2016 Mesh Blocks to Contracted Catchments Linkset

This code repository contains a Linkset - a specialised Dataset linking objects in two other Datasets.

This Linkset contains spatial associations between Mesh Block class objects in the 2016 version of the Australian Statistical Geography Standard (ASGS 2016) and Contracted Catchment class objects in the Geofabric.

Both Mesh Block and Contracted Catchment features are represented spatially as polygons of varying area, consequently the following 3 spatial relations are present:

• within - A Mesh Block may be wholly within Contracted Catchment
• overlaps - A Mesh Block may overlap a Contracted Catchment
• contains - A Mesh Block may be wholly contained within a Contracted Catchment

These three relationships are expressed using predicates from the GeoSPARQL Standard and correspond to 'Simple Features' relationships, as defined by the Open Geospatial Consortium.

Additionally, a new relationship (RDF predicate) transitive overlaps is defined within the LocI project's GeoSPARQL Extensions Ontology and this is also used. This relationship allows for absolute area overlaps to be indicated by having two overlapping polygons indicate the polygon of their intersection.

The formal definition of what a Linkset is, is provided by the Location Index (LocI) project within its project ontology, see:

• http://linked.data.gov.au/def/loci

Figure 1: A Mesh Block (red outline) overlapping two Contracted Catchments (blue and purple ploygons)

Repository Contents

• data.ttl - this Linkset’s main data files.
• header.ttl - this Linkset’s data.ttl header information, stored separately for ease of access
• example data:
  • example-data-overlaps.ttl - 10 Statements from the Linkset where a Mesh Block overlaps a Contracted Catchment, for ease of access, in RDF (turtle) format, as per the main data file
  • example-data-within.ttl - 10 Statements from the Linkset where a Mesh Block is wholly within Contracted Catchment, for ease of access, in RDF (turtle) format, as per the main data file
• LICENSE - the data license assigned to this Linkset’s content
• overlaps.png - the image above
• methods/ - a folder containing information (prose and code) about how this Linkset was generated

Purpose

This repository contains a Linkset. Linksets are specialised Linked Data datasets that link objects, such as Addresses or Contracted Catchments, in one Linked Data dataset to objects in another.

Publishing relationships between Datasets as distinct Linksets allows for the independent management of Dataset-to-Dataset relationships.

Linksets for Spatial Relationships

Where LocI objects across multiple datasets have spatial relationships that we wish to represent, we create Linksets with spatial (topological) relationships such as touches, within, overlaps etc. using terms formalised in the (GeoSPARQL Standard](https://www.opengeospatial.org/standards/geosparql).

Linksets for Dataset versions

Some LocI Datasets, such as the ASGS, have multiple, independently delivered versions (the ASGS is released as a Linked Data Datasets in both 2011 and 2016 versions). Linksets can be used to link between these versions of a Dataset too. This allows for information such as correspondence tables (links between ASGS versions, published by the Australian Bureau of Statistics) to be published as Linked Data independently of any other Dataset.

This Linkset

This Linkset - GNAF Current Addresses to Geofabric Contracted Catchments Linkset - is a spatial relations Linkset linking GNAF Current Addresses (points) to Geofabric Contracted Catchments (polygons) by indicating which Contracted Catchment each Address is within.

This Linkset states, per Address and with other details, something like this:

Address GAACT714845933
is within
Contracted Catchment 7155143

...and that this particular link was made on the 6th of February, 2015 using a Parcel Level matching method.

How is a Linkset’s data organised?

Linksets include the main facts of relations between objects in two datasets - what the IDs of two objects are and how they are related - and they also include information about how links were created, such as what spatial intersection method was used to establish a topological relation. Linkset generation might have employed multiple methods to make all the object-to-object links within it so a Linkset may relate multiple methods and give the particular method used for each link.

Other per-link information may be recorded too: if the links within a Linkset are generated over a significant period of time then the each link may have a created time; if different people/organisations contributed different links then each link may reference their specific contributor.

Linkset content sections

Linksets use a highly condensed, but still (sort of) human-readable data format to include many (millions) of links. Linkset data files contain:

• A header
  • Basic information about the Linksets - what, who when
  • Links to methods used in the generation of the Linkset
• A (long) set of Statements
  • One Statement per link
  • Link type - how the two objects relate, spatially or otherwise
  • Link metadata - time of creation (if important), method used and who created it (if known) etc.

Linksets include all their information in one potentially very large file but they also include the header information in a stand-alone text file - header.ttl.

They also include a few (perhaps 10) example Statements in a stand-alone text file - example-data-… .ttl (numbered as there may be many).

Linkset files

In addition to the main Linkset data file and the header.ttl and example-data.ttl files, there are usually several other files within a Linkset, including this README file. General Linkset files include:

• data.ttl - the main Linkset data file.
  • since this could be very large (1.5GB+), it is often compressed (data.ttl.gz) and sometimes split into parts (data01.ttl.gz, data02.ttl.gz...)
• header.ttl - the Linkset’s data.ttl header information, stored separately for ease of access
• example-data.ttl - a few statements (perhaps 10) from the Linkset for ease of access
• README.md - this file: a description of this Linkset and general Linkset information
• LICENSE - the data license assigned to this Linkset’s content

This specific Linkset’s files are listed in above in Repository Content.

Linkset data format

In its long list of statements, this Linkset expresses each link like this:

• Statement MW1 says:
  • Address GAACT714845933
  • is within
  • Contracted Catchment 7155143
  • Link generated via SpatialIntersection Method

In RDF code this link is expressed as:

:mw1 a rdf:Statement ;
  rdf:subject:     address:GAACT714845933 ;
  rdf:predicate: geo:sfWithin ;
  rdf:object       catchment:7155143 ;
  loci:hadGenerationMethod: :SpatialIntersection 
.

With contractions used to save data volumes resulting in:

:mw1
  s: g:GAACT714845933 ;
  p: w: ;
  o: b:7155143 ;
  m: :SpatialIntersection ;
.

See the file example-data-within.ttl for the first 10 Statements of the Linkset expressed like this and see the header.ttl file to explain all the contractions.

Linkset metrics

Linksets always contain similar information - links between objects in datasets - and a standard set of metrics can be calculated for any Linkset. These metrics, set by the LocI project, are:

• Number of links
• Number of items in Dataset A (from) not linked
• Number of items in Dataset B (to) not linked
• Number of link creation methods used
• Numbers of uses of each link-creation method

Metric calculation

A series of queries to calculate Linkset metrics is being prepared here: https://github.com/CSIRO-enviro-informatics/linkset-metrics

This Linkset’s metrics

Rights & License

The content of this API is licensed for use under the Creative Commons 4.0 License. See the license deed all details.

Contacts

LocI Project technical owner:
Nicholas Car
CSIRO Land & Water, Environmental Informatics Group
nicholas.car@csiro.au

Linkset creator:
Ashley Sommer
CSIRO Land & Water, Environmental Informatics Group
ashley.sommer@csiro.au