[NatureNS] GPS deal - spatial resolution

Date: Tue, 20 Oct 2009 08:49:51 -0300
From: Gerald Ruderman <naturens@zdoit.airpost.net>
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Steve,

What I learned from a surveyor who needs about 1 foot accuracy (maybe
different than what he meant): Find a geodetic marker nearby and measure
its location using your GPS. This will give a correction. There is
expensive equipment which will transmit this correction real time to the
GPS. How quickly this correction changes over time is unclear to me.

Stephen Shaw wrote:
> Hi Fritz or others,
> Thanks -- it sounds like a good deal.
> 
> What's the best GPS spatial resolution you can get in the field these
> days, for
> close-scale local mapping?  My recollection is that a few years ago it was
> quoted as no better than about 15 feet, even with several satellites
> optimally
> in view, which is a bit coarse for an application I was thinking of.  Is it
> still in the same ballpark or has it improved?
> 
> If you were to stand in one place and do repeated determinations and then
> average these numbers, would you get closer to determining more accurate
> spatial coordinates for that spot?  You might expect this if the source of
> spatial uncertainty were `random` internal noise in the receiver, or
> something
> similar. Or does the source of uncertainty in the GPS reading lie
> elsewhere?  I
> think I asked this once before a few years ago but don't remember getting a
> clear reply, and never bought a unit.  There was even a story that the
> spatial
> resolution is intrinsically much finer than is available to the public, but
> that the US military has deliberately detuned the public output so it
> was less
> precise, presumably so it could not be used for accurate nefarious
> targetting
> -- I'm guessing that this was an urban legend?
> 
> Steve
> (Halifax)

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