<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: Arial; font-size: 12pt; color: #000000'><P>Here's a good reference from NGA/NIMA/DMA, the agency responsible for WGS84</P>
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<P><A href="http://earth-info.nga.mil/GandG/publications/tr8350.2/Addendum%20NIMA%20TR8350.2.pdf">http://earth-info.nga.mil/GandG/publications/tr8350.2/Addendum%20NIMA%20TR8350.2.pdf</A></P>
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<P><BR>----- Original Message -----<BR>From: "Richard Greenwood" <richard.greenwood@gmail.com><BR>To: "PROJ.4 and general Projections Discussions" <proj@lists.maptools.org><BR>Sent: Wednesday, December 17, 2008 7:32:43 AM GMT -06:00 US/Canada Central<BR>Subject: Re: [Proj] Google Earth Accuracy<BR><BR>On Wed, Dec 17, 2008 at 3:08 AM, Mikael Rittri<BR><Mikael.Rittri@carmenta.com> wrote:<BR>> Andrew,<BR>> I may have misunderstood you. And maybe the Starfire differential service<BR>> really does give you WGS84 coordinates, rather than some ground-attached<BR>> variant like ETRS89 or NAD83.<BR>><BR>>> The variation between WGS84 and ITRF of the day varies because of<BR>>> plate tectonics. That is WGS 84 framework was set at a time in history,<BR>>> but ITRF if calculated daily. The variable is the plate movements between<BR>>> them.<BR>><BR>> I still think you are wrong here. Let me quote:<BR>> "WGS84 is maintained by the US. Like the ITRS it is a global system.<BR>> Initially it was coincident with ITRS to an accuracy of about 1.5 meters.<BR>> It is currently maintained within 10cm of the ITRS."<BR>> http://www.ogp.org.uk/pubs/373-10.pdf (section 2)<BR>> If the differences were caused by tectonic plate movements, then it<BR>> would be quite difficult to maintain the two systems to within 10cm.<BR>> You would need to connect all continents by very strong steel chains, to<BR>> prevent them drifting apart more than 10cm from their original position ;-)<BR>><BR>> No, if the difference between WGS84 and ITRF varies from day to day, I<BR>> think that such differences are caused by something like athmospheric<BR>> conditions, not tectonic plate movements. (Radio waves slow down in the<BR>> athmosphere, by an amount depending on air pressure and humidity, I think.)<BR>><BR>>> I'm not expert all I know is that we have to write some pretty special<BR>>> code to account for this variation so that when we use this GPS for<BR>>> locating with WGS84 coordinates we can find them.<BR>><BR>> All right. If your special code is something like a time-dependent<BR>> 7-parameter datum transform (like those given in section 6.5 of<BR>> http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html ),<BR>> then you are indeed correcting for tectonic plate movements. But I would<BR>> say that if so, then it is not the difference between WGS84 and ITRS<BR>> that you are correcting for. Rather, your original coordinates must<BR>> be expressed in something like ETRS89 or NAD83, not WGS84 in its<BR>> strict sense.<BR>> On the other hand, if your special code is something else,<BR>> then I don't know what it does...<BR>><BR>> Disclaimer: I am a GIS programmer, not a surveyor.<BR>> Maybe some surveyor on this list can give a better answer.<BR>><BR>> Best regards,<BR>> --<BR>> Mikael Rittri<BR><BR>I am a surveyor, but not a geodesic. My understanding is the same as<BR>Mikael's: WGS84 and ITRF are very nearly the same. I don't know why<BR>they are not identical, Mikael says<BR> "Radio waves slow down in the athmosphere, by an amount depending<BR> on air pressure and humidity, I think."<BR>It is generally electromagnetic disturbances much higher in our<BR>atmosphere that distort GPS radio waves. The disturbances are in the<BR>ionosphere and are effected by sun spot activity and solar flares,<BR>among other things. Maybe this is the reason for the difference<BR>between WGS84 and ITRF, but I am not sure.<BR><BR>WGS84 and ITRF are not fixed to a plate, they are geocentric and<BR>fixed to the center of the earth. NAD83 is fixed to the North American<BR>plate. Many software, including Proj, do not distinguish between NAD83<BR>and WGS84 so it is a common mistake to refer to the two datums<BR>interchangeably. And since they did start out as matching, it did not<BR>used to matter much. Right now there is over a meter of difference<BR>between WGS84 and NAD83, so it is becoming more important for all of<BR>us to understand and respect the difference between geocentric datums<BR>(ITRF and WGS84) and plate-based datums.<BR><BR>-- <BR>Richard Greenwood<BR>richard.greenwood@gmail.com<BR>www.greenwoodmap.com<BR>_______________________________________________<BR>Proj mailing list<BR>Proj@lists.maptools.org<BR>http://lists.maptools.org/mailman/listinfo/proj<BR></P></div></body></html>