skip the GPS totally, unless you have one of those $1200+ systems designed to be used for race telemetry which has an inertial correction.
standard GPS only updates every 5 seconds I think, and they are deliberately detuned by the us military.
If you want accurate GPS tracks, either use topofusion (can use custom elevation files) or google earth/maps to make a GPX/kml file that you can import.
also, you can use mwsnap and hugin to make background pics, and then use a elevation script to elevate the track you make (pre0.7BTB),(elevation data varies in quality depending on location, outside USA is limited data, if using us gov data, better local sources May be available)
edit: btw, a soccer field is a great way to calibrate your background image (scale it) since if its official it uses standard measurements for the line markings.