When GPS was first developed (Late 70's) the DATUM used was the World Geodetic System, 1972 (WGS-72). The next release was indeed in 1984 (WGS-84). The early GPS receivers had over 200 datums stored in permanent memory. In its most basic form a datum defines the center of the earth and the equation of the earths ellipsoid.
>From Wikipedia: A *geodetic datum* (plural *datums*, not *data*) is a reference from which measurements are made. In surveying <http://en.wikipedia.org/wiki/Surveying>and geodesy <http://en.wikipedia.org/wiki/Geodesy>, a *datum* is a set of reference points on the Earth's surface against which position measurements are made, and (often) an associated model of the shape of the earth (reference ellipsoid <http://en.wikipedia.org/wiki/Reference_ellipsoid>) to define a geographic coordinate system<http://en.wikipedia.org/wiki/Geographic_coordinate_system> . The user, mostly military or marine, would choose the datum he/she wanted to use. This would match up the local map with the GPS derived position. The difference could be quite large (hundreds of feet), especially important in the Z (vertical) direction! Michael / K7HIL On Tue, Sep 13, 2011 at 2:23 PM, jmfranke <[email protected]> wrote: > I believe that should read WGS 84 not WPS84. > > John WA4WDL > > ------------------------------**-------------------- > From: "Chris Albertson" <[email protected]> > Sent: Tuesday, September 13, 2011 5:03 PM > To: "Discussion of precise time and frequency measurement" < > [email protected]> > Subject: Re: [time-nuts] Averaging Location for Position Hold > > 2011/9/13 Miguel Gonçalves <[email protected]>: >> >>> Hi all! >>> >>> I am installing a timing GPS unit over a new location where I already >>> have a >>> NMEA GPS with PPS (let's call it unit A). The NMEA GPS is logging every >>> 16 >>> seconds its GPGGA string. >>> >> >> The Oncore UT+ can does it's own site survey automatically. That's >> the best way. >> >> You can only compare the GPS location with Google if both are using >> the same "system". The most common one today is WPS84 but you need to >> check. >> >> The problem is that the Earth is not a Sphere and different systems >> assume non-sphere shapes. Getting this wrongs gives about the size >> error you observed, more or less. >> >> Also, can you really trust Google Earth as an authoritative source? >> I'm not sure. An interesting test would be to go find a USGS >> benchmark or a section marker near you then enter it's location into >> Google. See if Google hits the marker. >> -- >> >> Chris Albertson >> Redondo Beach, California >> >> ______________________________**_________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/** >> mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> >> and follow the instructions there. >> >> > > ______________________________**_________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
