A Most Peculiar Test Drive

Re:A Most Peculiar Test Drive by lutusp on Thursday February 14, 2013 @06:05AM (#5218902)

Yes, all well understood. The problem comes up in cluttered terrain and an ordinary GPS unit that's in motion (no differential GPS option). In such a case, if the receiver encounters obstacles in its satellite line of sight, it may switch to more favorable satellites and recompute its position. As a result, the apparent position will jump around within the normal error bound (see below). I know, I've had the experience any number of times.

> GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres)

Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence):

http://www.gps.gov/systems/gps/performance/accuracy/

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Re:A Most Peculiar Test Drive by Robin_Message on Thursday February 14, 2013 @06:16AM (#5218934)

Good point about changing satelites. I guess an open parking lot would be fine, but who knows.

I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.

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Re:A Most Peculiar Test Drive by lutusp on Thursday February 14, 2013 @06:54AM (#5219033)

> I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate.

It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.

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