[Proj] Google Earth Accuracy

support.mn at elisanet.fi support.mn at elisanet.fi
Mon Dec 22 02:56:12 EST 2008


Here is some text, most borrowed from the "WGS84 Implementation Manual"


The precise satellite laser ranging technique has led to a precise worldwide
terrestrial coordinates system, called the International Terrestrial Reference
System (ITRS). Satellite Laser Ranging (SLR) and Very Long Base-Line
Interferometry (VLBI) techniques guarantee a precision of 1-3 cm
over distances up to about 5000 km.

The ITRS is maintained by the IERS and the realization of
the ITRS is the International Terrestrial Reference Frame (ITRF).
Plate tectonic movement was incorporated in that coordinate system using
results of recent measurements and a global geophysical model. Thus, it is a
model with ***changing coordinates*** due to movements of tectonic plates on
which the ground stations are located. This reference system
provides the fundamental position of the Earth to within 10 cm and the
orientation of the axes to correspondingly high accuracies. Since 1988, the
IERS has defined the mean spin axis, the IERS Reference Pole (IRP) and the
zero meridian and the IERS Reference Meridian (IRM).


Notice that the movement of some ITRS points (coordinates) are usually
only few centimeters a year maximum due to the plate movements.

Since WGS84 follows ITRS, it's coordinates are also moving. And so if one
includes the date of the point measurement, it is possible to calculate
backwards, if nessesary, the original location of that point.

Regards: Janne.

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