The GPS unit on my boat regularly claims an estimated position error of 4
feet after it has acquired its full complement of satellites.  This is a
fairly new mid-price GPS unit using up to nine satellites and WAAS.  So my
recreational GPS supposedly obtains fifth-decimal-place accuracy.  It was
running under an unobstructed sky, which is common when boating.  Careful
use of a good GPS unit should be able to achieve this level of accuracy on
land as well.

>From http://www.gps.gov/systems/gps/performance/accuracy/ the raw accuracy
of the positioning information from a satellite is less than 2.4 feet 95% of
the time.  The accuracy reported by a GPS unit is degraded by atmospheric
conditions; false signals, e.g., bounces; and the need to determine position
by intersecting the raw data from several satellites.  Accuracy can be
improved by using more satellites and multiple frequencies and by
comparing to a signal from a receiver at a known location.

The web page above claims that accuracy can be improved to a few centimeters
in real time and down to the millimeter level if a device is left in the
same place for a long period of time.  I think that these last two
accuracies require a close-by receiver at a known location and correspond
to what is said in [4].

peter



On 08/30/2017 06:53 PM, Nick Wilson (Quiddity) wrote:
> On Tue, Aug 29, 2017 at 2:13 PM, Stas Malyshev <smalyshev@wikimedia.org> wrote:
>> [...] Would four decimals
>> after the dot be enough? According to [4] this is what commercial GPS
>> device can provide. If not, why and which accuracy would be appropriate?
>>
>
> I think that should be 5 decimals for commercial GPS, per that link?
> It also suggests that "The sixth decimal place is worth up to 0.11 m:
> you can use this for laying out structures in detail, for designing
> landscapes, building roads. It should be more than good enough for
> tracking movements of glaciers and rivers. This can be achieved by
> taking painstaking measures with GPS, such as differentially corrected
> GPS."
>
> Do we hope to store datasets around glacier movement? It seems
> possible. (We don't seem to currently
> https://www.wikidata.org/wiki/Q770424 )
>
> I skimmed a few search results, and found 7 (or 15) decimals given in
> one standard, but the details are beyond my understanding:
> http://resources.esri.com/help/9.3/arcgisengine/java/gp_toolref/geoprocessing_environments/about_coverage_precision.htm
> https://stackoverflow.com/questions/1947481/how-many-significant-digits-should-i-store-in-my-database-for-a-gps-coordinate
> https://stackoverflow.com/questions/7167604/how-accurately-should-i-store-latitude-and-longitude
>
>> [4]
>> https://gis.stackexchange.com/questions/8650/measuring-accuracy-of-latitude-and-longitude
>

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