[talk-au] The Paradox of Postcodes (Was Re: Victorian Vicmap Address Import Proposal - Suburb and Postcode discussion)

[Andrew Harvey]

On Thu, 17 Jun 2021, at 6:08 PM, Adam Horan wrote:
> 
> The ABS have an interesting factsheet on postcodes and their own 'Postal Area' interpretation (POA).
> https://www.abs.gov.au/websitedbs/censushome.nsf/home/factsheetspoa 
> It starts with this statement:
> *"A postcode is a four digit number used by Australia Post to assist with mail delivery. Australia Post does not currently define geographic boundaries for postcodes. However, a number of organisations, such as PSMA Australia Limited, create geographic boundaries that aim to define the geographic extent of the mail delivery area for each postcode. Defining postcodes with a geographic boundary is an imprecise process, and this is demonstrated by the fact that there are variations in boundaries released by different organisations."*
> **
> Some postcodes cross state boundaries, one example is 3644 which covers Cobram in VIC and Lalalty in NSW
> https://auspost.com.au/postcode/lalalty
> https://auspost.com.au/postcode/cobram/vic/dgee
> https://www.google.com/maps/place/VIC+3644/ 
> 
> There are also regions with no postcode, eg parts of the wilderness in West Tasmania.
> 
> Some postcodes cover non-contiguous areas eg 3585 which is in two parts https://www.google.com/maps/place/VIC+3585/
> 
> In VIC at least shapefiles for postcodes exist, I didn't search more broadly. The VIC data is aligned to property boundaries.
> https://services.land.vic.gov.au/SpatialDatamart/dataSearchViewMetadata.html?anzlicId=ANZVI0803003521&extractionProviderId=1 
> and https://discover.data.vic.gov.au/dataset/postcode-boundaries-polygon-vicmap-admin

This does sound like addr:postcode on each address object is the way to go and correctly capture the postcode of each address. We can still have postal_code's on admin boundaries where the the vast majority of addresses within that boundary have that postcode.

Stage 1 of my proposed import adds the missing postal_code tags on these relations https://gitlab.com/alantgeo/vicmap2osm/#stage-1-postal_code. This can coexist with addr:postcode.

It's a fair point that Vicmap's own postcode field shouldn't be taken as 100% correct, it looks like it might have been assigned based on postcode boundaries so might still suffer issues because of this, but where addr:postcode is not already mapped, most of the time the Vicmap one will be correct.