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

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#",
     "qudt": "http://qudt.org/schema/qudt#",
     "skos": "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/",
       "@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
>
>