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RE: Units of Measure (BP, Coverages, SSN,Time?)

From: <Simon.Cox@csiro.au>
Date: Fri, 1 Jul 2016 06:10:07 +0000
To: <rob@metalinkage.com.au>, <m.riechert@reading.ac.uk>, <public-sdw-wg@w3.org>
Message-ID: <f9b3da8d24c5440d932c7d648db3f02b@exch1-mel.nexus.csiro.au>
Ø  If OGC has adopted UCUM as a BP (can someone make a definitive statement on this …

OGC’s endorsement of UCUM comes from

1.      It is recommended in WMS [1]

2.      Ditto GML [2]

3.      There is a branch of the www.opengis.net/def/<http://www.opengis.net/def/> URI set for UCUM - http://www.opengis.net/def/uom/UCUM/ but just redirects to the UCUM spec [3]

But that is purely pragmatic, as it seemed to be the best thing around at the time.
It has a fragile governance arrangement, and URIs are not de-referenceable.

[1] http://www.opengeospatial.org/standards/wms version 1.3 clause C.2.
[2] http://www.opengeospatial.org/standards/gml v3.2.1 clause 8.2.3.6<http://www.opengeospatial.org/standards/gml%20v3.2.1%20clause%208.2.3.6>
[3] http://unitsofmeasure.org/ucum.html


From: Rob Atkinson [mailto:rob@metalinkage.com.au]
Sent: Friday, 1 July 2016 1:46 AM
To: Maik Riechert <m.riechert@reading.ac.uk>; Rob Atkinson <rob@metalinkage.com.au>; SDW WG Public List <public-sdw-wg@w3.org>
Subject: Re: Units of Measure (BP, Coverages, SSN,Time?)

Thanks Maik,

If i read this right, this example assumes the client understands qudt - then uses the semantics of qudt:symbol to map instances (Cel)  in another namespace to this.  UCUM uses http://purl.oclc.org/NET/muo/ucum/unit/temperature/degree-Celsius as the id - but the information to map to that is not present. Is "Cel" just a dummy example - would you actually want to say "degree-Celsius" - and in turn want the OGC redirect to respect that and redirect
http://www.opengis.net/def/uom/UCUM/degree-Celsius to   http://purl.oclc.org/NET/muo/ucum/unit/temperature/degree-Celsius?


What about the original assumption of using QUDT - why not use UCUM or another in the first instance. Coming from the outside and trying to identify a best practice, what exactly is this example saying?

If OGC has adopted UCUM as a BP (can someone make a definitive statement on this - it should be present in the BP when we talk about vocabulary re-use - a list of vocabularies in use in the OGC space) then we should start with that perhaps? If we are saying the BP requirement is to allow an emerging body of QUDT usage to interoperate then we need perhaps to recommend publishing the mappings as a resource - whatever we think is BP we need to communicate clearly to the average user who wont have years of exposure to the history and details to draw on - and will most likely simply want to maximise interoperability of a few cases.

Cheers
Rob

On Fri, 1 Jul 2016 at 01:00 Maik Riechert <m.riechert@reading.ac.uk<mailto:m.riechert@reading.ac.uk>> wrote:

Hi Rob,

I just wanted to throw in a slightly different/complementary view on this.

While it is useful to have URIs for any kind of unit, I think it is even more useful to have a symbolic coding in a certain coding scheme for those units, because then clients with support for that scheme can easily parse the unit, and transform it and the associated numbers. One scheme example is UCUM (http://unitsofmeasure.org/ucum.html). OGC gave it a URI as well: http://www.opengis.net/def/uom/UCUM/


In my opinion you would have something like that (JSON-LD):

{
  "@context": {
    "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"<http://www.w3.org/1999/02/22-rdf-syntax-ns>,
    "qudt": "http://qudt.org/schema/qudt#"<http://qudt.org/schema/qudt>,
    "skos": "http://www.w3.org/2004/02/skos/core#"<http://www.w3.org/2004/02/skos/core>
  },
  "rdf:value": 27.5, // for example purposes only
  "qudt:unit": {
    "@id": "qudt:DegreeCelsius",
    "skos:prefLabel": { "en": "Degree Celsius" },
    "qudt:symbol": {
      "@type": "http://www.opengis.net/def/uom/UCUM/"<http://www.opengis.net/def/uom/UCUM/>,
      "@value": "Cel"
    }
  }
}
So the main point is that the value of "qudt:symbol" has a custom data type, in this case http://www.opengis.net/def/uom/UCUM/.


Cheers

Maik



Am 30.06.2016 um 15:14 schrieb Rob Atkinson:
Hi,

I'm looking into the BP aspects around defining data dimensions as a framework for evaluating and contributing to various SDW threads. One which seems to cut across, but I havent seen an explicit treatment of the UoM problem. I know I may have missed previous conversatiosn - but I dont see any treatment in the current reviewable docs.

Specifically, if I was to follow the W3C Data on the Web Best Practices I would be led via BP #2

"To express frequency of update an instance from the Content-Oriented Guidelines developed as part of the W3C Data Cube Vocabulary efforts was used."

to this statement:
"To express the value of this attribute we would typically use a common thesaurus of units of measure. For the sake of this simple example we will use the DBpedia resource http://dbpedia.org/resource/Year which corresponds to the topic of the Wikipedia page on "Years".

If we have a Time ontology - surely we would be pointing to that as a recommendation for temporal units of measure.
Likewise, i would have thought that OGC would have an interest in binding CRS with their in built units of measure to spatial dimensions.
One could argue that without interoperability at this level there is a question why the OGC would have any involvement in Web standards - but if there is a counter-argument then I feel this needs to be front-and-centre of the BP to explain to a potential user what they can expect, and where they are going to be left with making all the significant decisions.

If we have Time and CRS UoM, then we may be able to get away with not specifiying a vocabulary for other UoM for measurements. Are there any obvious dimensions that need UoM vocabularies?

When I specify O&M profiles, (my driving use case), I'll need to specify the UoM for measurements - is there any recommendation regarding which vocabulary to choose?   And for CRS based dimensions?

Rob Atkinson



Received on Friday, 1 July 2016 06:11:28 UTC

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