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

Thu Sep 4 01:59:40 PDT 2008

Thanks for your replies. 

Hamish wrote: 
> Are you looking at this dataset or something else 
> like gas exploration maps from NZ's GNS? 

Well, actually, no.  I am just writing code to convert an OGRSpatialReference
to the corresponding thing in Carmenta Engine.  To create test examples for 
various projections, it is convenient to call 

    OGRSpatialReference.importFromEPSG( int nCode );

and when I was looking for examples of Mercator, I found EPSG:3752, 
which I thought was interesting, until I realized that I had no idea 
of what the result ought to be. 

> 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  <>

Hm. I don't think this can be correct, since the number 41 does not occur anywhere.

By the way, there is no way in Carmenta Engine to set a non-zero 
"Latitude of natural origin" for a Mercator projection. (A standard 
parallel can be set, though.) I was thinking that maybe I could emulate 
a non-equatorial natural origin by fiddling with the falseNorthing 
instead. But if this is a very unusual way to define a Mercator 
projection, I suppose I ought to throw an exception instead. 

Best regards,
       Mikael 

--
Mikael Rittri
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mikael.rittri at carmenta.com
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-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Hamish
Sent: den 3 september 2008 01:55
To: proj at lists.maptools.org; geraldi.evenden at gmail.com
Subject: Re: [Proj] What is the meaning of the "Mercator 41" projection inEPSG:3752 ?

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

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