[PROJ] Using +nadgrids with cs2cs (version 4.9.3)

[Even Rouault]

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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