|Review Comment: |
This manuscript was submitted as 'full paper' and should be reviewed along the usual dimensions for research contributions which include (1) originality, (2) significance of the results, and (3) quality of writing.
This paper presents QB4SOLAP, a new vocabulary for describing spatial data cubes using RDF, and spatial OLAP operators that can be translated into spatial SPARQL queries. It is a substantial re-working of a previously published conference paper with much new material. The authors present a very thorough set of formal definitions for the elements of the QB4SOLAP vocabulary built on the previously defined QB4OLAP vocabulary, and SOLAP operators over these elements. The SPARQL generator functions are well-defined as well -- a link to working service that performs these functions would be a nice addition to this work, but is not currently included.
Overall, the formalisms appear sound and are clearly written, but my main critique of the paper is that it remains unclear to me what the key advantages of translating SOLAP operations to SPARQL are? To me this is a key missing component of the paper. Some kind of evaluation is required that demonstrates that using QB4SOLAP is a viable alternative to existing SOLAP technologies, either in terms of efficiency of processing or in terms of expressivity. If it is the case that you can just reproduce the kinds of operations that can already be done in a traditional SOLAP tool that is built using spatial indexing, then it needs to be shown that it is efficent to do so also with spatial SPARQL queries over RDF. Otherwise, some examples of the kind of reasoning that is only possible with a QB4SOLAP defined MD data cube is in order. Without these, it begs the question why would someone go to the trouble of encoding their spatial datacube in RDF in the first place?
With regard to the organization of the paper, putting the related work section at the end instead of after the introduction, and including almost no references in the introduction, gives the impression that the Spatial OLAP operations being defined are new to this work. In fact, SOLAP has been around for over 15 years with many of these operators defined previously, in some cases quite formally. The authors need to differentiate their work from this previous work more clearly. Many key references for SOLAP are missing with this regard, including:
Sonia Rivest, Yvan Bédard and Pierre Marchand "Toward Better Support for Spatial Decision Making: Defining the Characteristics of Spatial On-Line Analytical Processing (SOLAP)"
Joel da Silva, Valerria C. Times, Ana Carolina Salgado "A Set of Aggregation Functions for Spatial Measures"
Leticia Gómez, Sophie Haesevoets, Bart Kuijpers, Alejandro A. Vaisman "Spatial aggregation: Data model and implementation"
Ines Fernando Vega Lopez, Richard T. Snodgrass, and Bongki Moon "Spatiotemporal Aggregate Computation: A Survey"
Overall, this is a nicely written paper but as stated above, I believe it requires some more justification for why this work will have impact on users of spatial data warehouses.
A summary table listing all the main definitions (and their number), operators, and generators would aid the readability of the paper.
pg.1 "This does not allows" -> "This does not allow"
pg.2 "Third, we provide algorithms for generation spatially" -> "Third, we provide algorithms for generation of spatially"
pg.2 "This paper extends a previous conference paper very significantly..." -- Not sure this sentence needs to be in the paper, seems more like a comment to the editor.
pg.2 I take issue with the classification of Intersection, Union operators as "Spatial Aggregation" operators, although this appears to be usage in other papers as well. These are not aggregations in a spatial sense, rather in the sense of record aggregation.
pg.2-3 Need citations to RCC8 and DE-9DIM
pg.18 delete box character at end of paragraph in column 2.