And if you change 20351 (AGD84
Z51) to 20251 (AGD66 Z51) in Micha's example conversion you get exactly
the same result, which shouldn't happen. Again, I believe this is
because none of the towgs84 parameters for AGD66 & AGD84 are
included in Proj4 or QGIS. The GDA94 definition is probably OK.
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On 22/09/10 22:50, Micha Silver wrote:
On
09/22/2010 11:59 AM, Brett Adams wrote:
Gents,
Guess I'm wondering what the next step on this topic is. I don't know
what to do next.
Currently QGIS cannot separate ADG66/ AGD84 /GDA94 in either export or
import. If it can, then I haven't seen it and if its that fickle, its
way too dangerous for me.
Not keen on going to another package as they take time to learn. Is it
even an option to sponsor someone to sort this out for me?
Technically I'm way over my depth so can't offer many solutions myself.
I need some honest answers.
Thanks
Hi Brett:
Ken Norris indicated [1] that there might be a general problem
transforming from AGD66 or AGD84 (which use the aust_SA ellipsoid) to
GDA94 which is based on the GRS80 ellipsoid. He mentioned Oziexplorer
which does the transform correctly. To the best of my understanding,
proj4 (and thus QGIS) doesn't do any transform between the two.
Here's a trivial example. I took a coord pair from the file you posted
and ran it thru the proj4 cs2cs utility with epsg:20351 as the source
CRS (that's AGD84) and epsg:28351 as the target CRS (GDA94).
echo "275635 6411747" | cs2cs +init=epsg:20351 +to +init=epsg:28351
275635.81 6411759.44 0.00
Note that the resulting X-Y is offset by only about 12 meters. But
you're expecting a shift of nearly 200 m. So it seems proj4 is missing
here.
Normally datum transforms fit into either a 3 parameter or 7 parameter
model. It might be that the transform between those datums requires
something fancy, or localized that regular datum transforms cannot do.
So proj4 just doesn't handle it.
The best source for an authoritative answer would, of course, be the
Australian govt' mapping agency. If you learn anything more, please
let us know...
Regards,
Micha
[1]
http://osgeo-org.1803224.n2.nabble.com/Projection-of-Shapefile-export-tp5536736p5552105.html
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