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Precision and accuracy for spatial data. (Was: Re: Agenda requests - 14th January telcon)

From: Nikos Andronikos <Nikos.Andronikos@cisra.canon.com.au>
Date: Thu, 14 Jan 2016 01:09:14 +0000
To: "Little, Chris" <chris.little@metoffice.gov.uk>
CC: www-svg <www-svg@w3.org>, Kerry Taylor <Kerry.Taylor@acm.org>, Ed Parsons <eparsons@google.com>, "sdwwg@lists.opengeospatial.org" <sdwwg@lists.opengeospatial.org>, Satoru Takagi <sa-takagi@kddi.com>
Message-ID: <97F66AD1-F1FF-4466-8D31-101452D450E4@cisra.canon.com.au>
Hi Chris,

> On 13 Jan 2016, at 9:32 PM, Little, Chris <chris.little@metoffice.gov.uk> wrote:
>
> Hello Nikos, and WG members,
>
> I am usually travelling while the telcos happen (Thursday evening local time) so cannot attend. [Unusual lifestyle.]
>
> I have had a topic re SDW WG and SVG WG overlap, arising out of the proposals of Satoru Takagi, that was discussed in SDW WG and with Cameron McCormack. As the Spatial Data for the Web WG is very close to publishing its first public working draft, I have thought carefully about the topic and come to the conclusion that there is no real, non-trivial, overlap.
>
> The simple, ostensibly trivial, overlap is specifying that user coordinates in the ranges of (-180, +180) and (-90, +90) could be longitude and latitude. In particular, longitude/latitude coordinates from the WGS84 system, as used by GPS.
>
> A problem with this is the confusion of precision and accuracy. Whatever the precision of the specified coordinates, the accuracy of positioning on the actual earth's surface will only approach about a metre horizontally and may have errors of up to 100 metres vertically, because of assumptions about reference systems, tectonic plate movements and the 'height' of the earth's 'surface'. For example, North America and Europe are receding from each other by a couple of centimetres per year, whereas Australia is moving several centimetres per year in a northerly direction.
>
> I propose that somewhere in the SVG document is an non-normative statement like the previous two paragraphs.
>
> I am happy for you guys to debate in my absence, and I will help with any drafting of words if you decide to include something.
>
> I've included the SDW WG and the Chairs for their info. We may end up with a similar statement in the Best Practice.
>
> HTH, Chris

I’ve added the topic to the agenda for the next telcon.

(I’m really just sending this reply so that I can move the email to www-svg for archiving)

Nikos.

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Received on Thursday, 14 January 2016 01:09:51 UTC

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