Not sure if I would go for it, but… "Precision for the location of the center should be one percent of the square root of the area covered."
Oslo covers nearly 1000 km², that would give 1 % of 32 km or 300 meter or 0.3 arc seconds. On Mon, Nov 6, 2017 at 2:50 AM, John Erling Blad <[email protected]> wrote: > 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 < > [email protected]> 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 <[email protected]> >> 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_tool >> ref/geoprocessing_environments/about_coverage_precision.htm >> > https://stackoverflow.com/questions/1947481/how-many-signifi >> cant-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-accur >> acy-of-latitude-and-longitude >> > >> >> _______________________________________________ >> Wikidata mailing list >> [email protected] >> https://lists.wikimedia.org/mailman/listinfo/wikidata >> > >
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