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RE: Requirements for units of measure, accuracy and precision

From: Byron Cochrane <bcochrane@linz.govt.nz>
Date: Tue, 6 Sep 2016 11:33:24 +1200
To: "'Simon.Cox@csiro.au'" <Simon.Cox@csiro.au>, "'andrea.perego@jrc.ec.europa.eu'" <andrea.perego@jrc.ec.europa.eu>, "'frans.knibbe@geodan.nl'" <frans.knibbe@geodan.nl>, "'jeremy.tandy@gmail.com'" <jeremy.tandy@gmail.com>
CC: "'rob@metalinkage.com.au'" <rob@metalinkage.com.au>, "'jlieberman@tumblingwalls.com'" <jlieberman@tumblingwalls.com>, "'chris.little@metoffice.gov.uk'" <chris.little@metoffice.gov.uk>, "'public-sdw-wg@w3.org'" <public-sdw-wg@w3.org>
Message-ID: <666FB8D75E95AE42965A0E76A5E5337E15D7C7C597@prdlsmmsg01.ad.linz.govt.nz>
That was buried! I think it would be useful to provide a link in SDWBP to this (https://www.w3.org/TR/vocab-dqv/#ExpressDatasetAccuracyPrecision) if we hope for others to find this guidance.


-----Original Message-----
From: Simon.Cox@csiro.au [mailto:Simon.Cox@csiro.au]
Sent: Tuesday, 6 September 2016 11:14 a.m.
To: andrea.perego@jrc.ec.europa.eu; frans.knibbe@geodan.nl; jeremy.tandy@gmail.com
Cc: rob@metalinkage.com.au; jlieberman@tumblingwalls.com; chris.little@metoffice.gov.uk; public-sdw-wg@w3.org
Subject: RE: Requirements for units of measure, accuracy and precision

Thanks Andrea -

But gosh - you have to dig deep to find it. And the sub-head 'dataset precision and accuracy' does not exactly shout 'units of measure' or 'scale' does it? It looks like a side-effect rather than a central concern.

Am frankly amazed that this issue is not more central for quantitative data, which is a significant subset of general 'data on the web'.


-----Original Message-----
From: Andrea Perego [mailto:andrea.perego@jrc.ec.europa.eu]
Sent: Monday, 5 September 2016 7:15 PM
To: Frans Knibbe <frans.knibbe@geodan.nl>; Jeremy Tandy <jeremy.tandy@gmail.com>
Cc: Rob Atkinson <rob@metalinkage.com.au>; Joshua Lieberman <jlieberman@tumblingwalls.com>; Chris Little <chris.little@metoffice.gov.uk>; Cox, Simon (L&W, Clayton) <Simon.Cox@csiro.au>; public-sdw-wg@w3.org
Subject: Re: Requirements for units of measure, accuracy and precision

Just to note that UoMs are included in some of the examples of DQV, in particular in relation to spatial resolution - see:


For more details, see my earlier messages on this topics [1,2].

Please note that the examples above are some of those added by the DWBP WG following our comments on the "representation of precision and accuracy" - see thread starting here:


and related DWBP issue:


Since DWBP deliverables include UoMs only for spatial resolution, I think it's up to us to address the other relevant use cases - possibly by building upon the DQV approach, if appropriate.



On 02/09/2016 17:26, Frans Knibbe wrote:
> All,
> thanks for replying, and a special thanks to Rob for fencing off
> Jeremy's hijack attempt :-)
> An attempt to summarize and suggest steps to be taken:
>   * One of us could submit a comment to the DWBPWG that we feel
>     indication of UoM and uncertainty in quantitative data deserves
>     attention in their document. Is there a volunteer?
>     (I would like to note that an earlier DWBP comment had me suggesting
>     they should encourage data providers using significant digits, which
>     can be seen a good way to meet the requirement for indicating
>     uncertainty in quantitative data. After some discussion, the DWBP
>     decided the subject was out of scope for data on the web (see this
>     message
>     <https://lists.w3.org/Archives/Public/public-dwbp-comments/2016Jun/0012.html>),
>     because it applies to the broader domain of quantitative data (that
>     can exist outside of the Web too). So I think next to the DWBP
>     already being fixed there is little chance of the comment being
>     acted upon.)
>   * There seems to be agreement that the UoM thing should be tied to CRS
>     definitions. Can we solve the issue by listing an indication of UoM
>     as one of the types of 'useful information' about a CRS in the CRS
>     Definition requirement
>     <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#CRSDefinition>?
>     Of course I would need to list some other typical properties of CRSs
>     for good measure, but that should not be a problem.
>   * We add a BP requirement to the UC&R that says "The use of precision
>     that matches uncertainty in coordinate data should be facilitated
>     and encouraged". Note that we already havea requirement for
>     describing spatial resolution in the metadata
>     <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#SpatialMetadata>.
> I hope we can agree on making concrete changes to the UCR document (or
> not) soon.
> Greetings,
> Frans
> On 1 September 2016 at 13:41, Jeremy Tandy <jeremy.tandy@gmail.com
> <mailto:jeremy.tandy@gmail.com>> wrote:
>     @ Rob:
>     > I think we need to step back and decide if this is about
>     requirements or BP
>     Good point. I'll take the best practice question to a separate
>     thread ...
>     On Wed, 31 Aug 2016 at 23:44 Rob Atkinson <rob@metalinkage.com.au
>     <mailto:rob@metalinkage.com.au>> wrote:
>         I think we need to step back and decide if this is about
>         requirements or BP, and target the elements of conversation
>         better to which deliverable is affected.
>         IMHO there is a _requirement_ for a canonical mechanism to state
>         aspects of spatial properties.
>         I also think its a requirement for that mechanism to be common
>         with broader requirements for publishing metadata.  Its not a
>         uniquely spatial concern - but spatial is typified by this being
>         relevant at dataset, service, feature and property levels of
>         granularity and the need for users to be able to discover and
>         interpret this information regardless of where it is best placed.
>         If there is no consensus on an identifiable Best Practice - IMHO
>         we should note this as an unmet requirement in the BP document ,
>         and refer to the catch-all (reuse something!) BP
>         Rob
>         On Thu, 1 Sep 2016 at 02:27 Joshua Lieberman
>         <jlieberman@tumblingwalls.com
>         <mailto:jlieberman@tumblingwalls.com>> wrote:
>             Jeremy,
>             These may not be the terms we want to make a best practice
>             of. Precision (closeness of repeated measurements) and
>             accuracy (closeness to truth) are really concepts that are
>             impossible to compute directly, since one can’t either make
>             all possible repeated measurements, nor actually make true
>             measurements. What one can estimate and report is
>             uncertainty (the range within which the true value has some
>             likelihood of occurring if we accounted for all sources of
>             error) and resolution (predicted precision or fineness of
>             measurement).
>             The spatial question in this is whether one should report
>             multidimensional uncertainty and resolution, using such
>             things as error ellipses or fuzzy geometric boundaries. It
>             could be a real mess to make a map with fuzzy topographic
>             contours, but the variation in hypsometric uncertainty
>             between different published maps would probably shock people
>             if they were made aware of it.
>             Josh
>>             On Aug 31, 2016, at 11:13 AM, Jeremy Tandy
>>             <jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>>
>>             wrote:
>>             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
>>             <http://w3c.github.io/sdw/bp/#desc-accuracy>
>>             On Wed, 31 Aug 2016 at 12:32 Joshua Lieberman
>>             <jlieberman@tumblingwalls.com
>>             <mailto: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 <tel:%2B1%20617-431-6431>
>>                 jlieberman@tumblingwalls.com
>>                 <mailto:jlieberman@tumblingwalls.com>
>>                 On Aug 31, 2016, at 07:19, Little, Chris
>>                 <chris.little@metoffice.gov.uk
>>                 <mailto: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]
>>>                 *Sent:* Friday, August 26, 2016 1:42 AM
>>>                 *To:* Simon.Cox@csiro.au <mailto:Simon.Cox@csiro.au>;
>>>                 frans.knibbe@geodan.nl
>>>                 <mailto:frans.knibbe@geodan.nl>; public-sdw-wg@w3.org
>>>                 <mailto: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
>>>                 <mailto: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
>>>                     <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/
>>>                     <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
>>>                     <mailto:frans.knibbe@geodan.nl>]
>>>                     *Sent:* Thursday, 25 August 2016 10:50 PM
>>>                     *To:* SDW WG Public List <public-sdw-wg@w3.org
>>>                     <mailto: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
>>>                     <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
>>>                     <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____

Andrea Perego, Ph.D.
Scientific / Technical Project Officer
European Commission DG JRC
Directorate B - Growth and Innovation
Unit B6 - Digital Economy
Via E. Fermi, 2749 - TP 262
21027 Ispra VA, Italy


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