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

Thanks for the insights Simon.

It will take some care to turn this into a best practice recipe that doesnt
get broken immediately IMHO.
We can get out of jail from an engineering perspective by saying you should
use a URI identifier for UoM that allows content-negotiation to access one
or more representations.
Recommended representations include:
1) QUDT structural description
2) SKOS as a canonical means to describe labels and provide links to
alternative codes
3) UCUM specification if relevant for the UoM
4) OWL-class ?
4) Any representations defined by standards organisations relevant to the
community of practice

(Content negotiation can be driven by MIME-type in headers or by explicit
view parameters - need a separate BP around this that encompasses the UK
and other LDA examples - its a pattern that generally allows us to take on
a de-facto option and migrate to a de jure standard when it evolves - which
we see as the most common pattern just about everywhere.  We also either
need to specify a set of views and their corresponding OWL models , or a
way to bind any view to its relevant OWL model in a general way )

We can further recommend the UCUM URI structure.

If necessary we can deploy such representations - I dont mind taking on the
deploying using the URI redirection machinery I have deployed at
resources.opengeospatial.org. Would prefer someone to provide some endorsed
representations - HTML, JSON-LD, RDF  - for QUDT, SKOS and OWL-class.

Minimum would be for some examples (simple, derived-with UCUM equiv,
derived-without UCUM equiv). A complete set would be just as easy to deploy.



On Mon, 4 Jul 2016 at 19:23 Jon Blower <j.d.blower@reading.ac.uk> wrote:

> Ø  Ideally we would have a reliable set of URIs for UOMs which could
> leverage the UCUM algorithm to build the URI, and which would resolve to a
> QUDT-based representation of the unit of measure.
>
>
>
> +1
>
>
>
> Is it possible to use the UCUM symbol for the UoM the URI suffix? Or are
> there problems like character-encoding issues?
>
>
>
> Cheers,
> Jon
>
>
>
> *From: *"Simon.Cox@csiro.au" <Simon.Cox@csiro.au>
> *Date: *Monday, 4 July 2016 01:13
> *To: *"rob@metalinkage.com.au" <rob@metalinkage.com.au>, Jon Blower <
> sgs02jdb@reading.ac.uk>, Maik Riechert <m.riechert@reading.ac.uk>, "
> public-sdw-wg@w3.org" <public-sdw-wg@w3.org>
> *Subject: *RE: Units of Measure (BP, Coverages, SSN,Time?)
>
>
>
> Lets be clear about what QUDT and UCUM actually offer.
>
>
>
> QUDT -
>
> ·         primarily provides a model for descriptions of units of
> measure, and of quantity-kinds (a.k.a. qualities, or “observable
> properties”); the model is formalized using OWL, and thus provides an
> RDF-based syntax for description of a uom or a quantity-kind
>
> ·         also provides some lists (called ‘vocabularies’) of individual
> unit- and quantity-kind- descriptions, but which is very idiosyncratic and
> incomplete (includes a whole bunch of currencies!)
>
> ·         there are no rules for how the labels or symbols for units are
> built in the QUDT vocabularies; they are not aligned with the ISO or SI
> standards (e.g. the label for the unit of length is spelled ‘Meter’, and
> the symbol for the unit of temperature is ‘degC’), capitalization is
> inconsistent, and use of non-asci character set is variable
>
> ·         the maintenance arrangements for QUDT are private (TopQuadrant
> +  NASA) and the publication arrangements are flaky (QUDT v2.0 has been ‘on
> the way’ for about 3 years, and even though it is linked the qudt.org
> website, it has been 404 for over a year).
>
>
>
> UCUM –
>
> ·         Focuses on a rule for how to generate a symbol for a ‘derived
> uom’
>
> ·         uses a rigorous algorithm based on a theory of quantities and
> dimensional analysis, which starts from any base set of units in a rational
> system (SI, MKS, cgs, even pounds-feet-seconds if you want!)
>
> ·         UCUM provides a base set of symbols corresponding essentially
> with SI, plus symbols for the standard power of ten prefixes
> (micro/milli/kilo/mega etc). The base set has some fudging to get around
> the anomaly that the SI base unit for mass (kg) already has a power-of-ten
> prefix built in.
>
> ·         The algorithm and base set of symbols is such that symbols
> generated following UCUM are aligned with conventional usage, and with ISO
> 1000
>
> ·          There is some additional notation using {} and [] to allow for
> annotations and ‘conventional’ units, which I always get confused about.
>
>
>
> My assessment is that the QUDT Ontology v1.1 is good enough, (I was on an
> Ontolog telecon with Pat Hayes, Ralph Hodgson, Gary Berg-Cross a couple of
> years ago where that was the clear consensus) but the QUDT vocabularies are
> not. So we need another set of URIs denoting uoms, with the expectation
> that dereferencing one of these would result in a QUDT-based
> representation.
>
> Ideally we would have a reliable set of URIs for UOMs which could leverage
> the UCUM algorithm to build the URI, and which would resolve to a
> QUDT-based representation of the unit of measure. These representations
> should be built on-the-fly using the UCUM engine.
>
>
>
> Note that, using QUDT, a uom description is an OWL _*individual*_ (not a
> class), but with complete semantics, still supporting some reasoning. Rob –
> going with individuals doesn’t mean you have to us SKOS and certainly
> doesn’t lose semantic precision -  probably best not to casually suggest
> that!
>
>
>
> Simon
>
>
>
>
>
> *From:* Rob Atkinson [mailto:rob@metalinkage.com.au]
> *Sent:* Saturday, 2 July 2016 1:32 PM
> *To:* Jon Blower <j.d.blower@reading.ac.uk>; Rob Atkinson <
> rob@metalinkage.com.au>; Cox, Simon (L&W, Clayton) <Simon.Cox@csiro.au>;
> m.riechert@reading.ac.uk; public-sdw-wg@w3.org
> *Subject:* Re: Units of Measure (BP, Coverages, SSN,Time?)
>
>
>
> Hi Jon
>
>
>
> The encoding scheme issue raises a duality between class and instance -
> any UoM could be expressed as as either an instance (with SKOS encoding as
> a natural default) or a Class - RDFS or OWL being the default options. In
> addition a meta-model of UoM could be defined in RDFS or OWL and used to
> drive encodings of instances.
>
>
>
> Personally, I think that in the Web we should specify that a URI is used
> if one is available - and that an encoding of its details may be used as
> annotation. In the case of an "anonymous" UoM, then the encoding will still
> probably need to reference base units using URIs.
>
>
>
> The wrinkles are whether URIs are explicit, or encoded as items in a
> namespace - and whether any encoding scheme (model) may be used or one is
> recommended, and if the model itself needs to be explicitly referenced
> (presumably this applies to JSON-LD, RDFA etc as RDF will always use URIs
> to specify the model elements anyways.
>
>
>
> A worked example set with:
>
> 1) just URI from a well-known vocabulary (UCUM)
>
> 2) A encoded UoM with one URI, and a simple label
>
> 3) ditto, with a more complex set of details
>
> 4) ditto with more that one URI (e.g. UCUM and QUDT)
>
> 5) a blank/anonymous encoded UoM with base measures.
>
>
>
> Would we go so far as to recommend QUDT as the meta-model (as per example
> provided?) - or simply list a few in use and provide a couple of examples?
>
>
>
> This will cover the "follow-your-nose" cases - however there is the case
> of a data encoding where the UoM is specified in metadata. The question
> here then is defining a BP for this metadata.
>
> One option - we can use RDF-QB to define data structures and relevant UoM.
> I'm not sure there is an obvious alternative to ad-hoc metadata models and
> UoM specified any non-interoperable way that emerges.
>
>
>
> This option then speaks directly to the coverages metadata perspective
> (encoding of data using RD-QB becomes a trivial case - we simply state that
> if RDF encoding, then BP would be to use RDF-QB encoding consistent with
> the RDF-QB metadata for the set, and the interesting and more generally
> useful case is describing an existing or compact encoding usefully)
>
>
>
> Rob
>
>
>
> On Sat, 2 Jul 2016 at 02:20 Jon Blower <j.d.blower@reading.ac.uk> wrote:
>
> Hi Rob – yes, I think those are the missing bits, but, just to reiterate,
> it may not be (just) a “vocabulary” that we need (in the sense of a set of
> URIs), but a serialisation scheme for any unit.
>
>
>
> For concrete examples, we should look at where we need to use units. I
> think we have:
>
>
>
> 1.       As part of coordinate systems and coordinate reference systems
>
> 2.       As part of measured quantities (e.g. the range of a coverage),
> linked to observed properties etc
>
> 3.       …
>
>
>
> My last paragraph wasn’t very clear, sorry. I was trying to say that the
> different uses (coordinate systems, observed properties) might actually
> have different best practices in terms of the encoding of their units. We
> could feasibly decide that coordinate system units are best expressed as
> URIs, but the units of observed properties are better expressed as strings
> in a named serialisation scheme (like UCUM). Maybe, I don’t know – just
> raising the possibility.
>
>
>
> Cheers,
> Jon
>
>
>
>
>
> *From: *Rob Atkinson <rob@metalinkage.com.au>
> *Date: *Friday, 1 July 2016 14:39
> *To: *Jon Blower <sgs02jdb@reading.ac.uk>, Rob Atkinson <
> rob@metalinkage.com.au>, "Simon.Cox@csiro.au" <Simon.Cox@csiro.au>, Maik
> Riechert <m.riechert@reading.ac.uk>, "public-sdw-wg@w3.org" <
> public-sdw-wg@w3.org>
>
>
> *Subject: *Re: Units of Measure (BP, Coverages, SSN,Time?)
>
>
>
> This is the type of recommendation i think we need. Lets refine... the
> missing bits are:
> 1 guidance on what vocabulary.. even noting that different communities use
> different ones and naming them is a help.
> 2 provision of mappings if you want to interoperate across community
> choice here.. do you embed multiple uris, or provide sone sort of sameAs
> service?
> 3 concrete examples
>
> I dont quite follow the final paragraph and the implications for what the
> encoding would look like?
>
> Rob
>
>
>
> On Fri, 1 Jul 2016 11:12 am Jon Blower <j.d.blower@reading.ac.uk> wrote:
>
> Just to add a little to this – units of measure are very tricky in
> general. The overall requirement, I think, is to have an unambiguous
> serialisation scheme for units, including both base units (the easy cases)
> and the infinite number of derived units (the hard cases) – that is to say,
> a spec for serialising units to ASCII strings. This allows clients to
> convert between units, which is a primary use case for having “strongly
> typed” units.
>
>
>
> In terms of serialisations, I’m aware of UCUM and UDUNITS (the latter is
> used extensively in climate/met/ocean and is connected with CF). I don’t
> think either are perfect in terms of governance, and I’m not even sure that
> UDUNITS has a formal spec.
>
>
>
> Then there are URIs. QUDT has URIs for a lot of base and derived units,
> but it can’t possibly have them all, hence the need for a scheme that
> allows any unit to be serialised. So there will always be gaps, but I note
> that QUDT covers a lot of the common cases I can think of – so it’s not
> clear to me how important the gaps are.
>
>
>
> Typical clients will just want to display the symbol for the unit, so we
> should make sure that, if we use URIs, we also transmit the symbol, as I
> doubt that a typical web client will want to resolve the URI and look up
> the symbol. This is effectively what Maik is doing, by transmitting the
> symbol plus a URI for the unit **scheme** rather than a URI for the unit
> itself.
>
>
>
> (Question – does QUDT use UCUM as a means of generating the unit symbol?)
>
>
>
> There are a few tricky cases in science – e.g. salinity, which strictly
> has no units and is a very weird kind of quantity – and sometimes these
> tricky cases lead to poor practice in real data files – i.e. expressing
> units incorrectly or inconsistently. (and of course, poor practice can
> happen in real-world data files anywhere).
>
>
>
> I think an overall BP recommendation would be:
>
>
>
> 1.       Express units unambiguously if possible, using a named unit
> serialisation scheme or URI.
>
> 2.       Give the unit symbol, and perhaps a longer explanatory text
> string (e.g. a rdfs:label), to help simple clients understand the unit,
> even if they don’t want to resolve the full unit description.
>
> 3.       Also allow users to record “ad hoc” unit strings for fallback
> cases that don’t fit well with existing serialisation or URI schemes,
> making it clear that these are not really machine-understandable
>
>
>
> There may be cases where we can refine this further depending on the use
> case. For example, in CRS definitions, which tend to use simple units, it’s
> probably desirable to use well-known URIs to represent units. For recording
> the units of a measured quantity (e.g. the range of the coverage), I like
> methods like the one Maik suggested, as this maps more neatly to common
> practice in my community.
>
>
>
> Cheers,
>
> Jon
>
>
>
>
>
> *From: *Rob Atkinson <rob@metalinkage.com.au>
> *Date: *Friday, 1 July 2016 08:46
> *To: *"Simon.Cox@csiro.au" <Simon.Cox@csiro.au>, "rob@metalinkage.com.au"
> <rob@metalinkage.com.au>, Maik Riechert <m.riechert@reading.ac.uk>, "
> public-sdw-wg@w3.org" <public-sdw-wg@w3.org>
>
>
> *Subject: *Re: Units of Measure (BP, Coverages, SSN,Time?)
>
> *Resent-From: *<public-sdw-wg@w3.org>
> *Resent-Date: *Friday, 1 July 2016 08:47
>
>
>
> Perfect Simon - thanks.
>
> Its not that obvious trawling the docs what the pragmatic aspects are.
>
>
>
> So I would suggest then that a BP endorsed by OGC would have a minimum
> requirement that a mapping to UCUM is provided for any vocabulary used for
> UoM, to provide for compatibility with existing recommendations (can we
> call these BP?)
>
>
>
> If it helps I could set up a OGC resource for UCUM - with redirects for
> specific terms - instead of to the containing spec (thats the way UCUM
> works) - or to a SKOS resource with skos:exactMatch relationships to the
> UCUM terms.  I can also deploy a crosswalk to UCUM from another UoM vocab
> if we decide to recommend it.
>
>
>
> The onoging governance of such a resource in the context of the BP can be
> taken up as a action from the SDW to the OGC (what is the appropriate point
> of contact here? NA, OAB, TC, PC?)
>
>
>
> Rob
>
>
>
> On Fri, 1 Jul 2016 at 16:10 <Simon.Cox@csiro.au> wrote:
>
> 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/ 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
>
> [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>
> 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
>
>
>
>
>
>
>
>