Mercator projection in Grads

[Suru Polade]

Dear
        Sir,
               Thank you very much, for your nice reply, i now how to
convert from Marcator Space to Lat lon space,
                              Thank once more

Suraj


On 11/15/07, Eric Altshuler <ela at cola.iges.org> wrote:
>
> 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
>
> Â Â Â Â Â Â Â  Sir
>
>                I have emission field from WRF-Chem model
> over the Indian Region, at the resolution of 1/4 Deg,
>
> Â Â Â Â Â Â Â Â  with
>
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  True Latitude 1 =Â  Â 22.875
>
>                  Center    Lat    =    22.875
>
>                 Center     LON  =    82.875
>
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  DXÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  =Â Â
> Â Â 1/4 DegÂ
>
> Â
>
>           North West  corner Lat Lon ( 36.79, 66.77)     Â
> Â  Â Â Â Â 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
>
> Â
>
> 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
>
> Â
>
> so if giving 121 X 121 in ctl file then emission field is shifting from
> it's original lat lon position.
>
> Â
>
> i am attaching here data file gs file and ctl file, can you please go
> through it
>
> Â
>
> Â Â Â Â Â Â
>
> Â your sincerely
>
> Â
>
> Suraj
> Â
> 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.
> >
>
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