Larry, et al ...
I have been using GPS equipment for nearly 10 years at the Observatory
here, and have also had the telescope surveyed by a professional
branch of the Gov to obtain precise location coordinates for the
astronomical telescope.   

You are correct, that there are several datum references, and the
actual measurement you obtain will be with reference to a specific
coordinate reference.  It must be converted, as you have described,
to other references where that base is desired.

After all is said and done, my experience is consistent with the
professional specifications:  the Latitude and Longitude "readouts"
are consistently better than the elevation readouts.  I have a
GPS monitor running here continuously.  Most of the time (but not all
the time) the horizontal precision (or at least consistency) is very
good, within a few meters without averaging.  But the same is not
true for elevation, where the readings generally wander over a range
of about +/- 100 feet or so, and occasionally, much wider.   I think
the elevation measurements are probably reliable to several tens of
feet (maybe +/- 100 fet) only if the measurements are averaged over
a long period.  The last time I did this, I ran 4000 measurements
averaged over about 3 or 4 days, and was getting consitency down
to the ~25 foot level in elevation, maybe a little better.

The overall precision is very dependent on the number of satellintes
in use and their disposition ... you really want to strive for close
to horizon-to-horizon view for your GPS receiver over 360 degrees of
bearing.  But, with patience and lots of averaging, lesser view fields
can also be used.

However, even when everything is being down carefully, the nature of
the beast is that elevation precision will be poorer that horizontal
precision.

To gain some subjective feel for your situation, run your averaging
over, say, an hour for each of several days spread over, say, 2 weeks,
and then compare the results ...

Best regards to all

warren       W & B Observatory  (astronomical)
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