Database Reference
In-Depth Information
Ash Fleet Farm X 4875.
mm:/181113 X 4875.
Ash Fleet Farm Y 1287.
mm:/181113 Y 1287.
This simple system works well enough if a single point is required. If multiple
points are needed, then these simple properties are insufficient because it would not
be possible to determine which Lat corresponds to which Long. This also raises the
question of just how useful it is to represent geometry as pure RDF. Consider how
you might represent a line or polygon as a series of linked triples. It can certainly be
done, but to what advantage? It would only be necessary on the rare occasion when
it is required to associate further unique data to an individual coordinate. The alter-
native is to bundle up all the coordinates in some structured form so that it can be
unbundled when it is needed. Fortunately, the Open Geospatial Consortium (OGC)
has already done this for us as an extension to the standard datatypes that are sup-
ported by RDF and defined as part of their GeoSPARQL specification (Perry and
Herring, 2011). We will not go into all the nitty-gritty here, but focus on the essen-
tials of how the geometry is associated with the feature being described and then
how the geometry is represented. Using GeoSPARQL for querying spatial data is
discussed more in Section 8.3.
The OGC Simple Feature model represents a spatial object as an OGC fea-
ture, which is essentially an abstract representation of a real-world object. The
OGC feature is assigned a URI, and the property
geo:defaultGeometry
3
is
used to associate the geometry to the OGC feature. The namespace geo: expands
The geometry itself is represented using WKT (Well-Known Text) as defined
by the Simple Feature model International Organization for Standardization (ISO)
19125-1 (ISO 19125-1 2004). The WKT format (or serialization) represents points,
lines, and polygons as follows:
Point
Point (x, y)
Line
Linestring (x1, y1, x2, y2, … xn, yn)
Polygon
Polygon ((x1, y1, x2, y2, … x1, y1))
and
Polygon
((x1, y1, x2, y2, … x1, y1), (a1, b1, … an, bn, a1, b1))
The polygon has two formats, the first for simple polygons and the second for poly-
gons with holes, each hole being represented by a separate closed linestring. Each
linestring is closed by repeating the first coordinate pair as the last. The following
code snippet demonstrates how a WKT polygon can be associated with some feature
(featureX) in RDF:
1. <example:feature rdf:about = “example:featureX”>
2. <geo:defaultGeometry rdf:resource = “example:geometry”/>
3. </example:feature>
4.
5. <ogc:Polygon rdf:about = “example:geometry”>
6. <ogc:asWKT rdf:datatype = “
http://www.opengis.net/rdf#WKTLiteral”
>
8. Polygon((5.6 20.2, 5.6 20.7, 5.3 20.5, 5.3 21.9, 5.6 20.2))
