Clinton turn the encryption off some of the accuracy bits of the GPS signal at some stage (making military vs consumer less important)? _______________________________________________
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-- 
John F. Eldredge -- john at jfeldredge.com
"Reserve your right to think, for even to think wrongly
is better than not to think at all." -- Hypatia of Alexandria