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Re: Requirements for units of measure, accuracy and precision

From: Jeremy Tandy <jeremy.tandy@gmail.com>
Date: Wed, 31 Aug 2016 15:13:57 +0000
Message-ID: <CADtUq_0We5YJ3e_Q8fMuf3Jwr_o3Wx=rgTmVQ=rTvg4dTsGVJg@mail.gmail.com>
To: Joshua Lieberman <jlieberman@tumblingwalls.com>, "Little, Chris" <chris.little@metoffice.gov.uk>
Cc: Rob Atkinson <rob@metalinkage.com.au>, "Simon.Cox@csiro.au" <Simon.Cox@csiro.au>, "frans.knibbe@geodan.nl" <frans.knibbe@geodan.nl>, "public-sdw-wg@w3.org" <public-sdw-wg@w3.org>
Ignoring the UoM thing for now (although I can see @josh's point about it
being needed to define CRS), can folks tell me whether BP 5 "Describe the
positional accuracy of spatial data" [1] is on the right track to meet
requirements about precision and accuracy? If not, please can one of you
fine folks make some suggests content changes?

Thanks in advance. Jeremy

[1]: http://w3c.github.io/sdw/bp/#desc-accuracy

On Wed, 31 Aug 2016 at 12:32 Joshua Lieberman <jlieberman@tumblingwalls.com>
wrote:

> Given that there are specific spatial aspects to both precision and uom,
> for example a precision ellipsoid is a geometry and a unit of measure such
> as an altitude is relatively to a datum such as a geoid, it seems
> reasonable to me that these can be included SDW BP's without worrying
> overmuch whether DWBP will ever get to these sorts of multi-dimensional and
> affine space issues.
>
> Josh
>
> Joshua Lieberman, Ph.D.
> Principal, Tumbling Walls Consultancy
> Tel/Direct: +1 617-431-6431
> jlieberman@tumblingwalls.com
>
> On Aug 31, 2016, at 07:19, Little, Chris <chris.little@metoffice.gov.uk>
> wrote:
>
> Simon, Rob,
>
>
>
> I support submitting a formal public comment to DWBP, AND doing something
> specific to spatial.
>
>
>
> Chris
>
>
>
> *From:* Rob Atkinson [mailto:rob@metalinkage.com.au
> <rob@metalinkage.com.au>]
> *Sent:* Friday, August 26, 2016 1:42 AM
> *To:* Simon.Cox@csiro.au; frans.knibbe@geodan.nl; public-sdw-wg@w3.org
> *Subject:* Re: Requirements for units of measure, accuracy and precision
>
>
>
> When this was raised and we asked Phil for guidance, we were basically
> told DWBP is pretty much fixed, if we care it behooves us to address it
> even ifs its a more general issue. I think we just need to note its a more
> general issue, and provide what we feel is a BP otherwise there is really
> no useful path to implementation.
>
>
>
> Given precision is a first class concern of spatial data - as spatial
> resolution is going to be ever critical in the Internet of Things
> especially, we need to handle this.  Precision may be common to a dataset
> due to a common methodology, specific to a sensor, or time-varying - such
> as GPS coordinates. The BP needs to address how to attach this at this
> different levels of granularity. We need to cover both measured position
> and gridded coverages as well, and preferably not with completely different
> approaches. Precision is not generally an attribute of a CRS, unless we are
> going to define CRS for every possible grid - and that would be a fairly
> major departure from existing practice i suspect.
>
>
>
> UoM is a _similar_ case, and not inherently spatial, so i agree it can be
> localised to CRS , TRS,  but I see no harm in tying it in to the same
> pattern as spatial precision - whats needed is a BP to provide metadata
> about values, to meet the hard requirement of spatial CRS and precision,
> but its useful to suggest this would be applicable to UoM. Certainly having
> a completely different set of patterns for attribution of uom and CRS feels
> like sub-optimal practice.
>
>
>
> Rob
>
>
>
>
>
> On Fri, 26 Aug 2016 at 10:09 <Simon.Cox@csiro.au> wrote:
>
> Ø  The lack of information on units of measure and the apparent lack of
> concern for proper indication of uncertainty in numbers are widespread in
> spatial data and something should definitely be done about that. However, I
> maintain that both problems are more general than spatial data and are
> therefore out of scope for the UCR document. We have tried hard to limit
> the UCR requirements to only spatial data on the web.
>
>
>
> Frans – I agree with this assessment in principle. Should we try to pass
> it back up to the DWBP group?
>
>
>
> Best Practice 7 https://www.w3.org/TR/dwbp/#quality refers off to the
> Data Quality Vocabulary, which may mean they think it is dealt with, but
> I’m not convinced. It’s one more step away from the readers, at least, and
> none of the examples show units of measure, or suggest where they would be
> found. ‘units’ or ‘uom’ does not appear in their document (in the sense
> that we mean, anyway).
>
>
>
> However, this document is still in ‘Working Draft’ status, although it is
> certainly a pretty mature document by now. We could start by making a
> comment on their public list
> https://lists.w3.org/Archives/Public/public-dwbp-comments/ to the effect
> that
>
>
>
> ‘The requirement to know the units of measure for quantitative data (or
> another kind of ‘reference system’ for other kinds of data values) is not
> mentioned in https://www.w3.org/TR/dwbp/. Units are required in order to
> use quantitative data, so the Data on the Web Best Practices should include
> a recommendation on the topic. This affects the following benefit
> categories: Comprehension (C), Processability (P), Reuse (R)’
>
>
>
> But perhaps this is a place where some more formal coordination between
> the groups is warranted?
>
> @phila – has this issue ever been discussed in the DWBP group? A quick
> search of the email archive doesn’t turn anything up.
>
>
>
> --
>
> As a side comment – it is kinda remarkable how this gets overlooked. I
> raised it in the Open Knowledge forum on their Data Packaging/Frictionless
> Data initiative. As soon as I mentioned it there was acceptance that it is
> in scope, but somehow no-one had brought it up until then.
>
>
>
> Simon
>
>
>
> *From:* Frans Knibbe [mailto:frans.knibbe@geodan.nl]
> *Sent:* Thursday, 25 August 2016 10:50 PM
> *To:* SDW WG Public List <public-sdw-wg@w3.org>
>
>
> *Subject:* Requirements for units of measure, accuracy and precision
>
>
>
> Hello,
>
>
>
> In messages
> https://lists.w3.org/Archives/Public/public-sdw-wg/2016Aug/0172.html and
>
> https://lists.w3.org/Archives/Public/public-sdw-wg/2016Aug/0109.html the
> possibility of adding new requirements to the UC&R doc was brought forward.
> Those should be requirements that
>
>
>
> A) the units of measure (UoM) in spatial data should be made clear
>
> B) the precision of spatial data should be made clear
>
>
>
> From the looks of it, those requirements would at least be requirements
> for the BP deliverable.
>
>
>
> The topic was also discussed in the last SSN teleconference
> <https://www.w3.org/2016/08/23-sdwssn-minutes>. I thought it would be
> good to create a separate thread for these related issues.
>
>
>
> I will first repeat my initial response: The requirements are very good
> requirements. The lack of information on units of measure and the apparent
> lack of concern for proper indication of uncertainty in numbers are
> widespread in spatial data and something should definitely be done about
> that. However, I maintain that both problems are more general than spatial
> data and are therefore out of scope for the UCR document. We have tried
> hard to limit the UCR requirements to only spatial data on the web. This
> constraint is specifically mentioned in the section on methodology
> <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#Methodology>.
> If we were to neglect this constraint, then the amount of requirements
> could run out of hand quickly, the decisions on which requirements to
> include or not would become very arbitrary and the deliverable teams that
> are tasked with meeting requirements would inevitably find out that they
> are not in a position to meet the requirements because they are not in
> scope for their work. Of course the deliverable teams will work with
> additional requirements next to the ones mentioned in the UCR document.
> Those additional requirements will be based on general best practices. I
> think the UoM and precision requirements fall in that class.
>
>
>
> That said, perhaps there is a way to shape the requirements in such a way
> that they can be included in the UCR document, without violating the
> spatial scope constraint too harshly. After all, it has been done for other
> requirements too, to be honest.
>
>
>
> Let's start with the UoM requirement (A). I assume that is about the UoM
> of coordinate data only. I think this is already implicitly covered by the
> CRS requirements Linking geometry to CRS
> <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#LinkingCRS>,
> Determinable CRS
> <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#DeterminableCRS>
> and CRS definition
> <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#CRSDefinition>:
> If those requirements are met, it should always be possible to know the UoM
> of coordinates, because the UoM will be part of the CRS definition. Perhaps
> we should be explicit in mentioning that a CRS definition should include an
> indication of UoM?
>
>
>
> As for the requirement B, if we change the wording a bit the requirement
> could be made applicable to spatial data only and therefore be in scope:
>
>
>
> B2) The use of precision that matches uncertainty in coordinate data
> should be facilitated and encouraged
>
>
>
> With this kind of wording I think the BP editors have a fighting chance of
> meeting the requirement.
>
>
>
> Please share your thoughts...
>
>
>
> Regards,
>
> Frans
>
>
Received on Wednesday, 31 August 2016 15:14:39 UTC

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