Incorrect result and unable to build PROJ string for MGA 55 to WGS 84 transform in pyproj v3.1

I’m getting different results between PyProj v3.0.1 and v3.1 for the MGA 55 (EPSG 28355) to WGS 84 (EPSG 4326) transformation; in addition, the transformer in v3.1 can’t produce a PROJ string

I feel like this is an issue with PROJ v8

Code sample

import pyproj
t = pyproj.Transformer.from_crs("epsg:28355", "epsg:4326")
print(repr(t))
print(str(t))
print(t.transform(800000,6000000))

Problem description

The output from PyProj v3.1 is incorrect (as far as I can tell)

Expected output

Output from PyProj v3.0.1

<Concatenated Operation Transformer: pipeline>
Description: Inverse of Map Grid of Australia zone 55 + GDA94 to WGS 84 (1)
Area of Use:
- name: Australia including Lord Howe Island, Macquarie Islands, Ashmore and Cartier Islands, Christmas Island, Cocos (Keeling) Islands, Norfolk Island. All onshore and offshore.
- bounds: (93.41, -60.56, 173.35, -8.47)

proj=pipeline step inv proj=utm zone=55 south ellps=GRS80 step proj=unitconvert xy_in=rad xy_out=deg step proj=axisswap order=2,1

(-36.098349196454585, 150.33217717123762)

Actual output

Output from PyProj v3.1

<Unknown Transformer: unknown>
Description: unavailable until proj_trans is called
Area of Use:
- undefined

unavailable until proj_trans is called

(-36.09833644723916, 150.33218243077053)

Environment information

PyProj v3.1

• pyproj version: 3.1.0
• PROJ version: 8.0.1
• PROJ data directory: /media/linux-data/venvs/eggs/lib/python3.8/site-packages/pyproj/proj_dir/share/proj

PyProj v3.0.1

• pyproj version: 3.0.1
• PROJ version: 7.2.1
• PROJ data directory: /media/linux-data/venvs/spam/lib/python3.8/site-packages/pyproj/proj_dir/share/proj

Common

• Python version: 3.8.10 (default, Sep 28 2021, 16:10:42) [GCC 9.3.0]
• Operation System Information: Linux-5.11.0-38-generic-x86_64-with-glibc2.29

Installation method

pip wheel

[onewhaleid]

I believe this issue is related to changes in the Geocentric Datum of Australia (GDA94 -> GDA2020). There is more information available here: https://www.spatial.nsw.gov.au/surveying/geodesy/gda2020

The specific issue with transformations to and from WGS84 is discussed here: https://www.spatial.nsw.gov.au/__data/assets/pdf_file/0008/224396/WGS84_and_Australias_misaligned_web-maps_Information_Sheet.pdf

[snowman2]

@EpicWink, I am going to close this as I think the issue has been resolved.

If you want to ensure a specific operation is used instead of relying on the order determined by PROJ, you can always use Transformer.from_pipeline:

>>> tr = Transformer.from_pipeline("+proj=pipeline +step +inv +proj=utm +zone=55 +south +ellps=GRS80 +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1")
>>> tr.transform(800000, 6000000)
(-36.098349196454585, 150.33217717123762)