Re: Using +nadgrids with cs2cs (version 4.9.3)

Thanks Even

Glad to provide some nostalgia.

Cheers
Chris


> -----Original Message-----
> From: Even Rouault [mailto:[email protected]]
> Sent: Monday, 2 December 2019 10:19 p.m.
> To: [email protected]
> Cc: Chris Crook
> Subject: Re: [PROJ] Using +nadgrids with cs2cs (version 4.9.3)
>
> Chris,
>
> ah, the good old times... Actually not so good, as it shows how hard it could
> be to manage to do things correctly.
>
> So you want to do this:
>
> # PROJ < 6
>
> echo '173 -41 0' | cs2cs -v -f %.10f \
>       +proj=latlong +ellps=intl +nadgrids=nzgd2kgrid0005.gsb \
>    +to \
>    +proj=latlong +ellps=GRS80 +towgs84=0,0,0
>
> 173.0001712855        -40.9982540723 0.0000450267
>
> The left part is the "definition" of NZGD49: based on Intl 1924 and it
> transforms to WGS84 with nzgd2kgrid0005.gsb (there was a typo in the
> extension you used). You could also have used +datum=nzgd49 instead of
> +ellps=intl.
> Alone +datum=nzgd49 expands to "+ellps=intl
> +towgs84=59.47,-5.04,187.44,0.47,-0.1,1.024,-4.5993", and the +nadgrids
> +will
> override the +towgs84, but that's a confusing.
>
> The right part is the "definition" of NZGD2000: equivalent to WGS84 for our
> purpose. You *need* the +towgs84=0,0,0 argument so that both left and
> right side have a transformation term to WGS84
>
> Cross-checking with modern times:
>
> $ echo "-41 173 0" | src/cs2cs -f "%.10f" NZGD49 NZGD2000
>
> -40.9982540714        173.0001712855 0.0000000000
>
> The results are slightly different as in PROJ 6 there's no conversion in the
> geocentric domain. The grid is applied directly in the geographic 2D domain.
>
> Even
>
> --
> Spatialys - Geospatial professional services http://www.spatialys.com

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