Review Comment:
The paper proposes an interface that supports users in querying SPARQL endpoints, in particular those with data holding geographic and/or temporal data. Here, maps and timelines are used for visualization of the results.
While I like the general idea of the paper, and the paper itself is well written, I have a two major remarks on the contents. The first is on the presentation of the motivation and the related work, the second is about the user study.
The motivation and related work section are not well tailored to the problem. The problem statement "sucessful queries require exploration, and scalable exploration requires queries" holds true for any data, not just for time and space indexed Linked Open Data. Furthermore, most of the related work section (2.1 through 2.4) addresses general LOD interaction, while only a small paragraph (2.5) is devoted on actual problems related to time/space indexed data.
In addition to that, I do not agree with some of the statements made in that section. While the authors state that concatenations of relative clauses are difficult for end users, there is the gFacet tool [1] which allows the visual construction of such queries (and the paper even presents geographic visualizations of facets in such queries). Furthermore, the statement "we know of only one data type standard and triple store technology which allows handling of geospatial data on a level comparable to a GIS" leaves me confused why Virtuoso, which has built-in support for geospatial data [2], is not mentioned here. There may be some aspects missing to make it "comparable to a GIS", but this needs to be explained in more detail for the general audience of SWJ. In addition, I miss a reference to Linked Geo Data [3], which, after all, features a map-based browser.
In addition to that, the motivation would benefit from a clearer statement about the need for spatio-temporal interaction support in terms of the datasets that are available as LOD. The authors use a runinng example, which is fine, but at the same time seems to be a rather borderline case of Linked Open Data. Some general remarks about the number of LOD datasets using spatial and/or temporal dimensions would be appreciated. Further, a clearer statement about which interaction problems exist for such data which do not hold for LOD in general would strengthen the motivation.
In section 5.2, the authors discuss some related tools, while the user study only investigates SPEX. It would have been appropriate to include one of the other tools in the user study and compare the users' performance with both tools. In particular, Rhizomer would have been a good choice, since it shows the same characteristics in the feature matrix in Table 1, except for the support for space and time queries. By that comparison, the authors could have shown that their approach brings an advantage over the state of the art. Without that comparison, all that the user study shows is that users can work with the tool, and that they are able to solve some tasks in some time. This does, on the other hand, not prove that the work presented in this paper is a significant advancement over the state of the art.
I suggest that the authors reorganize the related work section, shortening the parts that address interaction with LOD in general, fleshing out the particular challenges that exist for geospatial linked data, and emphasizing their contribution in the area of spatiotemporal data, and more thoroughly including recent works on linked data interfaces w.r.t. temporal and spatial data. In particular, a recent survey [4] lists quite a few approaches that have special interaction means tailored towards spatial and temporal data which are not mentioned in this paper.
Furthermore, the user study should be extended and compare to related tools. If that is infeasible, at least a control group should be used which gets the same tasks and the same tool, but with the support for spatial and temporal querying being disabled. Only such a study would unarguably prove the utility of the proposed solution.
Minor remark: in section 2, the authors seem to mix up metadata and schema for LOD, which are usually not used synonymously.
[1] Heim et al.: gFacet: A Browser for the Web of Data.
[2] see, e.g., http://docs.openlinksw.com/virtuoso/rdfsparqlgeospat.html#rdfsparqlgeosp...
[3] Auer et al.: LinkedGeoData: Adding a spatial dimension to the web of data.
[4] Pena et al.: Linked Open Data Visualization Revisited: A Survey
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