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Re: Some more stuff about coordinate reference systems [SEC=UNCLASSIFIED]

From: Bruce Bannerman <B.Bannerman@bom.gov.au>
Date: Sun, 7 Aug 2016 22:49:35 +0000
To: Jon Blower <j.d.blower@reading.ac.uk>, "public-sdw-wg@w3.org" <public-sdw-wg@w3.org>
Message-ID: <D3CDF786.2C912%B.Bannerman@bom.gov.au>
Thank you Jon.

Martin has done a great job with his presentation at [2].

I know that our web developers would like things spatial to be Œeasyı, but
that has consequences.

Bruce


On 5/08/2016, 19:59, "Jon Blower" <j.d.blower@reading.ac.uk> wrote:

>Hi all,
>
>Iıve 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
>Martinıs 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 donıt use the
>library. He also has written a presentation [2], which is a great
>explanation of why we donıt 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
>
Received on Sunday, 7 August 2016 22:50:11 UTC

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