[GRASS-user] polar projection with grid lines

[Moritz Lennert]

Le Sun, 28 May 2017 08:29:48 -0400,
Ken Mankoff <mankoff at gmail.com> a écrit :

> Hi List,
> 
> I'm trying to set up a polar projection and draw some grid lines. I'm
> having trouble a) getting the grid lines to the pole and b) rotating
> it so a different longitude is the center longitude. I'm using the
> following code:
> 
> grass72 -e -c epsg:3413 ./tmp
> grass72 ./tmp/PERMANENT
> 
> xy0=$(m.proj -i coordinates=0,40)  # set corners to 40° N
> xy1=$(m.proj -i coordinates=180,40)
> x0=$(echo ${xy0} | cut -d"|" -f1)
> y0=$(echo ${xy0} | cut -d"|" -f2)
> x1=$(echo ${xy1} | cut -d"|" -f1)
> y1=$(echo ${xy1} | cut -d"|" -f2)
> 
> g.region s=${y0} n=${y1} w=${x1} e=${x0}
> 
> d.mon start=wx0
> d.erase
> d.grid -w size=10:0
> 
> 
> The d.grid manual suggests calling 2x near the poles, but even doing
> that (e.g. d.grid -w size=1 direction=east-west) does not improve
> where the grid lines are drawn. Also, if I wanted Russia, not
> Greenland, pointing "down" on the map, how do I rotate this
> projection?
> 


Can't help you with you grid problems right now, but for "rotating"
your projection, you would have to create a new location with modified
projection parameters. Just change the lon_0 value to whatever meridian
you want as central:

Original EPSG 3413 :
+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0
+datum=WGS84 +units=m +no_defs  <>


Centered on Novosibirsk:

+proj=stere +lat_0=90 +lat_ts=70 +lon_0=83 +k=1 +x_0=0
+y_0=0 +datum=WGS84 +units=m +no_defs  <>

You can then create the new location using the "custom PROJ.4
parameters" option for defining the projection and pasting the above
definition.

Moritz