This is really useful, thanks. Also, perhaps this is one of those use cases where “Good Enough” is good enough http://c2.com/cgi/wiki?GoodEnough and http://queue.acm.org/detail.cfm?id=1988603
Caveat emptor and all that -- Puneet Kishor Just Another Creative Commoner http://punkish.org/About > On Aug 8, 2016, at 12:55 AM, Bruce Bannerman > <[email protected]> wrote: > > FYI, info on the Apache SIS library. > > Martin Desruisseaux of Geomatys has done a good job with his presentation at > [2] that describes why spatial reference systems are important. > > Bruce > > > > ---------- Forwarded message ---------- > > On 5/08/2016, 19:59, "Jon Blower" <[email protected]> wrote: > > >Hi all, > > > >I¹ve been talking offline with Martin Desruisseaux of Geomatys, a CRS > >guru. He has given me lots of information that I thought this group might > >be interested in, to help better understand issues around CRSs. This is a > >complicated area so I apologise if I make any mistakes in transmitting > >Martin¹s wisdom! > > > >Firstly, a couple of useful references. Martin is in the Apache SIS > >project, which provides a Java library for spatial data, including very > >comprehensive treatment of CRSs and conversions between CRSs. The SIS > >developer guide [1] is still in development but already has a lot of > >useful information about CRSs, which is useful even if you don¹t use the > >library. He also has written a presentation [2], which is a great > >explanation of why we don¹t always use WGS84 lat-lon, the importance of > >different datums and some of the issues in transforming between CRSs. > > > >Secondly, a useful point that is not always understood by newcomers. We > >usually talk about latitude and longitude, but often forget about the > >third dimension (ellipsoidal height). When converting between CRSs that > >use different datums, we need the third dimension as well. EPSG > >guidelines say that, if the height is missing, reasonable assumptions are: > > > >1. Height = 0 (i.e. we are standing on the surface of the ellipsoid) > >2. The height is given by a digital elevation model (i.e. we are standing > >on the surface of the planet) > > > >These two assumptions will, of course, lead to different answers for *all > >three* coordinates in the ³new² system (and both assumptions might be > >wrong). This could be important on the Web, because we frequently give > >latitude and longitude, but no information about ellipsoidal height. This > >means that if we convert these coordinates into a new system, we will get > >an uncertain position (and even the horizontal positions in the new > >system are uncertain, not just the height). [Does the Best Practice > >document mention this?] > > > >Finally, the process of converting coordinates between CRSs that use > >different datums can be very involved. Few open-source libraries do it > >³properly² (this is one of the gaps that SIS hopes to fill). I have a lot > >more information from Martin on this point but I think this email is long > >enough already! > > > >Hope this is helpful! > >Jon > > > > > >[1] http://sis.staging.apache.org/book/en/developer-guide.html > >[2] http://home.apache.org/~desruisseaux/SIS/2016-05.odp > > > > > _______________________________________________ Discuss mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/discuss
