GPS Unit horizontal accuracy

[Guest jmlane]

Can some of you experienced GPS unit users give me some idea as to the horizontal accuracy of the hand held units being used by geocachers. Is there a difference in accuracy between the $200 units and the $500 units or do the more expensive just have more features? I have used backpack models with the pole antennae and found them to be very accurate. I would guess, the handheld units are somewhat less accurate.

Thanks

[Guest Moun10Bike]

In general, you can expect about 15 meter (49 feet) RMS, which in English means that you can assume that the GPS is correct to within 15 meters 63% of the time (experts, correct me if I'm wrong here). Of course, the position of the satellites in the sky, signal reflection, and signal blockage can all affect this accuracy. With DGPS (differential GPS) you can get within 1-5 meters RMS.

 

All of the consumer units priced between $200 and $500 will give you essentially the same accuracy (although as WAAS continues to get implemented, you will see those that are WAAS enabled start to distance themselves from the others in accuracy). As you surmised, the price differences generally involve the number of extra features. The backpack models you have used and commercial-grade receivers are significantly more expensive, and cost well over $1000.

[Guest peter]

quote:

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Originally posted by Moun10Bike:

In general, you can expect about 15 meter (49 feet) RMS, which in English means that you can assume that the GPS is correct to within 15 meters 63% of the time (experts, correct me if I'm wrong here).

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Yes, but this is the 2-sigma accuracy, so you should be within that distance 95% of the time. Measurements have suggested that the 15m figure is conservative and with a clear skyview consumer GPS units are accurate to better than 10m over 95% of the time. Of course, if you have reflections from buildings or terrain, or have portions of the sky blocked, then the accuracy can be much worse.

[Guest trilobites]

Here is a repeat of a positional accuray test I did on an Etrex Summit last month.

 

Over a 40 day period the positional accuracy of an Etrex Summit was tested by taking waypoints at a known USGS benchmark [MCMC76] in fron of my office in Rolla, MO. A Etrex summit test of an Etrex Summit at a USGS bench in front of our USGS office in Rolla, MO

Readings were taken at various times of the day between 0600 and 1830 with a sample size of 80. The Etrex was warmed up for 1.5 minutes and held at random compass headings (if you hold it the same way each time a notable shift if the readings is observed).

 

Actual coordinate is

610270, 4201245

avg Etrex reading was

610269.4, 4201246.5

with std deviations of -0.6, and 1.5

 

What does this mean? From Quantile plots...

 

5% of the readings were within 1 meters

25% of the readings were within 3 meters

50% of the readings were within 5 meters

70% of the readings were within 6 meters

80% of the readings were within 8 meters

95% of the readings were within 13 meters

 

Thus, the positional error is just about what the manufacturer states. Most of the time between 4 and 7 channels were locked and the

display generally indicated an error of 14 to 22 feet. Based on this "quick" test, the error listed on the Etrex screen is a pretty good approximation of the actual error.