With resepct to meters/feet how close will a GPS device get me, say if device (like iPad), is "common". Say, could I upload a particular coordinate that has two-tree houses in my back yard and discern one from the other? I want to build an app which discerns between plots separated by just a few feet. Possible and plausible?
This gets at some fundamental spatial data quality concepts, and the qualities of the device being used. There is both accuracy and precision to be considered, and these are not the same thing - precision refers to how much variation exists in repeated measurements by your measuring system (i.e., how reproducible and reliable measurements are), while accuracy refers to how far off of the 'correct' value a given measurement is. I suspect, though I'm not certain, that a consumer-grade GPS receiver like the ones in iPads or smartphones these days are reasonably accurate, but not very precise. The real answer would lie in something like the full specifications sheet for the device.
Yes, probably you could get an app using an iPad and the normal satellite-provided GPS system to differentiate between two locations something like ten feet apart, but not dependably every time, and with considerable coordinate variation each time.
Some apps do a little better than this most of the time by making some assumptions and using ancillary data. For example, if you know exactly where two trees are, you could have your app 'snap' the current position of the iPad to whichever tree is closest to the reading currently given by the GPS receiver. That's the sort of thing that automobile GPS receivers do.