Mercator projection in Grads

[Eric Altshuler]

Dear Suru Polade,

In the Mercator projection the grid interval in latitude is not constant. It varies from north to south. If you use a constant grid interval in your YDEF statement, your plot will be incorrect. You need to specify each individual latitude like this:

YDEF 121 levels LAT_1 LAT_2 LAT_3 ... LAT_121

To find out the values of the latitudes, you need to transform the location of each grid point in Mercator-projection space to lat-lon space. There are formulas that can be used to do this, but I'm not familiar with the case where the standard latitude is not on the equator (22.875N in your case). If the original source of your data is in netcdf or grib format, the metadata may include the latitude values.

Your model may have resolution of "X" km but this does not mean the grid interval is the same everywhere in your domain. Also, the post-processed output data may have a different resolution and/or be on a different grid than your model's native grid.

Best regards,


Eric L. Altshuler
Assistant Research Scientist
Center for Ocean-Land-Atmosphere Studies
4041 Powder Mill Road, Suite 302
Calverton, MD 20705-3106
USA

E-mail: ela at cola.iges.org
Phone: (301) 902-1257
Fax: (301) 595-9793

----- Original Message -----
From: "Suru Polade" <gr.suru at GMAIL.COM>
To: GRADSUSR at LIST.CINECA.IT
Sent: Wednesday, November 14, 2007 12:48:38 AM (GMT-0500) America/New_York
Subject: Re: Mercator projection in Grads




Dear

        Sir

               I have emission field from WRF-Chem model over the Indian Region, at the resolution of 1/4 Deg,

         with

                 True Latitude 1 =   22.875

                 Center    Lat    =    22.875

                Center     LON  =    82.875

                 DX                  =    1/4 DegÂ

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          North West  corner Lat Lon ( 36.79, 66.77)            Â

          South West corner Lat Lon (7.33, 66.77)

          North East   Corner Lat Lon (36.79, 98.98)

          South East Corner  Lat Lon ( 7.38, 98.98)

Â

if with 1/4 deg we are calculation number of Grid  it will comeÂ

       North South Grid number  117Â

       West East Grid number    129

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but in WRF-SI, Dx we need to give in Km

if considering 1 Deg = 110 Km then dx will come 27.5 km

that will create 121 X 121 Grid with same center and corner lat lon for Mercator projection Â

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so if giving 121 X 121 in ctl file then emission field is shifting from it's original lat lon position.

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i am attaching here data file gs file and ctl file, can you please go through it

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     Â

 your sincerely

Â

Suraj
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On 11/13/07 , Brian Doty < doty at cola.iges.org > wrote:

A Mercator grid is typically rectangular in lat/lon, and can be dealt
with using the "levels" keyword on the ydef statement in the
descriptor file... Brian

On Nov 13, 2007 , at 5:43 AM, Suru Polade wrote:

> I have a pre-projected field in a Mercator projection
>
>Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Is there any way in grads to work with Mercator
> projection, as this projection is not there for pre-projected data.
>