[ncl-talk] grib projection conversion

[Micah Sklut]

Thanks Rick. I think this transform_coordinate function will take care of
my needs.

In my case the gfs data has a latitude array of 1440, and longitude array
of 721.  So, does this mean, I need to pass transform_coordinate a 1d array
for X that shows latitude values for every coordinate? Meaning 1440x721
values? And, subsequently for Y? And, I"ll pass a 1d array for Z with all
zeroes that have 1440x721 values.

On Tue, Dec 3, 2019 at 5:13 PM Rick Brownrigg <brownrig at ucar.edu> wrote:

> Hi,
>
> NCL has an undocumented interface to the Proj4 cartographic projection
> library that could produce coordinates in a Mercator projection, given a
> list of lat/lons:
>
>    transform_coordinate(srcProj:string, dstProj:string, x[*]:double,
> y[*]:double, z[*]:double)
>
> Where srcProj/dstProj are the "proj4 strings" describing the source and
> destination projections. In your case, the srcProj would be something like:
>   "+proj=lonlat +ellps=sphere"     # I've encountered some instance where
> things fail without specifying the ellipsoid
>
> dest:
>
>   "+proj=merc"
>
> The x/y/z arrays are the same length; the z can be set to zeros if real
> data unavailable. The conversion happens "in place".  More on proj4 here.
> Particularly if your area of interest is not global, there are likely other
> parameters to the Mercator you might want to add, like center of
> projection, etc:
>
>     https://proj.org/
>
> (Note, NCL's binding to proj4 is specifically the older v4.xxx libraries,
> not the newer v.6. series. The proj-strings should be compatible however)
>
> Finally, I don't understand the bit about starting with a 1440x721 and
> ending up with a 1440x1440. It almost sounds like you'll need to regrid to
> that density in lat/lon space prior to projecting.
>
>
> Rick
>
>
> On Tue, Dec 3, 2019 at 3:01 PM Micah Sklut via ncl-talk <ncl-talk at ucar.edu>
> wrote:
>
>> Hi,
>>
>> I'm interested in the best way to convert lat/lon and corresponding data
>> values from GFS grib data into Mercator projection values. The use case is
>> not for mapping in NCL, but taking an input grib file and converting values
>> to an output grib file.
>>
>> The GFS data I have is 0.25x0.25, so 1440x721 values, so I believe I'm
>> looking for the Mercator projection to provide 1440x1440 values, where the
>> lat/lons are spaced according to the Mercator projection. Is there a
>> conversion function in NCL that will do this?
>>
>> Thanks!
>>
>> --
>> Micah Sklut
>>
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>

-- 
Micah Sklut
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