Accuracy of Tracks From a Garmin Etrex GPS Receiver: Saved Tracks Vs. Tracks in Memory

GPS has become interesting for hiking now that the accuracy degradation (SA) of the GPS signals has been turned off. The accuracy of GPS position determinations is now routinely quoted as ~15-40'.

Unfortunately, the manual for the Garmin Etrex GPS Receiver gives no information at all about the tracks stored in memory or the tracks stored in saved tracks. Hence I have begun to analyze the accuracy of those saved tracks. The Etrex software version used for this analysis is 2.05. (Note that there are free later software updates.)

Here are some characteristics of the saved tracks, most of which were a surprise to me:

Here is an example of the accuracy degradation for a Grizzly Flats hike in the San Gabriel Mountains. I used the Etrex in normal mode to record a 100 mile journey from my home in San Diego County to the Upper Dark Canyon Trailhead at mile marker 30.02 on the Angeles Crest Highway. I then used the Etrex in battery save mode to record the hike. (Normal mode is necessary to obtain accurate tracks for car travel, but battery save mode is sufficient for low-speed travel.) In the hike, we traveled to the saddle above Grizzly Flats, but then took an excursion along the Fire Road to Mt. Lukens before returning to the saddle and going on to Grizzly Flats.

I downloaded the memory without doing any saving of tracks. I then saved the memory as a track, and downloaded the saved track. The 1440 points from the original track was compressed to 125 points in the saved track!

A comparison of the two tracks for the "to" part of the trip (the "return" part is not shown for clarity) shows that the saved track is an extremely poor approximation of the original track stored in memory. There are two places along the track where the saved track is off by ~650'!

Note that this large error may be due to this being a portion of a track with a 100 mile journey. I have seen other examples where the saved track is an excellent approximation to the track in memory, when the entire track in memory spanned only a few miles.

See the topo map for a comparison of either track to truth. Note that even the track in memory did not record the switchback shown on the topo map.

Go to:

Copyright © 2000-2001 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last update: 13 July 2000 (minor updates 2 January 2001).