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Re: How can I declare that the coordinates I publish are only approximations?

From: Frans Knibbe | Geodan <frans.knibbe@geodan.nl>
Date: Wed, 02 Oct 2013 12:35:28 +0200
Message-ID: <524BF6F0.9050801@geodan.nl>
To: public-geosemweb@w3.org
Hello,

Significant figures could be problematic if the precision is on the left 
side of the decimal point. In that case we have to use scientific 
notation. That means that with XML-based data we need to check if the 
coordinates are based on a data type that supports scientific notation. 
I believe the types /xsd:float/ and /xsd:double/ support scientific 
notation. I also believe that the /gml:pos/ element consists of 
/xsd:double/s, so I think that any GML geometry using the currently 
preferred way of using /gml:pos/ for coordinates can be written using 
scientific notation. Whether all software handling the geometries can 
handle scientific notation is another matter. I have little experience 
with that, because luckily I never had to work with geometries where the 
significant figures were on the left side of the decimal point. In that 
case it is simply a matter of rounding the coordinates to the right 
number of figures after the decimal point.

I also tried to check other means of encoding geometries in RDF. I think 
WKT should also support scientific notation, but I could not find proof 
in the time I spent on the matter. Encoding geometries as sequences of 
other point geometries, as advocated by the neogeo vocabulary, should 
also work with scientific notation because the individual coordinates 
are of type/xsd:float/ in the W3C basic geo vocabulary.

Regards,
Frans


On 30-9-2013 14:08, John Goodwin wrote:
>
> Hi,
>
> That’s a fair point. I did ask a colleague who is more of a GML expert 
> than me if there was a way to express significant figures in GML. She 
> wasn’t aware of a way.
>
> Regards,
>
> John
>
> *Dr John Goodwin*
>
> *Principal Scientist*
>
> *Research, Ordnance Survey *
>
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>
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>
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>
> *From:*Frans Knibbe | Geodan [mailto:frans.knibbe@geodan.nl]
> *Sent:* 27 September 2013 09:12
> *To:* public-geosemweb@w3.org
> *Subject:* Re: How can I declare that the coordinates I publish are 
> only approximations?
>
> Hello Lars,
>
> Aren't all coordinates in geographical data sets approximations?
>
> I think the right thing to do is to use the appropriate amount of 
> significant digits <http://en.wikipedia.org/wiki/Significant_figures> 
> for the coordinates. Is it possible for you to quantify the amount of 
> approximation?
>
> Regards,
> Frans
>
> On 12-9-2013 13:16, Svensson, Lars wrote:
>
>     All,
>
>       
>
>     A few months ago I asked for advice on how to represent coordinates for maps and got very valuable insights. Now I have a related question: When libraries and archives digitise historic maps they also create metadata for them, including a best-effort attempt to determine the map's geographic coverage. It goes without saying that the determined coordinates are mere approximations. When we publish that data in RDF, is there a way to express in geosparql (or WKT) that the specified coordinates are only approximations? I looked in the geosparql vocabulary and WKT but could not find any information.
>
>       
>
>     Thanks in advance,
>
>       
>
>     Lars
>
> --------------------------------------
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Received on Wednesday, 2 October 2013 10:36:03 UTC

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