Ok, let me explain. I was flying on Saturday surveying a very large recently extinguished area of burnt bush for re-ignition points. I have two GPS instruments in the aircraft, one Garmin and one Lowrance. Both use an identical map, the Jeppesen Aviation Pacific Region Chart. Both GPS instruments were set to record 'trails' at the same intervals, and these were downloaded to a PC for analysis. Using a tool called 'GPSBabel' both files were converted to the same format and overlaid on a chart. Now here is the funny bit, they didn't match exactly, they varied by as much as 1/2 nautical mile. Both recorded at the same time, in the same aircraft, on the same chart data......why the difference? :?