[ncl-talk] grib projection conversion

[Micah Sklut]

Perhaps it's converting to meters rather than degrees. The poles (90, -90),
come out as infinity, which makes sense for Mercator. I'll check out the
proj documentation to see what output options there are.

On Wed, Dec 4, 2019 at 11:56 AM Micah Sklut <micahs2005 at gmail.com> wrote:

> 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
>>>>>
>>>>> _______________________________________________
>>>>> ncl-talk mailing list
>>>>> ncl-talk at ucar.edu
>>>>> List instructions, subscriber options, unsubscribe:
>>>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>>>
>>>>
>>>
>>> --
>>> Micah Sklut
>>>
>>>
>
> --
> Micah Sklut
>
>

-- 
Micah Sklut
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