# Documentation on the projection parameters

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#### littleho_song

##### New member
I couldn't find a clear description of the pole_lat, pole_lon, stand_lon from the documentation. Can WRF help provide one?

Plus, the plotgrids.ncl still is not able to plot the rotated lat-lon projection.

Thanks

Hi,

You can find descriptions of the 3 parameters in chapter 3 of the Users' Guide:
http://www2.mmm.ucar.edu/wrf/users/docs/user_guide_v4/v4.1/users_guide_chap3.html

If you do a search for "pole_lat", for example, there are 9 places where it is found. There are a couple of paragraphs and diagrams that explain it in detail, and there are formal definitions toward the end of the chapter.

We are aware that plotgrids.ncl does not work for the rotated lat-lon grid. This is the case with most visualization programs - they aren't able to handle the rotation of the grid. This has been a long-term problem (for many years) and unfortunately we don't have the resources to try to correct the problem at this time.

That is exactly what I was saying:
"The parameters pole_lat and pole_lon specify the latitude and longitude of the geographic north pole within the model’s computational grid, and stand_lon gives the rotation about the earth’s axis. In the context of the ARW, the computational grid refers to the regular latitude-longitude grid on which model computation is done, and on whose latitude circles Fourier filters are applied at high latitudes; users interested in the details of this filtering are referred to the WRF Version 3 Technical Note, and here, it suffices to note that the computational latitude-longitude grid is always represented with computational latitude lines running parallel to the x-axis of the model grid and computational longitude lines running parallel to the y-axis of the grid."

But the pole_lat is not the north pole latitude at all and stand_lon gives the rotation, but what direction is the rotation?

An image to show that would be great

Hi,
Can you tell me what domain you are interested in and why you want to use the rotated lat-lon grid for it? I'd like to get a better idea of this so that I can try to explain the details of this for your particular purpose.

Thanks

right now, interested in the European domain -

Can you explain why you would like to use a rotated lat-lon grid, instead of something like a lambert conformal projection?

That is typical requirement for Cordex

Can you clarify what you mean when you say "the pole_lat is not the north pole latitude at all"?

The rotation is counter-clockwise about the geographic North Pole.

To get a clearer understanding of the parameters pole_lat, pole_lon, and stand_lon, it may be helpful to try setting up a few global domains using the following geogrid settings.

Firstly, no rotation at all, with the geographic North Pole corresponding to the computational pole:
Code:
`````` map_proj = 'lat-lon',
stand_lon = 0.0,
pole_lat = 90.0
pole_lon = 0.0``````

Secondly, shifting of the geographic North Pole to computational (lat,lon) = 45.0, 0.0:
Code:
`````` map_proj = 'lat-lon',
stand_lon = 0.0,
pole_lat = 45.0
pole_lon = 0.0``````

Finally, rotation about the shifted North Pole by 60 degrees:
Code:
`````` map_proj = 'lat-lon',
stand_lon = 60.0,
pole_lat = 45.0
pole_lon = 0.0``````

What should be the time step value in case of lat-lon projection? Usually we use 6*dx or less if the dx in km. But now dx is in degree (i.e. 0.2x0.2). 6*0.2 becomes 1.2 sec now.

@rabbanidu93,
Will you please post your question as a new topic? The subject is somewhat different and will help future users to search for it more easily if it's not buried in the correspondence of another topic. It should probably go under the WRF section (instead of WPS), since the question refers to the WRF namelist. Thank you!

kwerner said:
@rabbanidu93,
Will you please post your question as a new topic? The subject is somewhat different and will help future users to search for it more easily if it's not buried in the correspondence of another topic. It should probably go under the WRF section (instead of WPS), since the question refers to the WRF namelist. Thank you!

Yes. I have already posted it there. thanks.