On Sat, 03 Mar 2012 19:43:47 -0800, Jeff Liebermann wrote:

http://www.maptools.com/UsingUTM/mapdatum.html

Interesting quotes from that helpful reference (with my comments in
parenthesis).

"In the Continental United States the difference between WGS 84 and NAD
27 can be as much as 200 meters." (I wonder how they handle the constant
creep which occurs out here near the San Andreas fault line).

"Every map that shows a geographic coordinate system such as UTM or
Latitude and Longitude with any precision will also list the datum used
on the map." (I'd change "will" to 'should' based on my experience the
past two weeks on the web)

"The Global Positioning System uses an earth centered datum called the
World Geodetic System 1984 or WGS 84." (That's what I prefer.)

"For all practical purposes there is no difference between WGS 84 and NAD
83." (Good to know.)

"On a USGS topographic map ... The datum will always be NAD 27... A
dashed cross in the SW and NE corners of the map gives a visual
indication of the difference between the two datums." (This is good to
know.)

"If you are engaged in a mission that requires more [than several hundred
meters] precision, then your datums should match." (Since we're siting
antennas on private hilly land, we probably want two or three meters
accuracy in position and a half-meter to a meter in elevation accuracy so
our datums must match.)

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.

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.