On Monday, March 5, 2012 7:33:45 AM UTC-8, alpha male wrote:
On Mon, 05 Mar 2012 15:30:57 +0000, alpha male wrote:

On Sat, 03 Mar 2012 19:43:47 -0800, Jeff Liebermann wrote:
(I wonder how they handle the constant
creep which occurs out here near the San Andreas fault line).

What I mean by that is that it's a right-slip fault, and it moves by
centimeters to inches each year (sometimes in feet to yards, both in
elevation and in position) ... but ... how do they know if the west side
moved north or if the east side moved south?

I wonder what they use for their frame of reference since it depends on
which side of the fault you're on if you want to say the west moved north
or that the east moved south.

WGS84 is a geographic coordinate system: it's referenced to the Prime Meridian and the Equator, not to any ground landmarks locally.

the digital elevation data is "accurate as of the date of collection". If a fault moves or someone engages in a big earthmoving operation, then the data set won't reflect reality.

As to the practical problem of determining plate movement? Easy if you have GPS, because GPS is referenced to WGS84, and WGS84 is referenced to a specific geoid, anchored at the equator and the prime meridian. In turn, one can use celestial landmarks to calibrate it.