[aprssig] gps accuracy?

Mark Earle wa2mct at mearle.com
Mon Nov 23 10:00:45 EST 2009

n4lbl wrote:
> I've always presumed that the accuracy reported is at some particular
> confidence interval but I surely don't know that.  If the GPS says the
> accuracy now is 10 feet that sounds like a 100% confidence interval is
> implied but I have trouble believing that.
Something else to consider - what map datum is being used for the 
"known" point? For instance, if I stand at a USGS benchmark, my GPS 
might report 15' error of position. But, what map datum is my GPS using, 
vs the Benchmark coordinates? This must be accounted for.

My Nuvi350, as far as I can find, the map datum is not selectable. My 
old GPS45XL offers many to choose from. This is handy, for instance, 
when doing tower siting work - my GPS datum can agree with the FCC 
databases, no conversions required after the fact.

      ) )    de WA2MCT Mark
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