[aprssig] gps accuracy?

Wes Johnston, AI4PX wes at ai4px.com
Mon Nov 23 10:53:02 EST 2009

The points are my property irons and were in SC State Plane (with
international foot) and I had to convert to WGS84.  Around these parts using
NAD27 gives an error of about 125' to the north if I recall correctly.
Further, the azimuth from the property iron to the point varied by about 120
degrees depending on which point I was at.   It wasn't like I was
consistently off 20 feet to the north for example.

Just to be more specific about my question, I was hoping for someone who had
recently been to the middle east where the US would want to have a degraded
service.  Maybe such a person could tell me if SA was on over there like it
was everywhere in the 1990's?

Footnote: the international foot is based on the redefined inch that is
exactly 2.54cm.  The imperial inch and US inch were "averaged" to 2.54cm in
1959 or so.  So in other words, we here in South Carolina are progressive
enough to recognize and use a modified inch derived from metric, but we
won't actually _use_ metric.  sigh....  for what it's worth, the difference
in the old inch and the "new" inch makes about a 6 foot difference across
the state from the westmost to the eastmost point.  I think AZ is the only
other state to use the international inch.  We sure make things hard on


On Mon, Nov 23, 2009 at 10:00, Mark Earle <wa2mct at mearle.com> wrote:

> 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
>    ( (     Echolink   221408    Grid Square EL17HQ
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