[Gdal-dev] problems with datum transform AGD66 to GDA94

[Andrew Williams]

OK, now that's a bit different. In this magnitude we're often talking wrong datum.

You will see this sort of magnitude when you 'assume' your source datum is AGD66 and convert to WGS84 when in fact the source datum was WGS84 all along.

Also vice versa.

However I just noticed you may have gotten the sign on one of the conversion parameters wrong.

You had

+proj=longlat +a=6378160 +rf=298.250000000055
+towgs84=-29.193,41.212,130.73,-0.246,-0.374,-0.329,-2.955 +no_defs

Based on the GDA technical manual http://www.icsm.gov.au/icsm/gda/gdatm/gdav2.2.pdf

The towgs84 parameters should be +towgs84=-29.193,-41.212,130.73,-0.246,-0.374,-0.329,-2.955

Note the minus added to the second parameter.

Also note that depending on the way these parameters are expressed also depends on how the sign is used. In some applications I have reversed all the signs and achieved the desired outcome.

Try the revised parameters. If that doesn't work, reverse the signs and try again.

-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org [mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Michael Sumner
Sent: Wednesday, 7 November 2007 10:49 AM
To: gdal-dev at lists.maptools.org
Subject: Re: [Gdal-dev] problems with datum transform AGD66 to GDA94

Thanks Andrew,

That's what I thought - but the differences between the GDAit and ogr2ogr
conversions are 100-200m.

That is worse than I thought we could expect. I simply don't know how
else to specify the conversion for this transformation.

Cheers, Mike.

>Mike,
>Based on your DSE link, one of the things you may be missing is that
GDAit is using a >local grid distortion model as well as doing a point to
point conversion from AMG to MGA. >The local distortion model uses a grid
of observed values to determine additional local >shifts. GDAit is
considered the most correct conversion for AMG to MGA and depending on
>location this can be a metre in magnitude.



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