[Proj] What is the meaning of the "Mercator 41" projection in EPSG:3752 ?

[Hamish]

Mikael Rittri wrote:
> > I am trying to understand the meaning of EPSG:3752,
> > "WGS 84 / Mercator 41", used around New Zealand.

note that "used around NZ" does not mean that it is widely used within NZ.

I seem to recall that the NZ Marine Environment Classification (MEC)
used some thing similar:

PROJCS["Clarke_1866_Mercator",GEOGCS["GCS_Clarke_1866",DATUM["D_Clarke_1866",SPHEROID["Clarke_1866",6378206.4,294.9786982]
],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Mercator"],PARAMETER["False_Easting",0.0],PARAMET
ER["False_Northing",0.0],PARAMETER["Central_Meridian",100.0],PARAMETER["Standard_Parallel_1",-46.0],UNIT["Meter",1.0]]

http://www.mfe.govt.nz/publications/ser/marine-environment-classification-jun05/

In the mailing list archives you'll find my struggles to understand why
they used that. If it of great concern


This is dealing with NZ's offshore territorial waters (EEZ) which spread
out over huge parts of the pacific due to the many offshore islands and
lack of neighbors. The dataset is primarily a 1km raster grid, so working
to the 0.1m (at sea!) isn't really too much of a practical concern.

Are you looking at this dataset or something else like gas exploration
maps from NZ's GNS? In my wanders I have not seen it used outside the MEC
dataset and do not understand the benefits or reasons for using that.

fwiw, the ESPG entry is:
# WGS 84 / Mercator 41
<3752> +proj=merc +lon_0=100 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs  <>


Gerald:
> Seems a bit odd that NZ would be using Mercator.  They used to have a
> unique TMish under +proj=nzmg and now I think I read that they went to
> a new projection.  Forgot the reference but probably TM.

The old one used nationwide is NZ Map Grid using the NZGD 1949 datum.
The new one used nationwide is "NZ TM" using the NZGD 2000 datum.

see epsg codes 27200 and 2193. see also Clifford Mugnier's Grids & Datums
article in PE&R from May 2005 for a nice history.

Especially NZMG is not meant to be used far offshore. It blows up by
the time you get to the subantarctic islands.


> Plain old Mercator as a projection for New Zealand seems a
> bit strange. I can't imagine a poorer projection.

If it helps (I suspect it doesn't), perhaps it is better to think of
this as for the southwestern pacific rather than the NZ mainland.


Hamish