Re: Roughtly converting DOP to metric error

Bitstring <vPudnUqIffIJ4zbUnZ2dnUVZ_rfinZ2d@xxxxxxxxxxxxxxxxxxxxxxx>, from the wonderful person Roy Lewallen <w7el@xxxxxxxxx> said
speleoluc@xxxxxxxxx wrote:
I found such info pages googling for a simple answer, but I can't help
but think that when I turn on my GPS 76, I don't have to input various
parameters to get a position error in meters. Can't I do the same
thing from the NMEA data I get from the GPS, or is the GPS basing its
precision estimation on data the DOP value doesn't convey?

I seriously doubt that anyone has actually tested one under a variety of conditions (partially obscured sky, multi-path, wet forest canopy, etc.) at accurately known locations for extended periods, so I don't think anyone knows how good the Garmin accuracy estimate is. If you run the DOP through a random weighting function or one based on astrological charts to create a nice looking number, I'll bet no one will ever be able to prove your result isn't as good as Garmin's. And it might just be.

Well I now have about 1000 trails up/down the road outside my house (standard 2 way blacktop, so what, 25-30ft wide?) and while the =precision= may have something to do with Garmin's number (yes, I AM more likely to show in the fields instead when the satellite config is lousy), and while it can show the difference between the two sides of the road under most circumstances (on the same day), the actual positional =accuracy= is much worse than the number the GPS (a 60CSX with external antenna) displays.

So the number Garmin gives may be the +/- sigma or even two sigma, but it sure as heck isn't 'worse case' (whatever that means). On some occasions I've been shown as 100m away from where I know damn well I am, with an indicated error on the handset of 3m. Typically that'll correct within a few minutes, but sometimes it'll take 15 or 30.

Multipath? Screwed up satellite signal? Bad firmware in the handset?? Not really possible to say, and those are all errors that you can't figure out from looking at DOP math.

One thing I can deduce is that Garmin scales the 'error' in an averaged point based on the (sqrt of, I imagine) number of fixes averaged (1 per second, except when the 60CSX goes into cloud cuckoo land every now and then). And it'll do that - show increasing precision/accuracy over time, even when the actual position values it is averaging are getting further apart (try it while walking along, for instance).

I wish they'd do some stats on the individual results instead. And for EPE I wish they'd look around all the possible satellites available and do some stats on the resulting cloud of PVT solutions.

--
GSV Three Minds in a Can
13,737 Km walked. 2,584 Km PROWs surveyed. 46.6% complete.
.

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