Glittertinden, a mountain in Norway have a geopos 61.651222 N 8.557492 E, alternate geopos 6835406.62, 476558.22 (EU89, UTM32).

Some of the mountains are measured to within a millimeter in elevation. For example Ørneflag is measured to be at 1242.808 meter, with a position 6705530.826, 537607.272 (EU89, UTM32) alternate geopos 6717133.02, 208055.24 (EU89, UTM33). This is on a bolt on the top of the mountain.
There is an on-going project to map the country withing 1x1 meter and elevation about 0.2 meter.

One arc second is about 1 km, so five digits after decimal point should be about 1 cm lateral precision.

Goepositions isn't a fixed thing, there can be quite large tidal waves, and modelling and estimating them is an important research field. The waves can be as large as ~0.3 meter. (From long ago, ask someone working on this.) Estimation of where we are is to less than 1 cm, but I have heard better numbers.

All geopos hould have a reference datum, without it it is pretty useless when the precision is high. An easy fix could be to use standard profiles with easy to recognize names, like "GPS", and limit the precision in that case to two digits after decimal point on an arc second.

Note that precision in longitude will depend on actual latitude.

On Fri, Sep 1, 2017 at 9:43 PM, Peter F. Patel-Schneider wrote:
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:
> 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
>

_______________________________________________
Wikidata mailing list
Wikidata@lists.wikimedia.org
https://lists.wikimedia.org/mailman/listinfo/wikidata