Re: 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

-- 
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