[Proj] Google Earth Accuracy

Richard Greenwood richard.greenwood at gmail.com
Wed Dec 17 13:32:43 UTC 2008

On Wed, Dec 17, 2008 at 3:08 AM, Mikael Rittri
<Mikael.Rittri at carmenta.com> wrote:
> Andrew,
> I may have misunderstood you. And maybe the Starfire differential service
> really does give you WGS84 coordinates, rather than some ground-attached
> variant like ETRS89 or NAD83.
>> The variation between WGS84 and ITRF of the day varies because of
>> plate tectonics. That is WGS 84 framework was set at a time in history,
>> but ITRF if calculated daily. The variable is the plate movements between
>> them.
> I still think you are wrong here. Let me quote:
>        "WGS84 is maintained by the US. Like the ITRS it is a global system.
>         Initially it was coincident with ITRS to an accuracy of about 1.5 meters.
>         It is currently maintained within 10cm of the ITRS."
>         http://www.ogp.org.uk/pubs/373-10.pdf (section 2)
> If the differences were caused by tectonic plate movements, then it
> would be quite difficult to maintain the two systems to within 10cm.
> You would need to connect all continents by very strong steel chains, to
> prevent them drifting apart more than 10cm from their original position ;-)
> No, if the difference between WGS84 and ITRF varies from day to day, I
> think that such differences are caused by something like athmospheric
> conditions, not tectonic plate movements. (Radio waves slow down in the
> athmosphere, by an amount depending on air pressure and humidity, I think.)
>> I'm not expert all I know is that we have to write some pretty special
>> code to account for this variation so that when we use this GPS for
>> locating with WGS84 coordinates we can find them.
> All right. If your special code is something like a time-dependent
> 7-parameter datum transform (like those given in section 6.5 of
> http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide6.html ),
> then you are indeed correcting for tectonic plate movements. But I would
> say that if so, then it is not the difference between WGS84 and ITRS
> that you are correcting for.  Rather, your original coordinates must
> be expressed in something like ETRS89 or NAD83, not WGS84 in its
> strict sense.
> On the other hand, if your special code is something else,
> then I don't know what it does...
> Disclaimer: I am a GIS programmer, not a surveyor.
> Maybe some surveyor on this list can give a better answer.
> Best regards,
> --
> Mikael Rittri

I am a surveyor, but not a geodesic. My understanding is the same as
Mikael's: WGS84 and ITRF are very nearly the same. I don't know why
they are not identical, Mikael says
    "Radio waves slow down in the athmosphere, by an amount depending
     on air pressure and humidity, I think."
It is generally electromagnetic disturbances much higher in our
atmosphere that distort GPS radio waves. The disturbances are in the
ionosphere and are effected by sun spot activity and solar flares,
among other things. Maybe this is the reason for the difference
between WGS84 and ITRF, but I am not sure.

WGS84 and ITRF are not fixed to a plate,  they are geocentric and
fixed to the center of the earth. NAD83 is fixed to the North American
plate. Many software, including Proj, do not distinguish between NAD83
and WGS84 so it is a common mistake to refer to the two datums
interchangeably. And since they did start out as matching, it did not
used to matter much. Right now there is over a meter of difference
between WGS84 and NAD83, so it is becoming more important for all of
us to understand and respect the difference between geocentric datums
(ITRF and WGS84) and plate-based datums.

Richard Greenwood
richard.greenwood at gmail.com

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