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Apparently, you can use it for surveying. Apparently, using a local reference point can improve the accuracy (differential?). I have seen people with big dish antennas on their back.

The landowner had his boundary surveyed by a professional, who used a Trimble setup with an on-site base station that supposedly communicates with the GPSSats, plus a proprietary Trimble RF network. Then the (big, clunky) Trimble hand units communicate with the base station.

I definitely got better accuracy by plotting the lengths and headings t from the deed description -- using Google Earth -- than he did with the GPS (or compass & chain, for that matter...) I demonstrated that I could "close" an irregular 12-leg 20 acre tract boundary to within 0.1 foot -- repeatedly -- using Google Earth's "line" measurement tool.

In fact, in that heavily-wooded, hilly old iron ore mine site ,the surveyor finally gave up on the GPS and went back to brush-cutting, transit, compass & chain...