How accurate do we expect the GPS to be?

[Guest zilla]

I was just curious as to the degree of accuracy we Geocacher's expect? I have placed caches and had people have problems finding the cache.. I have looked for caches and had problems. It seems that maybe we expect survey quality accuracy out of a consumer GPS.. Most of the time I consider the accuracy of my GPS to be adequate. But then I am not looking for a cache, but rather a larger item such as a lake, mountain, trail junction, etc. With the advent of WAAS we have a lesser degree of error, but then again do I really need that accuracy if I wasn't looking for a cache? Are we expecting too much? Personally I think that there are too many variables for us to be able to go directly to a cache, and it seems that when we get to the last 50 feet or so we go from high-tech, to low-tech. Lastly, how much accuracy can we expect to see in the future?

[Guest Iron Chef]

I usually expect an error of a 15-20 feet radius from where I stand, but of course this depends on the conditions of where I am standing. As for in the future? I dunno... the sky is the limit, because after all, 640K of memory is all that we'll ever need ;~)

 

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-Iron Chef

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agefive.com/geocache/ ~ Fe-26

Lets Drive Fast and Eat Cheese!

[Guest Vagabond]

Found 47 caches since Feb 17 maybe 7 were dead on most have been under 10 feet usually 3 to 7 ft a few I've had to range out to find them, furthest maybe 60 or 70 ft. And I'm using a Lowrance Global Nav 212 about 3 or 4 years old

[Guest Wheels]

Depending on your equipment, and especially if you use the DGPS service around the coasts, you can usually get within 10 feet. However, repeatability (going back to the exact same spot) has always been a problem for the GPS system (unlike the LORAN-C system, which is coastal-oriented). The bigggest issues are the number and configuration of satellites visible, number of channels in the receiver, and intervening items which change the propogation characteristics. Also, some lower-end receivers do a slow sequence through the satellites. If you want to check the accuracy of your equipment, find a local NGS benchark ( http://www.ngs.noaa.gov/datasheet.html ) and see what posit your GPS unit displays. Then compare to the actual NAD-83 (WGS-84) posit laid out on the datasheet. Also, a previous post in another thread (Ron Streeter's 'GPS accuracy revisited') suggested doing a backyard calibration to see how well your unit can find the same spot IT picked out, which is a pretty good idea.

[Guest Ron Streeter]

This is the url for the GPS Accuracy Revisited thread.

 

http://forums.Groundspeak.com/ubb/Forum2/HTML/000510.html

[Guest CharlieP]

From my reading on the subject, at a 95% confidence level, 15 feet is the *best* you can hope for, with good reception and good sat geometry. As reception and geometry varies from the ideal, the error becomes greater. The Estimated Position Error number reported by most GPS units may require some interpretation, since it may be based on 95% confidence or some less stringent criteria. This site shows how accuracy varies with sat geometry, the HDOP number is the one to watch, and multiplying it by 15 feet is probably a better estimate with SA off.

 

http://www.ualberta.ca/~norris/gps/DOPdemo.html

 

CharlieP