Re: [Wikidata-l] Precision of globe coordinates

Big picture it is important to recognize the importance of geodetic datums. The market leading datum is WGS84 or GPS because it is (1) good enough for military work, and (2) GPS hardware is everywhere. Before the space age, geodetic datums were determined by optical observations forming a network across a region. Islands far from the coast would get their own datum because the position of the island itself relative to the mainland is uncertain. In today's globalized world we have more reasons for datums. I often see road crews using specialized GPS equipment with large antennas. These systems establish a datum around base station(s) which can be precise to the centimetric range in the most advanced systems. Measurements made with that kind of system will be NOT be precisely comparable with measurements made in other places, but you could dumb down the claim to "WGS84" because it would still be that good. Then there is quality in the sense of conformance to requirements; we might muck with some coordinates to make data useful. For instance, if you use a quality hand GPS to survey stars on the Hollywood Walk of Fame and then plot the points on Google maps, you discover two MUST requirements are missing (i) the stars are all in the correct order, and (ii) on the correct side of the street which from the viewpoint of a pedestrian is a lot more important than the fact that my images of Hollywood and Vine are rotated a bit relative to the Big G's. Thus, an augmented reality mobile app for the Hall of Fame would require its own geodetic datum. I spend more time walking in the woods than I do in L.A., and in the woods there are similar but different concerns. If you walk on a path that closely follows a creek, for instance, a GPS trace may not agree with the actual sequence of creek crossings -- something that would drive me nuts if I was using a map while hiking in the woods. You're supposed to fix topological problems like that when you upload to Open Street Maps, so that is another sense of a privileged datum. On Tue, Jan 13, 2015 at 10:15 AM, Serge Wroclawski <emacsen(a)gmail.com> wrote: -- Paul Houle Expert on Freebase, DBpedia, Hadoop and RDF (607) 539 6254 paul.houle on Skype ontology2(a)gmail.com http://legalentityidentifier.info/lei/lookup