UCR ISSUE-23: phrasing of the spatial operators requirement

Hello,

This thread can be used for discussion of UCR issue 23
<https://www.w3.org/2015/spatial/track/issues/23>: phrasing of the spatial
operators requirement
<http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#SpatialOperators>.
Currently the requirment reads: "There should be a recommended way for the
definition and use of spatial operators. Spatial things can have spatial
relations: topological relations, directional or distance relations.
Operators based on these relations (e.g. 'Contains'. 'Intersects',
'Nearest') should be well defined and easy to use.". This issue is about
clarity of the requirement, keeping in mind that people with different
backgrounds should understand the requirement.

Questions that can be asked are:

1) Is it clear what is meant by 'operators'?  Is the difference between
operators and relationships clear (compare GeoSPARQL fucntions and
relationships)? For spatial relationships there is a separate requirement
<http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#SpatialRelationships>.
Perhaps we should add something about the purpose of spatial operators (to
filter, select or derive data)?

2) Should we specify that we understand operators to work on numerical
(coordinate) data only? Could there be a need to support a query like 'give
me all oases in the Sahara desert' when there is no way to compute the
answer based on geometry (because the Sahara desert has no clear boundary)?
Or put in another way: Would it be a problem if we leave the way operators
should be implemented open, for phrasing the requirement?

Next to these questions, if you think there are other ways in which the
requirement is unclear or could be improved, please share your thoughts!

Regards,
Frans