Publishing Linked Data - Linked Data: A Geographic Perspective - page 116

7.5.3 r elatIonal D atabaSe S oUrceS
Merea Maps stores its data in a GIS that is implemented as a software layer on top
of an RDB, an architecture that is common to many GIS systems. Merea Maps has
the option of outputting the data from its GIS into CSV or XML and converting it
to RDF using one of the RDF-izer tools discussed. However, since it is continuing
to use its GIS as the mainstay of its business, and the data is continuously updated
as new houses are built, the landscape changes, and so on, it would be better for it
to publish a Linked Data view of its RDB. This has the advantage that there is no
disruption to the existing business uses of the database or GIS, and the Linked Data
remains in sync with the latest version of Merea Maps' GI data.
The simplistic approach to an RDB to RDF mapping (Berners-Lee, 1998b) is

• An RDB table record is an RDF node.

• The column name of an RDB table is an RDF predicate.

• An RDB table cell is a value.

Each table represents an RDFS class, and foreign key relationships between tables
are made explicit by including a triple relating one RDFS class to another. A sim-
plistic example of an RDB table is shown in Table 7.2. The data in Table 7.2 can be
converted to the following RDF (shown in Turtle format here):

@prefix rdf: < http://www.w3.org/1999/02/22-rdf-syntax-ns# >.
@prefix rdfs: < http://www.w3.org/2000/01/rdf-schema# >.
@prefix mm_address: < http://id.mereamaps.gov.me/addresses >.
mm_address:0001 House_Number “39”;
Street “Troglodyte Road”.
mm_address:0002 House_Number “45”;
Street “Isis Way”.

As we can see, since the Geo IDs are unique identifiers (and primary keys of
the table), they can be reused within URIs. Each row of the table represents one
resource, and each column heading is used as a property for a triple. The geometry
data (a Binary Large Object or BLOB 11 type) can be represented as the ogc:Polygon
type, introduced in Section 6.4. The main problem that arises, however, is that
the table structure of Merea Maps' original data from Table 7.1 does not match
the RDFS ontology that has been developed in Section 7.4.3 to more accurately
represent the domain.

TABLE 7.2
Simplistic Example of a Relational Database Table

Geo ID House_Number Street Geometry

00001 39 Troglodyte Road [BLOB]

0002 45 Isis Way [BLOB]