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Re: Requirements for units of measure, accuracy and precision

From: Jeremy Tandy <jeremy.tandy@gmail.com>
Date: Thu, 01 Sep 2016 11:41:35 +0000
Message-ID: <CADtUq_2tcpVOWPJMPO35LFwSFPBJ0Hj2d4M2+jDtfA9+xKO=Ug@mail.gmail.com>
To: Rob Atkinson <rob@metalinkage.com.au>, Joshua Lieberman <jlieberman@tumblingwalls.com>
Cc: Chris Little <chris.little@metoffice.gov.uk>, "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>
@ 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> 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>
> 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>
>> 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
>>
>> 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 Thursday, 1 September 2016 11:42:17 UTC

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