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
This is a much stronger assumption than the 2 ellipsoids are identical. This is an assumption that the 2 *datums* are considered identical because you don't care about positional accuracy below 3 meters. This approximation is only used during the step where geodetic datum changes are used, not for forward or reverse map projections > > 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 If you go from GDA94 to MGA50, GRS80 will be used as the ellipsoid for map projection purposes: $ projinfo -s GDA94 -t "GDA94 / MGA zone 50" -o PROJ [...] EPSG:17350, Map Grid of Australia zone 50, 0 m, Australia - 114°E to 120°E PROJ string: +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=utm +zone=50 +south +ellps=GRS80 If you go from GDA94 to UTM50S, WGS84 will be used as the ellipsoid for map projection purposes: $ projinfo -s GDA94 -t EPSG:32750 -o PROJ --spatial-test intersects [...] unknown id, GDA94 to WGS 84 (1) + UTM zone 50S, 3 m, Australia - GDA PROJ string: +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=utm +zone=50 +south +ellps=WGS84 Which accounts for those submillemtric differences in projected coordinates. But remember that if you went from GDA94 to WGS84 or a projected CRS based on WGS84, you have an inaccuracy of 1 - 2 meters... So if you care about submillemetric accuracy, you must remain in the same datum (WGS84, UTM50 / WGS84) or (GDA94, MGA50 / GDA94) Even -- Spatialys - Geospatial professional services http://www.spatialys.com
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