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Geometry Data Service

The Geometry Data Service (GDS) is a lightweight API that has been developed in Loc-I to facilitate

  • minting URIs for geometries to enable Linked Data functionality
  • content-negotiation for formats and representations of the geometry
  • search by point for relevant geometries across the geometry store
  • parameterise the profile of the geometry representation for a Loc-I Feature (geometry of a feature with a resolution in metres, bounding box, centroid, simplified vs. complex geom, projection/CRS)

Geometry Data Service architecture

The current Geometry Data Service uses the pyLDAPI to implement the Linked Data APIs.

Scope

Mint URIs for geometries and provide an API for it

  • Only store (geo-referenced) geometries
  • Scoped to Earth initially

Implies

  • Ability to embed a URI Reference in a Loc-I feature instance
  • Selecting a predicate to associate a feature with the geometry URI reference (GeoSPARQL or something else?)
  • Returning the geometry representation from somewhere (from a remote service or local geodatabase cache)
  • Content negotiation by profiles

Out of scope

Performing extensive utility calculations on geometries (i.e. ArcGIS GeometryService).

Content negotiation interfaces

The Geometry Data Service implements and extends conneg arrangement proposed by (Regalia et al. 2017). Conneg interfaces the GDS implements are shown below:

MIME Type Description Returns
text/html Web interface <!DOCTYPE html><html lang="en">...
text/plain Well-Known Text POLYGON((113.1016 -38.062 ...))
application/json GeoJSON {"type":"Polygon","coordinates":...}
application/geo+json GeoJSON (coming soon...) {"type":"Polygon","coordinates":...}
text/turtle RDF Turtle representation using GeoSPARQL _:geom1 a geo:sfPolygon , geo:Geometry ; geo:asWKT "POLYGON((113.1016 -38.062 ...))"^^geo:wktLiteral .

GDS also implements some content profiles (refer to W3C Conneg by Profile):

Content profile name Returns Notes
geometryview (default) Provides the geometry as-is Useful for using the full resolution geometry
centroid Generates the centroid for the geometry Useful for when you only need the centroid
simplifiedgeom Applies a function to "simplify" the geometry Useful for rendering the geometry but don't need the full resolution (smaller payload also)

The combination of the two sets of interfaces allows users to query for the geometry resource to suit the needs of the application.

Usage scenarios

Linking from a description of a feature

Rather than embed a literal to the geometry in the Geosparql Feature instance, the GDS may be used to provide a description of the geometry via a URI reference.

Example: ASGS 2016 Meshblock Feature

<http://linked.data.gov.au/dataset/asgs2016/meshblock/20663970000> a asgs:MeshBlock,
        geo:Feature ;
    geo:hasGeometry <http://gds.loci.cat/dataset/asgs16_mb/20663970000> ;
.

It is up to the client/user to resolve the URI ref to the Geometry via standard HTTP plus conneg (by profile and format).

Find the respective feature(s) from the geometry

Given a geometry description, we can follow links back to respective feature. An example of the RDF/Turtle format response for the Geometry URI http://gds.loci.cat/geometry/asgs16_mb/20663970000:

<http://gds.loci.cat/geometry/asgs16_mb/20663970000> a geo:Geometry,
        sf:MultiPolygon ;
    geox:isGeometryOf <http://linked.data.gov.au/dataset/asgs2016/meshblock/20663970000> .

We can then traverse back to the Feature via the value for the geox:isGeometryOf property.

References and related work