I have recently been given permission from the company that I work for to upload the GPS traces from our fleet of vehicles. These are twenty or so trucks and 4WDs that travel the length and breadth of the south-west of Western Australia, often along small roads and farm tracks, etc. They each have a GPS tracker taking readings at 30s intervals and sending these (when possible; there's a delay when out of range) back to a central server via the GSM network.
I get hold of these traces as week-by-week NMEA files (containing all vehicles' data), and convert them to GPX with this gpsbabel command. Then I upload them. They're about 15000-20000 points each.
I'm going to try to get the sampling frequency increased, because there's lots of areas for which these points are too far apart. Although, I guess as the density of points increases (i.e. from multiple trips along the same roads) the topography will become clearer.
Now, the tracing begins! :-) If anyone wants to help....