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