GPS Accuracy in a real-world geocache test

[+Alan2]

Here’s the 4-month result in real geocaching GPS accuracy. The set-up was that each cache finder to a NGS benchmark, cache http://www.geocaching.com/seek/cache_details.asp?ID=10814 would report their GPS coordinates at the benchmark to the cache web page. The benchmark center was on a 7 foot wide, 2 ½ foot high granite slab. You would have to reqach to set the GPS in the center and to read it. Benchmark is in an open area although there are a couple of small trees nearby. The actual NGS coordinates are 40 44' 34.46640"(N) 073 50' 42.31409"(W) which places it in Flushing Meadow Park, Queens, NYC.

 

The following GPS’s were reported used including my own Vista when I set up the cache. Readings were taken with 13 different GPSr’s on 12 different days between 11/25/01 and 3/24/02 at different times of the day. Weather conditions were not reported.

 

Garmin Vista (4)

Garmin etrex (2)

Garmin Summit

Eagle Expedition II

Magellan Map 330

Magellan 300

Unknown

“Cheap” Garmin

Garmin Street Pilot

 

Summary of results. Difference calculated from NGS coordinates. WGS84 coordinates in DD MM.MMM were converted to UTM using Mapsource program. Distance from benchmark calculated taking the square root of the sum of the squares of the difference between the UTM latitudes and UTM longitudes in meters and converting to feet.

 

12 readings under 12 feet

5 readings between 13-25 feet

1 reading at 40 feet (note that this was the Magellan 300 which is an older unit and reads to 2 decimal places only - about 50 feet accurate I’ve been told).

 

All were single readings except for Eagle (13 readings = 3.3 feet avg.) and one of the etrexs (7 readings = 25 feet avg.). There was one WASS reading reported (my Vista at 3.3 feet).

 

I think the results speak mountains as to the accuracy of the equipment we are using.

 

I want to thank everyone who helped contribute the data. Hope you enjoy the cache site as well

 

Comments on the data or results would be appreciated?

 

Alan

[+GPS Guy]

Interesting results. Nice job.

I wonder how things would turn out if the experiment were repeated under heavy tree cover?

[+GOT GPS?]

You can check out my NGS BenchMark cache here:

"NGS Markers & GPS Accuracy Check" by Geoff (GC2AC7)

 

It has alot of info from several cachers.

 

My home page about GPS units and information

[+Alan2]

quote:

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Originally posted by GOT GPS?:

You can check out my NGS BenchMark cache here:

http://www.geocaching.com/seek/cache_details.asp?ID=10951

 

It has alot of info from several cachers.

 

http://members.aol.com/geoffr524/5_Rubik.gif http://members.aol.com/geoffr524/myhomepage/howto.html

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The results at yours parallel mine. Nice report. What was the cover like at your benchmark?

 

Mark G makes a good point about cover. In my case at least it was open and you could set the GPS down to let it "settle". But this shows that we all could get better readings in the field if we take the time and set the GPS down for a moment. It's still amazing to me that with all the complaints about "loose bearings" etc, that so many cachers using so many different models can actually get so close!

 

Someone want to set up a experiment with an NGS benchmark in heavier cover and report the results in a few months?

 

Alan