Re: GPS for Dummies
"If the cheapy GPS receivers in a mobile phone are only accurate to 10m, how does it (usually reliably) know which lane I'm in"
Sorry, which GPS is actually giving you an indication of which *lane* you're in? It might know "northbound" vs "southbound" or similar (which is carriageway information, not lane) but it knows that purely from your actual direction. Sat navs "snap-to" roads, allowing them to be very accurate. Try coming off a road onto a farm track and watch it lose its way quite quickly.
Even when not snapping-to a known location where it expects you to be (roads), it can use fuzzy logic to determine where you are, as the 10m accuracy will vary over time, and a GPS knows that you'll roughly travel quite consistently. If your reading jumps from 5m side to side, it's more likely the error has varied rather than you've moved erratically.