[ft-l] Fw: Accuracy of Garmin "Field" GPS Units

Joe Dabes kabjnd at msn.com
Fri Dec 18 18:52:26 CST 2009


Greetings,

Carrying my three field GPS units, I hiked the same 8.4 mile "A" loop in the Citrus Tract of the Withlacoochee S. F. on separate days (overhead canopy light to moderate).  The three units all have the Garmin high sensitivity internal antennas that have been available for ~3 years now.  All three were set to gather track points every 4 yards.

Etrex Legend HCx:  Mounted on top of the left shoulder strap of my day pack
Oregon 400t:  Carried in my left hand about a foot in front of my waist
GPSmap 60CSx using internal antenna:  Mounted on top of the right shoulder strap of my day pack
GPSmap 60CSx using external amplified Gilsson antenna: Antenna mounted on top of my hat

Here are the results for the maximum distance between the two tracks for each unit over the 8.4 mile repeats:

Etrex Legend HCx: 301 feet
Oregon 400t: 42 feet
60CSx without external antenna: 16 feet
60CSx with external amplified antenna: 15 feet

Three conclusions:

1) GPSmap 60CSx is clearly the most accurate.

1) Etrex Legend HCx can be very inaccurate.  I have seen the maximum several times at ~500' and once ~1000'.  Perhaps the problem is with my particular unit?  I don't think so, as two others have sent me track data using this same model unit and the tracks differed at times several hundred feet from the same trail that I GPSed with my 60CSx with ext. antenna.  This unit is popular as it sells for ~$100 less than the most popular 60CSx.

2) For the 60CSx under light to moderate canopy it seems to make little difference whether the external antenna is used (as long as the unit is mounted high, such as the top of a shoulder strap).  However, under the heavy canopy we have summers along the FLT, I know that using the external antenna does increase accuracy, sometimes considerably.

-Java Joe, Finger Lakes Trail Conference GIS Guy, now in our winter home near Ocala FL

P. S.  If anyone is interested in examining the track data for these four tests over this 8.4 mile loop, I would be happy to send it as .gpx files.
P. P. S.  I determined the maximum distance between the two tracks by zooming Garmin MapSource in to the maximum (80' scale) and scrolling over the tracks for the entire 8.4 miles, and using the "measure" tool measure distance between the two tracks.

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