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I see that NGS benchmarks are used to monitor the movement of geologic plates.

Is one of the benchmarks assumed to be in a fixed location, with movements of all other benchmarks measured with respect to it, or are all benchmarks moving as a result of plate tectonics?

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up vote 5 down vote accepted

The IERS reference meridian is a weighted average of ground-based monitoring stations. Thus, tectonic monitoring must involve the motion of one's own plate relative to this global average motion.

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@Kirk Thank you for responding with such alacrity! However, in order to encourage contributions, it might be a good idea in general to hold off for a few days before accepting responses. I for one would love to have someone come along with an amplification or critical remarks or even a totally different answer, especially anyone with direct experience in tectonic monitoring. –  whuber Oct 20 '10 at 21:27
    
Ok, sounds good. Also realized this only answers the longitude aspect. I presume this meridian begins/ends at the poles of rotation. Can we assume that the poles of rotation are always at +/- 90 degrees with respect to GPS? –  Kirk Kuykendall Oct 21 '10 at 19:35
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@Kirk These systems are geocentric. After establishing a meridian, the only thing left is to determine the earth's orientation (which precesses slowly, which means time is an essential parameter in any high-accuracy reference system). The orientation of the earth's axis is defined with reference to a celestial reference system. Some information is available at itrf.ensg.ign.fr/itrs_itrf.php . –  whuber Oct 21 '10 at 19:58
    
interesting, I wonder if this celestial reference system is what this guy needs gis.stackexchange.com/questions/2459/… I get a strange error for the php link you posted, is it a public site? –  Kirk Kuykendall Oct 22 '10 at 15:40
    
@Kirk I'm getting that error now too. It was a public site yesterday! Just back up to itrf.ensg.ign.fr and follow a sequence of links labeled "ITRS and ITRF". –  whuber Oct 22 '10 at 15:47
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