[OSGeo-Discuss] Fwd: FW: Some more stuff about coordinate reference systems

P Kishor punk.kish at gmail.com
Mon Aug 8 01:42:28 PDT 2016

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

> On Aug 8, 2016, at 12:55 AM, Bruce Bannerman <bruce.bannerman.osgeo at gmail.com> 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" <j.d.blower at reading.ac.uk> 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
> >

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