RE: Requirements for units of measure, accuracy and precision

Hi Frans,

I am happy with your proposal to restrict Rob A’s requirement to UoM of CRS elements, to keep within our scope.  However your phrase “CRS definition should include an indication of UoM” may need some engineering to get the correct level of looseness and prescription.

For B I suggest “use of precision that is appropriate to the uncertainty in coordinate data should be facilitated and encouraged”
Is better, but not sure that that is the best ‘line of attack’. I am sure Rob could suggest a better wording.

Chris

From: Frans Knibbe [mailto:frans.knibbe@geodan.nl]
Sent: Thursday, August 25, 2016 1:50 PM
To: SDW WG Public List
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