Without wishing to start an argument, I'm sure panel mounted GPS are supremely accurate, but I'm not so sure about handhelds, where the airspace database could be out of date for instance. I'm talking basically of penetrations or near penetrations where the pilot claims he is up to 5 miles away from regulated airspace, and I'm positive it's not the fault of ATC radars, 'cos we often crosscheck with adjacent units and they agree about the position. I dimbly remember a thread somewhere about innacuracies in Jeppesen supplied GPS databases (might have been on another forum other than Pprune).
My own Garmin for instance, is pretty accurate as regards airspace, but the road database seems to only get it right for the M25, other roads being about a mile adrift, but then it's not supposed to be used at ground level anyway without a DGPS signal.
Ni Thomas; the days of blobs on radar displays are long gone; we get plot extracted symbols nowadays which a processer compares with the SSR plot and oh god I'm boring myself TDM take over.