[Proj] New Zealand Map grid projection

[Hamish]

Claire wrote:
> I've got a problem with the results of the NZMG projection.
> Using proj 4.7.1 (23 sept 2009), I got :    
>       >proj -I +proj=nzmg    
>    
>      #2864916.8 6160570.9    
>      #177d8'19.853"E39d41'14.816"S    
>    
>   But when I check my results with the Land Information New Zealand (LINZ)
...
>      I got :    
>
>      That is more than 6 minutes difference in Latitude !...    
>      Both seems to use the same datum NZGD49.    


Hiya Claire, hope you are enjoying being back on dry land! :-)


The `proj` program does not apply datum shifts, you need to use `cs2cs`
instead.

the expanded version looks like:

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=longlat +datum=wgs84

a shorter version looks like
 cs2cs +init=epsg:27200 +nadgrids=nzgd2kgrid0005.gsb  +to  +init:epsg=4326


Also if working on/near land* you'll want to use the NTv2 distortion grid,
not the 3 or 7 term +towgs84 parameters. Make sure that both sides of the
"+to" include a +datum definition.

(* don't use NZMG out of view of a trig station if you can help it, it
goes a bit wonky offshore)


regards,
Hamish
(Otago)