[Proj] New Zealand Map grid projection

[Hamish]

Claire wrote:
> The result is actually the same as without specifying the
> datum, so I guess the datum shift has not been performed...
>
>  $ cs2cs +proj=nzmg +datum=nzgd49 +lat_0=-41.0 +lon_0=173.0  +x_0=2510000 +y_0=6023150 +a=6378388 +rf=297 +no_defs +nadgrids=nzgd2kgrid0005.gsb +to_meter=1.0  +to +proj=utm +zone=60 +south +datum=WGS84    
>      2864916.8 6160570.9    
>      511905.89 5606922.76 0.00    

that's strange, the same command for me that gives the correct result,
511924.57	5607117.97 0.00

(proj 4.7.1 on debian/stable)


> The right answer (from LINZ) should be :    
> 511924.6   5607118.0	  

hmm, I get close to that:

$ cs2cs +proj=nzmg +datum=nzgd49 +to +proj=utm +zone=60 +south +datum=WGS84
2864916.8 6160570.9
511923.33	5607116.10 18.93

(`cs2cs -ld` shows default 7-term transform will be used with nzgd49)


# using the grid file I match LINZ's result:
$ cs2cs +proj=nzmg +datum=nzgd49 +nadgrids=nzgd2kgrid0005.gsb \
    +to +proj=utm +zone=60 +south +datum=WGS84
2864916.8 6160570.9
511924.57	5607117.97 0.00

# the 7-term transform is identical to the default (as expected):
$ cs2cs +proj=nzmg +datum=nzgd49 +towgs84=59.47,-5.04,187.44,0.47,-0.1,1.024,-4.5993 \
    +to +proj=utm +zone=60 +south +datum=WGS84
2864916.8 6160570.9
511923.33	5607116.10 18.93

# the 3-term transform is nearby too:
$ cs2cs +proj=nzmg +datum=nzgd49 +towgs84=54.400,-20.100,183.100 \
    +to +proj=utm +zone=60 +south +datum=WGS84
2864916.8 6160570.9
511923.55	5607117.97 54.31


also be careful you don't fall off the eastern edge of the grid
file, as I recall that's not very far off the East Cape, so if
the data goes far off east perhaps it is better to use the 7-term
transform throughout.


??,
Hamish