Some comments on the spatial ontology (sdwgeo)

Hello Josh,

Many times during the F2F meeting in Lisbon the idea that work on an agreed
spatial ontology is very important was confirmed for me. So I had a look at
the ontology in WebProtégé
<http://webprotege.stanford.edu/#Edit:projectId=fa09f9df-1078-4c17-a16c-ae83695ff431>
in its current state. You wrote that comments are welcome. I thought a
message like this would be the best way to share such comments,
although WebProtégé has its own comment system - it could be that comments
in WebProtégé go unnoticed and besides that all decision making should be
publicly recorded for eternity.

So below are some comments and questions. Please excuse me for any stupid
comments, I am not an ontologist and there are probably a lot of things I
misunderstand.

And I hope that more people can find the time to look at this crucial piece
of work.

   1. Most importantly: Thank you for setting up the ontology!
   2. Earlier I asked about starting with the GeoSPARQL ontology and work
   from there. You answered that is not practical because WebProtégé does not
   seem to support refactoring. Still, it seems to me that the base classes
   and properties defined in GeoSPARQL 1.0.1 (gspql:geometry,
   gspql:SpatialObject and gspql:Feature) should be in the new ontology
   somewhere, if only for ensuring backward compatibility.
   3. I wondered if topology should be included in the ontology (see my
   earlier message to the list
   <https://lists.w3.org/Archives/Public/public-sdw-wg/2016Sep/0190.html>),
   but I noticed it's already in there (in the TopoModel class). I am happy to
   see that.
   4. SpatialThing is an important class, but its definition is not clear.
   It refers to ISO 19109, but that definition is not something everyone can
   look up. How about definitions like "Something that has some kind of
   spatial presence", or the current definition in the BP document, taken from
   the Basic Geo vocabulary: "Anything with spatial extent, i.e. size, shape,
   or position. e.g. people, places, bowling balls, as well as abstract
   regions like cubes."
   5. Continuing the point above, can SpatialThing be defined as some sort
   of equivalent of geo:SpatialThing?
   6. I assume the intention of the SpatialModel class is that it can
   represent a model of a SpatialThing. Shouldn't it say so in its
   definition/comment?
   7. If Extent is not defined as it usually is understood (an indication
   of the space a spatial thing occupies), but as a synonym of dimensionality,
   then why is Extent a subclass of SpatialModel?
   8. If Extent is not meant to be used to indicate the space a spatial
   thing occupies (e.g. a minimal bounding rectangle), then which part of the
   ontology is meant for that?
   9. In a general view of spatial relations there are three types:
   topological relations (e.g. within, crosses), distance relations (e.g. at,
   near to, far from)  and directional relations (e.g. north of, upstairs
   from, behind). Would it make sense to define these types as subproperties
   of spatialRelation, and let the current set of subproperties be
   subproperties of e.g. topologicalRelation?
   10. In Lisbon we had some discussion about the computability of spatial
   relationships, specifically topological relationships. In my view, both
   SpatialThings and Geometries can have spatial relationships. In the first
   case, they can be used as assertions, in the second case they are
   computable. If this view makes sense, is it useful to define two sets of
   spatialRelations, one for spatial things and one for geometries?
   11. Another suggestion made in Lisbon: could we regard the
   spatialRelation 'equals' as meeting the requirement to express subject
   equality
   <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#SubjectEquality>
   ?
   12. I see that the property hasSerialization has three subproperties:
   asGML, asJSON and asWKT. But GML, JSON and WKT have very different levels
   of expressiveness. For example, WKT has no way of expressing CRS or
   resolution. JSON, on the ohter hand, is a very general format so it is not
   so clear what the serialization would look like. Or did you mean GeoJSON?
   13. Is there an entity in the ontology that can be used for expressing
   the array of coordinates that can be used to define a geometry?
   14. I find it quite hard to see how the parts of the ontology are
   related. I think understanding the use of the ontology would be helped a
   lot with some examples (resource descriptions in RDF). I would like to try
   to make some examples, but what would be a good place for that? A new wiki
   page? Or is it better to start with a proper HTML document in GitHub that
   explains how to use the ontology, something that can be turned into a more
   or less official document?
   15. Can other people edit the ontology? Perhaps others can contribute
   resource descriptions (labels and comments in different languages).
   16. Why is LinearReference a separate class? Isn't it the same as a 2D
   CRS?
   17. I see a property 'resolution' has been defined. But it does not seem
   to be related to other entities. Will it be a property of SpatialModel?
   18. Can the ontology be related to the Location Core Vocabulary
   <https://www.w3.org/ns/locn>? That would give the opportunity to refer
   to SpatialThings by address or toponym. For example, could dcterms:Location
   be defined as a equivalent class or subclass of SpatialThing?
   19. Can the range of spatialDimension be specified as one the integers
   1, 2 or 3?


This is what I've come up with now. Probably questions will disappear or
form when understanding increases. Having a set of examples of how the
ontology can be used would probably help a lot for that understanding, and
I think that working of implementation examples from our own fields of work
could be a fun & fruitful group activity.

Greetings,
Frans