Someone seems to have forgotten that the thread was directed at GPS use for cars.

And then there's the matter of how and why one learned the particular navigation techniques that get used later on. Although the principles are the same whether you're tramping over rough terrain with a topo map, driving in a city with a street directory or flying an aircraft and concentrating on conversations with Air Traffic Controllers, their application requires different techniques because of the different rates at which information (and distraction) is presented and the consequences of "getting it wrong".

As one who learned in the topo map situation in a country where map reliability was always to be questioned I became very comfortable with transferring what I saw between the ground and the map; even with the best (commercial) topo maps here you can't pinpoint to better precision than 100m. That's metres for you Americans. In Britain, with a longer history of mapping, many places can be pinpointed to better than 10m. Where NATO wanted to drop things down chimneys from another country, the accuracy may be even better.

But most of us can't afford the sort of GPS that can routinely, reliably and rapidly pinpoint a position to better than 20m at driving speed although many could do it at walking speed. In older cities with streets, lanes and mews spaced according to horse and buggy rules (or even walking rules) lack of precision becomes important. In more recent cities, spaced with large cars and trucks in mind, such lack of precision is less important. And that's just the precision behind the instrument's calculations; on top of that you also have the accuracy of the supplied map to contend with.

In various student exercises I get them to select a datum, record what the GPS displays as its location, go and plot a series of other point and then come back to their datum and check the GPS reading of its location again. They are usually surprised at the variation (10 to 25m off) and how that depends on the time of day and the models of GPS used. Great fun. At this level I suspect the precision is sufficient for any navigation, just not for fine mapping but you should do similar checks on intended purchases, just to be sure.

Flying is a different activity entirely and not particularly relevant to driving and the context of this thread, I'd suggest.

Cheers, Rowan