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Re: Some thoughts on http://www.w3.org/TR/sdw-bp/

From: Ed Parsons <eparsons@google.com>
Date: Thu, 16 Mar 2017 21:12:39 +0000
Message-ID: <CAHrFjckg7QkaoBRRNokwVJF0YAcCt23fdLJ0YGW_9Q9WxO69ow@mail.gmail.com>
To: Peter Parslow <Peter.Parslow@os.uk>, "public-sdw-comments@w3.org" <public-sdw-comments@w3.org>
Hi Peter,

You make a very good point about precision, but the examples are actually
what you are returned... I'm not sure we want to get into questions of
precision for the intended audience who are generally looking to express a
location globally within a few metres...

Of course this leads us nicely into the discussion of samePlace as....


On Thu, 16 Mar 2017 at 12:54 Peter Parslow <Peter.Parslow@os.uk> wrote:

> Hi all,
> This is becoming a really useful resource - as an introduction to the
> subject area, as well as a set of best practices. Sorry to have not been
> more involved.
> I've been reading it again (along with other related articles), prior to
> attending next week's meeting, and a few things come to mind which could
> become improvements.
> 1. URI for Eddystone Lighthouse. It is of course fine to use a URI from
> Deutsche Nationalbibliothek, and a URL from Wikidata, if only to
> demonstrate that many URIs can identify the same spatial thing. But I think
> it would be good to honour the actual owners/operators of the lighthouse by
> using their reference:
> https://www.trinityhouse.co.uk/lighthouses-and-lightvessels/eddystone-lighthouse
> 2. in section 8 about CRS, it says that lat & long "can express a location
> to within a few metres". In fact there's nothing to stop a lat/long being
> as precise as you like, and I often find ones which are more precise than
> their accuracy should support. By which I mean that quite a lot of software
> defaults to serialising real numbers as decimals with six digits after the
> decimal place - which is something like 1cm on the ground. That accuracy
> can only be achieved with professional equipment, but a discussion of
> accuracy & precision may be out of place in this document (at least, at
> this section). But perhaps dropping the 'within a few metres' bit would be
> appropriate..
> And in fact, example 3 gives lat/long with seven and 12 decimal places:
> seven is millimetre accuracy; I'd have to Google the name for a unit small
> enough for 12 decimal places of a degree - probably smaller than an atom!
> See
> http://gis.stackexchange.com/questions/8650/measuring-accuracy-of-latitude-and-longitude/8674#8674
> Can I recommend reducing the number of digits in the examples.
> 3. Further on in section 8, I'm not sure that EPSG:4277 is "the UK
> National Grid"; my reading of the EPSG register is that this is the code
> for OSGB36, which is the geodetic reference system on which the project
> British National Grid is based. The code for the British National Grid is
> 27700.
> We don't call it "UK" national grid, because Ireland generally uses a
> different one.
> That's all for today; I may get time to read a bit more tomorrow.
> Peter
> Peter Parslow
> Principal Geographic Information Architect
> Products & Innovation, Ordnance Survey
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*Ed Parsons *FRGS
Geospatial Technologist, Google

+44 7825 382263 @edparsons
Received on Thursday, 16 March 2017 21:13:23 UTC

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