It looks like there's something screwey with the tracks I'm recording while driving around. I have a computer in my car which runs Kismet. I plug my eTrex Venture GPS into it, letting Kismet record a track from the NMEA stream (via gpsd) and then process Kismet's GPS logs into GPX files. But I just compared one of the Kismet data sets with a track downloaded directly from the eTrex with gpsbabel and the tracks are consistently offset from each other by about 100 metres or so. So now I've got to work out where on earth the error is being introduced. The GPS is set to WGS84 and I presume Kismet is too dumb to try and do any kind of datum conversion on the coordinates so I'm a bit stumped.