[ncl-talk] Map Reprojection, Spatial Aggregation and Regridding

Mon Dec 10 11:26:28 MST 2018

Hi Scott,

I talked this over with Mary, and the map projection shouldn't matter. The
regridder works in lat/lon space and knows how to handle curvilinear
lat/lon coordinates, so it doesn't care about the projected "meters" space
from the map projection transformation. Since you have the lat/lons for
each grid box, it's going to regrid your data to the same lat/lon points on
your destination grid (which you can plot via the same lambert settings).
Note that the lat/lons are for the center of the grid cell, and if you have
any missing/fill values in your source data, you need to supply a mask for
these points or you'll get some weird artifacts.

Hope this helps,

Bill

On Fri, Dec 7, 2018 at 12:47 PM Scott Capps <scapps at atmosdatasolutions.com>
wrote:

> Thank you.  Yes, I have lat and lon values and could use the ESMF
> regridding functions.  However, these lat lons are on different map
> projections and I wondering if the necessary reprojection is handled using
> the ESMF regridding software.
> On Fri, Dec 7, 2018 at 9:29 AM Bill Ladwig <ladwig at ucar.edu> wrote:
>
>> Hi Scott,
>>
>> You know what, I think you already have the lat/lon values for both your
>> source and destination grids at each grid point, so you can bypass that map
>> projection stuff and go right to the ESMF regridder. There's plenty of
>> examples here: https://www.ncl.ucar.edu/Applications/ESMF.shtml. You'll
>> be going from a rectilinear grid to a curvilinear grid. This example is
>> similar, but you'll want to go in the opposite direction:
>> https://www.ncl.ucar.edu/Applications/Scripts/ESMF_all_5.ncl
>>
>> Hope this helps,
>>
>> Bill
>>
>> On Thu, Dec 6, 2018 at 7:06 PM Scott Capps <scapps at atmosdatasolutions.com>
>> wrote:
>>
>>> Thank you.  I have looked at that function and I don't think it will
>>> work.  It would need to accept as input the 1D lat (y) and lon (x)
>>> variables and output two lat2d and lon2d variables.  Not sure what the "z"
>>> input is (I was not able to find the source code for this function).
>>>
>>> wgsfile = addfile("./territory_slope_2km_epsg4326.nc","r")
>>> x = wgsfile->lon
>>> y = wgsfile->lat
>>> printVarSummary(x) ; [lon | 385]
>>> printVarSummary(y) ; [lat | 359]
>>> ; To reproject to WRF, must relabel as spherical lat/lon (even though
>>> the file is in WGS84 ellipsoid):
>>> SrcProjStr = "+proj=latlong +a=6370 +b=6370 +towgs84=0,0,0 +no_defs"
>>> ; LCC WRF using truelats, center_lons, etc..
>>> DestProjStr = "+proj=lcc +lat_1=30.0 +lat_2=41.0 +lat_0=35.80001
>>> +lon_0=-120.095 +a=6370 +b=6370 +towgs84=0,0,0 +no_defs"
>>>
>>> transform_coordinate(SrcProjStr, DestProjStr, x, y, z)
>>>
>>> On Thu, Dec 6, 2018 at 1:53 PM Bill Ladwig <ladwig at ucar.edu> wrote:
>>>
>>>> Hi Scott,
>>>>
>>>> See this:
>>>> http://mailman.ucar.edu/pipermail/ncl-talk/2016-October/007059.html
>>>>
>>>> Remember when setting up the Proj4 Lambert Conformal string for WRF to
>>>> use a sphere with a radius of 6370000. I believe that function will give
>>>> you fractional x,y values, which you'll need to do some kind of
>>>> interpolation (or nearest neighbor if that's good enough) to do your
>>>> computation.
>>>>
>>>> Hope this helps,
>>>>
>>>> Bill
>>>>
>>>>
>>>> On Wed, Dec 5, 2018 at 1:58 PM Scott Capps <
>>>> scapps at atmosdatasolutions.com> wrote:
>>>>
>>>>>
>>>>> I have created a NetCDF file with data (terrain slope) on the WGS84
>>>>> 4326 map projection (ncdump -h output shown below):
>>>>>
>>>>> dimensions:
>>>>> lon = 27382 ;
>>>>> lat = 25569 ;
>>>>> variables:
>>>>> char crs ;
>>>>> crs:grid_mapping_name = "latitude_longitude" ;
>>>>> crs:long_name = "CRS definition" ;
>>>>> crs:longitude_of_prime_meridian = 0. ;
>>>>> crs:semi_major_axis = 6378137. ;
>>>>> crs:inverse_flattening = 298.257223563 ;
>>>>> crs:spatial_ref = "GEOGCS[\"WGS 84\",DATUM[\"WGS_1984\",SPHEROID[\"WGS
>>>>> 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0],UNIT[\"degree\",0.0174532925199433],AUTHORITY[\"EPSG\",\"4326\"]]"
>>>>> ;
>>>>> crs:GeoTransform = "-121.3651264724438 0.0002790528478797247 0
>>>>> 39.07036311976184 0 -0.0002790528478797247 " ;
>>>>> double lat(lat) ;
>>>>> lat:standard_name = "latitude" ;
>>>>> lat:long_name = "latitude" ;
>>>>> lat:units = "degrees_north" ;
>>>>> double lon(lon) ;
>>>>> lon:standard_name = "longitude" ;
>>>>> lon:long_name = "longitude" ;
>>>>> lon:units = "degrees_east" ;
>>>>> float Band1(lat, lon) ;
>>>>> Band1:long_name = "GDAL Band Number 1" ;
>>>>> Band1:_FillValue = -9999.f ;
>>>>> Band1:grid_mapping = "crs" ;
>>>>>
>>>>> // global attributes:
>>>>> :GDAL_AREA_OR_POINT = "Area" ;
>>>>> :Conventions = "CF-1.5" ;
>>>>> :GDAL = "GDAL 2.3.2, released 2018/09/21" ;
>>>>> :history = "Wed Dec 05 11:41:45 2018: GDAL CreateCopy(
>>>>> territory_epsg4326_slope.nc, ... )" ;
>>>>> }
>>>>>
>>>>> And, I would like to calculate the average terrain slope in each grid
>>>>> cell on my coarser WRF domain (2km resolution).
>>>>>
>>>>> Can NCL correctly handle the re-projection from the file above:
>>>>>
>>>>> Geodetic CRS: WGS84
>>>>> Datum: World Geodetic System 1984
>>>>> Ellipsoid: WGS 84
>>>>> Prime Meridian: Greenwich
>>>>>
>>>>> to my WRF Lambert Conformal Conic projection while calculating the
>>>>> average terrain slope in each overlapping, coarser resolution WRF
>>>>> gridcell?
>>>>>
>>>>> Any help or suggestions are appreciated.
>>>>>
>>>>> Thank you,
>>>>>
>>>>> Scott
>>>>> _______________________________________________
>>>>> ncl-talk mailing list
>>>>> ncl-talk at ucar.edu
>>>>> List instructions, subscriber options, unsubscribe:
>>>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>>>>
>>>>
>>>
>>> --
>>> *Scott Capps*
>>> Principal
>>> Atmospheric Data Solutions <http://www.atmosphericdatasolutions.com>
>>> scapps at atmosdatasolutions.com
>>> cell: (949) 910-4385
>>>
>>>
>
> --
> *Scott Capps*
> Principal
> Atmospheric Data Solutions <http://www.atmosphericdatasolutions.com>
> scapps at atmosdatasolutions.com
> cell: (949) 910-4385
>
>
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