|Review Comment: |
This manuscript was submitted as 'Ontology Description' and should be reviewed along the following dimensions: (1) Quality and relevance of the described ontology (convincing evidence must be provided). (2) Illustration, clarity and readability of the describing paper, which shall convey to the reader the key aspects of the described ontology.
The paper proposes a small ontology for collections of observations. It focuses on the notion of a collection.
It is not terribly clear what an observation is, although we learn that "an observation is generated by observing the physical reality" (it should be much clearer) but ssn:Observation is mentioned and appears to be the driving concept, as is that an observation is a dul:Situation (consistent with ssn). On the other hand, all the examples show an observation as a qb:Observation. The popular (but non-ontological) notion of observation in the OGC's O&M appears to be unknown to the authors. If an observation collection is indeed an important and distinguished concept, then observation needs clear definition (or at least we need to know what collections are *not* observation collections).
While the OGC notion of "coverage" is acknowledged (and there is, to my knowledge, no published ontology representation of it) the paper seems to fail to recognise that all three kinds of 'observation collections' identifed are actually coverages in the OGC sense. This appears to be a serious flaw (possibly inherited from reference 22).
I am afraid I have to disagree that the concept of "observation collection ...[has] been ignored... occasionally mentioned." On the contrary it is rare to find only single observations published on their own. This might be because the authors find there is no need for a whole new notion for collections, as you propose. Certainly, some publishers use the RDF datacube for the use cases you describe (except, possibly, trajectories). And I note that in all 3 of your worked examples you use things that are both qb:observation and your own obs:Observation (and you do not show any trajectories). The authors should have referenced other work that uses qb:observation as this paper does.
Which brings me to the main objection to the paper. I cannot see any value that the proposed ontology (and corresponding design pattern) brings to the representation of multiple observations. The paper's examples (which are only examples of the ontology applied, but do not offer any "convincing evidence") barely use the few terms in the ontology anyway.
To step through the terms in the ontology:
The paper makes the point itself that "collector" being the agent responsible for bringing the collection together, could just as well be re-used from almost any other ontology (eg DC).
There is also the 3 types of collections (see comment re coverage above -- one type? -- what does the typing do for us?).
There is the (borrowed) idea about 4 different types of time series. Without any objection to those (and indeed recognising the utility gained by labelling a time series as one of those -- which the paper entirely fails to justify) -- 4 simple un-original concepts seems a bit underdone. And only one of those, once, is used in the paper's examples.
Finally, there is the 3 types of trajectories (also a borrowed notion, also barely used in the examples, and also hard to see what they would be useful for).
The ontology is well organised with appropriate use of annotation properties.
The discussion of design principles is thorough, but confusing in places. e.g. spatial extent...can only be understood when talking about collections of observations" -- what about aerial photos?
The attempt to distinguish "single observation" from "observation collection" is self-contradictory "it cannot be both" whereas it is both by definition in the previous paragraph.
At one point we a are assured that observation collections are not sets due to the difference between abstract and concrete entities (which needs explanation) but later on it becomes apparent that observation collections are actually sequences -- so most certainly not sets! There is great confusion (p3) around the ideas of ordering when there may be multiple dimensions. It appears that the collector is obliged to choose exactly one total order over the collection, although the multiple alternative ways are "equally valid". As it turns out there does not seem to be any use made of the ordering anyway -- at least not in the examples presented. This is a pity, as there are indeed some useful things that could be done with it (or at least with an ordering over each dimension, separately, which is not considered in the paper).
Why must all observations of the collection be "generated by observing the same observed property"? Indeed the geolife trajectory example observes lat,long,alt and timeperiod --are these somehow all the same observed property? Or if not, what is the observed property in that example?
An observation can have a spatial location and this location is fixed to be the location of the sensor--- is this is a good design? What about an earth-observing satellite image?
The paper is well structured and well written in clear English. Reduced use of footnotes would aid readability, though. There is a very extensive reference list.
I urge the authors to check out the work of the W3C/OGC spatial data on the web working group,
especially the work on ssn and coverage.