[Proj] New Member - Question

[Robert McFaul]

Hello, I'm a new member to the list. My name is Robert McFaul and
I work for a geologic modeling software company. 

Got a question for you all regarding datums and surveys and the
earth-centered ellipsoids. Does a given point on the ground have 
the same latlong value for all surveys using the same earth-centered 
ellipsoid (GRS80, WGS84 etc.) regardless of the projection system used? 

In the past a surveyor would 'pin' the ellipsoid to the earth 
at a position of their choice creating their datum. Another surveyor
collecting the very same locations might use the same ellipsoid and 
projection system but the ellipsoid is 'pinned' to another location 
thus changing the latlong (and projection system) values of its survey 
points. These changes might be very slight or could be quite different
(hundreds of feet or meters difference). 

Am I correct in thinking that the earth-centered ellipsoids remove this
variation in datums?

Hope this is clear. If I've confused you a little email me and I'll 
finish the job:-)

Thanks.

robertm.
robert at dgi.com