Jerry,
The accuracy of the vertical with any gps unit isnt going to be entirely accurate. You have to look at the horizontal, and can pretty much believe that the vertical is going to be twice as bad. Its based off a single geoid that is preloaded onto the unit. so if you go out and take a point, and read its elevation, then build a large structure, stand atop it, and take a point, the elevation will not change. Plus too, Trimble is pushing and pushing their new floodlight stuff, when most base stations around the US cannot even post process it.
Dave