[Proj] Stereographic North Pole - EMEP Grid

[Oscar van Vlijmen]

> From: turek2 <turek2-poczta.fm>
> Date: Fri, 13 Oct 2006 22:54:28 +0200
> Subject: [Proj] Stereographic North Pole - EMEP Grid

> Has anyone defined EMEP coordinate system properly under GRASS?
> Any hints are welcome!

I can't give any PROJ/GRASS details, but I can give some clues.

With some preprocessing and postprocessing the spherical form of the PROJ
projection stere can be used.

The EMEP radius of the spherical earth = 6370000 m.
There are 3 different EMEP grids; let's concentrate on the 50 km official
one.
The following projection parameters should be used in this case:
lat0=90; lon0=-32; x0=8*50000; y0=110*50000; k0=1; lat_ts=60;
The x0 and y0 are defined in kilometers, the grid cell space is 50 km, hence
the multiplication factor of 50000.
For a
lat=15; lon=17;
degrees, the stere projection should produce something like:
easting = 7283626.857; northing = -483845.535;
These values have to be converted to EMEP grid coordinates ei,ej.
First divide them by 50000:
This gives values around: 145.6725, -9.6769
Then round to integers with the nint function to get cell numbers:
-> ei = 146; ej = -10.

It can be done without postprocessing by setting the radius of the sphere to
6370000/50000 m and keeping the x0,y0 in km:
lat0=90; lon0=-32; x0=8; y0=110; k0=1; lat_ts=60;
Libproj's stere then gives for lat=15, lon=17 an x,y of 145.6725, -9.6769
which still has to be rounded with a nint function if EMEP cell numbers are
required.

Read more about EMEP at:
<http://www.emep.int/grid/index.html>