Even if you make your own trails I still wouldnt trust it. The problem is GPS varies on what and how many satellites you are picking up so at any given time they could be anywhere from 5 to 30 or more feet off.
Does anyone know which of these systems our typical graphs use for their GPS?
GPS SYSTEM ____________________ EXPECTED GPS ACCURACY (METRES)
GPS with S/A activated____________________ ±100
GPS without S/A activated____________________ ±15
GPS with WAAS____________________ ±3
Differential GPS____________________ ±5