[ncl-talk] grib projection conversion

[Micah Sklut]

Thanks. I'll look at ndtooned(), but I did get the function to work, it
just returns values that are way outside of the expected lat/lon range.

Here's what my script looks like:

data = "~/wc_data/data/"

gfsFile = data + "gfs/gfs.t12z.pgrb2.0p25.f000.grb"

f = addfile(gfsFile,"r");
lat = f->lat_0;
lon = f->lon_0;

arraySize = dimsizes(lat) * dimsizes(lon)
x = new( (/ arraySize /), double)
y = new( (/ arraySize /), double)
z = new( (/ arraySize /), double)

latSize = dimsizes(lat)
lonSize = dimsizes(lon)

do j = 0, dimsizes(lat)-1
coordStart = j * lonSize
coordFinish = coordStart + lonSize - 1
x(coordStart:coordFinish) = lon(:)
y(coordStart:coordFinish) = lat(j)
z(coordStart:coordFinish) = 0
end do

;transform_coordinate(srcProj:string, dstProj:string, x[*]:double,
y[*]:double, z[*]:double)
transform_coordinate("+proj=lonlat +ellps=sphere", "+proj=merc", x, y, z)

print(x)

On Wed, Dec 4, 2019 at 11:50 AM Rick Brownrigg <brownrig at ucar.edu> wrote:

> Yes, you'll need to flatten your arrays into 1D. See the ndtooned() and
> related functions for ways to do that.
>
> On Wednesday, December 4, 2019, Micah Sklut <micahs2005 at gmail.com> wrote:
>
>> 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
>>
>>

-- 
Micah Sklut
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