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

From: Byron Cochrane <bcochrane@linz.govt.nz>
Date: Tue, 9 Aug 2016 09:15:17 +1200
To: 'Bruce Bannerman' <B.Bannerman@bom.gov.au>, 'Jon Blower' <j.d.blower@reading.ac.uk>, "'public-sdw-wg@w3.org'" <public-sdw-wg@w3.org>
Message-ID: <666FB8D75E95AE42965A0E76A5E5337E15D545244F@prdlsmmsg01.ad.linz.govt.nz>
A bit of informative humour


-----Original Message-----
From: Bruce Bannerman [mailto:B.Bannerman@bom.gov.au]
Sent: Monday, 8 August 2016 10:50 a.m.
To: Jon Blower; public-sdw-wg@w3.org
Subject: Re: Some more stuff about coordinate reference systems [SEC=UNCLASSIFIED]

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.


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
>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!
>[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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Received on Monday, 8 August 2016 21:16:36 UTC

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