Why does GPS Odometer mileage differ from uploaded GPX mileage?

[bbbullock]

I just completed a 10 day backpacking trip at Philmont Scout Ranch and the odometer on my GPSmap 60CSx showed we trekked 92 miles. Yet, when I uploaded the 10 GPX log files and combined them into a single file/trek in RoadTrip, it says that we only traveled 72 miles.

 

I thought I could rely on the GPS Odometer as an accurate measure of mileage. Why the large difference between the GPS Odometer and the mileage measured via the log files?

 

Brian

[photomankc]

The trip computer is usually different from the track log. The track log is going to average between log points while the trip computer function may be using different criteria to calculate distance traveled. I've found both are not superbly accurate. Many will continue to log motion when I'm stopped because they can't differentiate real movement from position drift. Other times they fail to pick up movement between samples and log it as stopped time then the track log averages out positions and turns between sample points. Switchbacks with sharp corners can carve off bits of your real path quickly. I've rarely seen them agree across several units I have owned even on the same trail at the same time.

[+DENelson83]

The trip computer's odometer most likely uses the measured doppler shift in the GPS signals, which can actually change with ionospheric conditions.

 

The track log distance measurement, meanwhile, can be tweaked using the track log settings. On my unit, track log record method and interval can be adjusted. I have my track log set to "Auto" record method and "Normal" interval. YMMV.

Edited July 6, 2009 by DENelson83