Free tool for linking non-RDF data: Ontop-spatial

Ontop-spatial is a new open-source software from the University of Athens that allows us to use linked data techniques on data that are not already stored as RDF.

Imagine that you use geospatial relational databases in order to store and process data from your daily work. What would you have to do if you wanted to create semantic views of your geospatial data using the latest technologies? Conventionally, you would have to perform the following steps:

  1. Create an ontology for your data
  2. Convert your geospatial data to RDF using a tool such as Geotriples
  3. Store this RDF dataset in a state-of-the-art semantic spatiotemporal RDF store like Strabon
  4. Express rich geospatial queries in stSPARQL/GeoSPARQL using a Strabon endpoint or the visualization tool Sextant
  5. Interlink your data to other datasets available in the Linked Open Data cloud, using the linking tool Silk

If your database is updated frequently - either manually or via a connected application - then you would have to perform these steps for every update in order to maintain semantic, interlinked, versions of your fresh data.

The work required to repeat this task of publishing data to the semantic web is clearly an expense that few will be willing to pay. Therefore, efforts in both industry and the research community concentrated on the following solutions:

  1. Establishing new languages to drive the mapping of relational data to RDF, such as the W3C standard R2RML
  2. Developing new approaches for translating SPARQL queries to SQL queries on-the-fly using ontologies and mappings

These two approaches have been combined into a new systems such as Ontop, which provide a good solution.

These existing approaches, however, do not take into account the particulars of geospatial data. So - for example - GeoSPARQL queries could not be expressed in Ontop.

In order to remedy this, we at the University of Athens have developed a system called Ontop-spatial, which is an extension of Ontop. Ontop-spatial creates virtual RDF geospatial views on top of geospatial databases as well as on-the-fly translations of GeoSPARQL queries to SQL queries using ontologies and mappings. All of this is done without converting and materializing the original data to RDF.

As Ontop-spatial can also be used as a GeoSPARQL endpoint, you can use it in the same way as you would use a Strabon endpoint:

  • To express GeoSPARQL queries (and save them).
  • To use it as input to the interlinking tool Silk.
  • To visualize the data using Sextant

Ontop-spatial is free, open-source software, distributed under Apache License. Currently Ontop-spatial supports PostGIS and SpatiaLite and more database management systems will be supported in the future. The following GeoSPARQL components are supported in Ontop-spatial:

  • Query rewrite extension,
  • core,
  • Topology vocabulary,
  • Geometry topology extension,
  • RDFS entailment extension,
  • Subset of the Geometry Extension. 

For more information check out the Ontop-spatial source code here: https://github.com/ConstantB/ontop-spatial

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