[Proj] Google Earth Accuracy

[Mikael Rittri]

Andrew Williams wrote:
> ITRF varies over time compared to WGS84 based on tectonics, correct? 

No. Neither ITRF nor WGS84 is attached to a tectonic plate. 
The difference between them is very small (I've heard that it is at most
something like 10 cm, but I don't remember where I heard it), and it does
not increase over time. The reason for the difference is just that 
coordinates in WGS84 cannot be measured as exactly as in ITRF.
(Source: http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide4.html )  

For WGS84-based mapping in Europe, we use ETRS89, which is 
attached to the Eurasian tectonic plate.  ETRS89 coincided with 
WGS84 on January 1, 1989, as if the WGS84 graticule had been
painted on the ground that day.  Since then, the ETRS89 (and Europe) 
has drifted away toward northeast by 2-3 cm/year.  So it is 
the difference between ETRS89 and WGS84 that increases due 
to continental drift. (Similarly for NAD83 in North America.) 

However, I am writing about WGS84 in a strict sense above. 
Many people who use GPS are familiar with WGS84, but they 
have never heard of ETRS89.  So these people will say "WGS84" 
when they really mean "ETRS89".  I believe differential GPS 
in Europe will really give ETRS89 coordinates, although most
users would call them WGS84 coordinates. 
 

Duncan Agnew wrote:
> Rather weirdly, Australia has chosen not to define a local frame, 
> even though it has a plate all to itself.

I am not a plate tectonician, but I believe this is wrong. 
The Geocentric Datum of Australia 1994 (GDA94) is attached
to the Australian plate. 
(Source: http://www.icsm.gov.au/icsm/gda/wgs84fact.pdf )

Best regards,
--
Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 Göteborg
Visitors: Sankt Eriksgatan 5
SWEDEN
mikael.rittri at carmenta.com
www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Andrew Williams
Sent: den 11 december 2008 05:36
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Google Earth Accuracy

Thanks Duncan. That "docking" scenario was how the system was sold to me by the developer of the hardware (Navcom).

Guess the brochure sounds better than the technically correct description;)

So just to make sure ITRF varies over time compared to WGS84 based on tectonics, correct?

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Duncan Agnew
Sent: Thursday, 4 December 2008 5:24 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Google Earth Accuracy

Since I am in the crustal-motion business I want to correct the previous posting about ITRF. This is "space-based" only in the sense that it uses data from outside the Earth (GPS and other satellites, along with distant radio sources).  It was not designed with space use in mind. Since ITRF and
WGS84 are both space-based in this sense, they align closely: after all, the center of the Earth and the rotation pole are both non-arbitrary.

ITRF was designed to be an ultraprecise reference frame for measuring motions of the Earth as a whole, and of different parts relative to each other.
Since
the frame is designed to be fixed relative to the velocities of all the tectonic plates, averaged over the whole Earth, any particular point will have time-varying coordinates in this frame--as is is in fact true for any frame not tied to a particular plate (WGS-84 included), though many softwares do not take account of this. The people who develop and update ITRF are working at the millimeter level.

Two "plate-fixed" frames are NAD (at least east of the Rockies), and EUREF (for Northern Europe). Rather weirdly, Australia has chosen not to define a local frame, even though it has a plate all to itself.

For more on ITRF, see http://itrf.ensg.ign.fr/

Duncan Agnew

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