Monday, May 9, 2016

Explaining holes in Postcodes 2016 coverage

When displaying Postcode Boundaries, Australia 2016 on a map you will notice that there is a lot of “empty space”. The expectation is that postcodes should cover the entire country, yet this is not the case for the reasons explained below.

Defining postcode boundaries should be a straightforward exercise since:
  • Australia Post officially declares that “postcode numbers are applied only to officially gazetted localities”, and since
  • gazetted localities cover, without overlaps, almost the entire continent, therefore
  • combining gazetted localities with a list of allocated postcodes should allow creating postcode boundaries that cover Australia without the gaps.
Simple, right? Not quite…

Australian Postcodes 2016 coverage

The reality is that:
  • There are places around Australia which do not have officially gazetted localities. For example, there are places with no permanent population (in the case of North West South Australia for example) or which have no formal recognition by local authorities for whatever reasons (in the case of Canberra surrounds). Hence, no postcodes can be allocated to those areas.
  • There are places which have gazetted localities but, for unknown reasons, those locations are not recognised by the Australia Post. So, they do not have postcodes assigned to them. This happens mainly in NT and Queensland but also for locations such as national parks or similar, mostly remote and uninhabited places.
  • There are places which are not officially gazetted localities so have no defined boundaries, yet they have assigned postcodes by the Australia Post. Point in case is the locality of Majura near Canberra with the allocated postcode number 2609. To make things more complicated, this locality also covers the officially gazetted locality of Pialligo (with the same allocated postcode number so, no big drama) but it also partially overlaps Canberra’s suburb of Watson which has a postcode number 2602.
  • Then there may be cases of locations that could be on the Australia Post postcode list but are not referenced with a postcode number in publicly available version of the list. So, such locations cannot be identified outright - unfortunately, Australia Post does not license freely its information.

As you can see, despite all the good intentions, the reality is quite messy. Therefore, the only way to deal with this complexity is to stick to a strict definition of what postcode boundaries should be. That is, to the Australia Post defined relationship between postcode numbers and locations where: “Postcodes are only allocated to localities officially gazetted by State land agencies”.

This way all major metropolitan areas are well covered with postcode boundaries and the issue of “gaps in coverage” is limited mainly to small, rural communities. The downside is a map that appears to have large chunks of Australia not covered in postcodes.

Using postcodes as a “widely recognisable and understood spatial reference to locations” is a deceivingly attractive proposition. In particular:
  • postcode numbers are part of an address, hence, theoretically, postcodes can be easily linked to specific locations;
  • everybody knows their postcode so it is very easy to solicit that information from clients or respondents to a survey, or from persons enquiring about a service, etc.
  • it is easy to use postcode groups to define sales territories or franchise areas,
  • postcodes are convenient for information aggregation purposes, without the need to resort to time consuming and potentially expensive geocoding of input data,
  • postcodes are large enough to provide good level of anonymity, so input data cannot be attributed to its sources (ie. inputs can be “confidentialised”),
  • at the same time, postcodes are small enough to highlight differences or similarities between local neighbourhoods. 

However, using postcodes boundaries is not without some serious issues due to:
  • ambiguity of how postcode numbers refer to locations,
  • existence of many versions of data at any particular point in time, and
  • their constant change.

You will find additional informaion on the limitations of postcode boundaries in our earlier articles on this topic:

Australian Postcodes User Guide
Comparing ABS Postal Areas 2011 and Postcode Boundaries 2016

All in all, gaps in Postcode Boundaries, Australia 2016 are the result of the methodology applied in creation of this dataset. MapDeck version of postcode boundaries may not suit all purposes and users should have a proper understanding of the limitations of this data set if they intend to make any use of it. 

Our recommendation is that, it is best to avoid postcodes (any version) and use instead more voluminous (in terms of count and data size) but more stable over time gazetted localities boundaries.