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

Joe Dabes kabjnd at msn.com
Mon Dec 21 18:55:52 CST 2009

Sandy, the Navigator (and others),

Good to hear from you.  Yes, GPS technology has progressed greatly, and you do get what you pay for.  The cheaper Etrex GPS units are not worth spending any $ on.

Citrus Tract of Withlacoochee S. F. -- I love the trails there!  Well thought out trails with four loops, and nice backpacking campsites.  I have spent probably 50 days in the last six years backpacking, camping, and hiking there.  Not much water anywhere there, but that mean no mosquitoes.  For backpacking I leave water (and beer) drops near the backpacking campsites.

Joan, the non-techie, asked what kind of animals I saw.  Well, I saw one 50-pound animal plenty -- Pumpkin, one of our two Golden Retrievers, along for the two hikes and camping out with me.  She flushed a giant gobbler turkey and a couple of deer.  And I enjoy the abundant long leaf pines, live oaks, and turkey oaks along the trail, along with winter flowers.

Java Joe
  ----- Original Message ----- 
  From: Navigator<mailto:ftcracker at aol.com> 
  To: ft-l at backcountry.net<mailto:ft-l at backcountry.net> 
  Cc: FT Listserv<mailto:ft-l at mailman.backcountry.net> 
  Sent: Monday, December 21, 2009 4:44 PM
  Subject: Re: [ft-l] Fw: Accuracy of Garmin "Field" GPS Units

  Interesting! I started using a Garmin Oregon 400t this summer, with WAAS enabled, and the difference between that and the $99 eTrex is night and day on accuracy of the track point curvature matching the trail. I'm happily re-hiking a lot of my old favorites to get more accurate GPS tracks to post on Florida Hikes for my subscribers. Can't say I'm up for hiking with an antennae on my hat, though! I leave that to you hard core GIS folks. I can generally straighten out the line by zooming into Google Earth and matching it against what I know about the trail on the ground, so my final KML is more accurate than the Garmin-gathered baseline GPX.  ;-)

  Cheers, Sandy aka Navigator

  Joe Dabes wrote: 

    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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