The European Union defines a ‘smart city’ as “a city well performing in 6 key fields of urban development, built on the ‘smart’ combination of endowments and activities of self-decisive, independent and aware citizens”. Amongst the key fields are smart mobility and a smart environment. Smart cities rely heavily on reliable, accurate and available data. Linked open data (LOD) is a way to make that data available and therefore to ‘enable’ the smart city.
Linked data (LD) refers to data that is made available in a structured way through the internet. Linked open data (LOD) takes the linked data concept one step further and exposes the linked data to everyone on the internet. LOD is based on the 5-Star Open Data scheme which was developed by Tim Berners Lee, one of the founders of the internet in the early 1980s.
The first step (equivalent to one star) is to make information available on the web (in any format) under an open licence so that it can be reused. The second step or star is to make it available in a structured, computer-readable format such as the ESRI shape format or an MS-Excel file. Even better (three stars) would be to do this in an open, non-proprietary format such as GML. Most datasets that are available today adhere to one, two or three stars, including the large variety of web services making data available through, for example, WFS.
But it is only with four and five stars that one enters the realm of LOD. So what makes this different? In LOD all ‘things’ are uniquely identified. The final step is to link all these objects together in such a way that the data can be navigated and that additional context is provided. This can be done within a single dataset by creating a link between a house and a parcel, but can also be done between datasets. By naming the links between the objects, context can be provided about the relationship as well as effectively creating what is called the ‘semantic web’ or ‘Web 3.0’. For example, when the municipality keeps track of new houses and the cadastre of the parcels on which they are built, there could be a link from the house to the cadastral parcel. Anybody interested in the house could not only see the information about the house but could also navigate towards the parcel data. And a wider context is also possible. If, for example, the house has featured in a movie, a link to the house (and as a result also to the cadastral parcel) could be established from the movie database.
LOD requires the implementation of a number of standards that are not often used outside the LOD community, with the exception of object identification. Object identification is done by so-called universal resource identifiers (URIs). An example of a URI is the way universal resource locators (URLs, e.g. http://gim-international.com) are used to find web pages. In LOD, however, the URI identifies a specific geographical object (road, building, cadastral parcel) rather than a specific web page. The URI needs to be stable over time; changes in the data should not change the URI (otherwise, the network of links would crumble). To keep the URI stable, governments may adopt a URI strategy for their linked data which describes how URIs should be created and navigated.
Two further World Wide Web Consortium (W3C) standards are required to make LOD work, namely the Resource Description Framework (RDF) and the query language SPARQL. RDF defines the XML structure to use when describing objects and linking them. For the links, ‘triples’ are used which describe how object types (and therefore objects) are related by identifying a ‘from’ object and a ‘to’ object as well as the relationship between them.