[Qgis-user] GDA94 and WGS84 transformations

[Stephane Goldstein]

Hi.

I'm having trouble understanding how QGIS handles the conversion from the
Australian datum to WGS84 in the UTM grid.

GDA94 (EPSG:4283) /  MGA94 Zone 5X (EPSG:2835x)
WGS84 (EPSG:4326) /  UTM South     (EPSG:3265x)

As far as I know the only difference between those two is the ellipsoid
flattening parameters:
GDA94: 298.257222101
WGS84: 298.257223563

When converting from WGS84 <--> GDA94 in degrees, I get exact same
coordinates

GDA94 115.82318346600005 -31.948283257999947
WGS84 115.82318346600005 -31.948283257999947

This is expected, since the CRSs transformation assumes the two ellipsoids
are identical:

EPSG:1150 GDA94 to WGS 84 (1)
Approximation at the +/- 3m level using inappropriate assumption that GDA94
is equivalent to WGS 84. Accuracy changed from 1m to 3m due to tectonic
plate motion over more than 15 years.
+proj=noop

However when going from degrees to UTM or MGA, I get different values:

GDA94 115.82318346600005 -31.948283257999947
To
MGA50 388777.573657*112836372* 6464692.233*647795394063*
UTM50 388777.573657*624074258* 6464692.233
*548169955611*
WGS84 115.82318346600005 -31.948283257999947
To
MGA50 388777.573657112836372 6464692.233647795394063
UTM50 388777.573657624074258 6464692.233548169955611

MGA50 388777.573657112836372 6464692.233647795394063
To
UTM50 388777.573657624074258 6464692.233548169955611

I don't know why UTM and MGA coordinates differ, since the transformation
used still assumes WGS and GDA have the same ellipsoid:

INVERSE(EPSG):16150, INVERSE(EPSG):1150
Inverse of UTM zone 50S + Inverse of GDA94 to WGS 84 (1)
+proj=pipeline +step +inv +proj=utm +zone=50 +south +ellps=WGS84 +step
+proj=unitconvert +xy_in=rad +xy_out=deg

I have been considering those CRSs identical and using them interchangeably
till now, but realized it is not the case after getting errors in my
process.

Any clarification is appreciated.

Cheers.
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