Substantial mismatches in 2020 and 2021 SA3

[MRWhitehead]

Re: australian_postcodes.csv and .xslx

Swathes of postcodes where sa3 code (2020) and SA3_CODE_2021 | SA3_NAME_2021 are mismatched.

I haven't quantified the extent of the issue. Just noticed that spreadsheet was not able to be used for any 2021 SA3 work.

[pnappa]

In addition to this, I notice some values are completely off & entire SA3s are missing, leading to maps having large holes in them (the Goulburn SA3 - 10105 - is missing).

I worked around this by using the ABS provided shapefiles for postcodes & SA3s, and deriving this myself based on largest overlapping area.

Here's the code (caveat, it's LLM generated), I just run it in a jupyter notebook:

# Import necessary libraries
import geopandas as gpd
import pandas as pd

# Download these from here:
# https://www.abs.gov.au/statistics/standards/australian-statistical-geography-standard-asgs-edition-3/jul2021-jun2026/access-and-downloads/digital-boundary-files
sa3_shapefile_path = 'SA3_2021_AUST_SHP_GDA2020/SA3_2021_AUST_GDA2020.shp'
postcode_shapefile_path = 'POA_2021_AUST_GDA2020_SHP/POA_2021_AUST_GDA2020.shp'

# Load the SA3 and Postcode shapefiles
sa3_gdf = gpd.read_file(sa3_shapefile_path)
postcode_gdf = gpd.read_file(postcode_shapefile_path)

# Ensure both GeoDataFrames use the same coordinate reference system (CRS)
if sa3_gdf.crs != postcode_gdf.crs:
    postcode_gdf = postcode_gdf.to_crs(sa3_gdf.crs)

# Calculate the intersection between each Postcode and SA3
intersections = gpd.overlay(postcode_gdf, sa3_gdf, how='intersection')

# Calculate the area of each intersection
intersections["area"] = intersections.geometry.area

# Group by Postcode and find the SA3 with the largest intersecting area
largest_overlap = intersections.groupby("POA_CODE21").apply(
    lambda x: x.loc[x["area"].idxmax()]
)

# Reset index and select relevant columns
largest_overlap = largest_overlap.reset_index(drop=True)
postcode_to_sa3_mapping = largest_overlap[["POA_CODE21", "SA3_CODE21"]]

# Save the mapping to a CSV file (optional)
postcode_to_sa3_mapping.to_csv("postcode_to_sa3_mapping.csv", index=False)

# Display the mapping
postcode_to_sa3_mapping.head()