[ncl-talk] Regridding from OSGB spatial coordinate to Latitude-longitude in rotated pole coordinates

Thu Feb 25 10:41:05 MST 2021

It helps to give NCL the correct lat/lon array information.

See attached

On Thu, Feb 25, 2021 at 10:23 AM Dave Allured - NOAA Affiliate via ncl-talk
<ncl-talk at mailman.ucar.edu> wrote:

> That script looks pretty good.  You can see from the error messages that
> NCL is trying to understand @lat2d and @lon2d, but something is not quite
> right with them.  We need to see printVarSummary for tas, tas at lat2d, and
> tas at lon2d.  It is possible that dimensions are mismatched between data
> and coordinates, for some reason.
>
> There is also this note in that doc page:  "If your data variable is a
> two-dimensional (2D) array ordered lon x lat, you will need to reorder it
> to be lat x lon."  Please check and see if this is the problem.
>
>
> On Thu, Feb 25, 2021 at 10:00 AM S Br <sbr.climate at gmail.com> wrote:
>
>>
>> Hi Dave,
>> Many thanks for your reply. Sorry I missed to provide the details.
>> I have run the following NCL script  and came across the errors given
>> below.
>> You can see the attached plot that it draws value throughout the globe.
>> It is supposed to be drawn only over the UK.
>> Am I doing something wrong?
>>
>> ;********
>> srcFileName="tas_rcp85_land-rcm_uk_12km_01_mon_198012-208011.nc"
>>
>> ;---Read File
>> f=addfile(srcFileName,"r")
>> print(f)
>>
>> ;---Get the source lat/lon grid
>> tas=f->tas(0,0,:,:)
>> tas at lat2d=f->projection_y_coordinate
>> tas at lon2d=f->projection_x_coordinate
>>
>> ;---Plot data
>>   wks = gsn_open_wks("x11","map")
>>
>>   res              = True
>>   res at gsnMaximize  = True     ; maximize plot in frame
>>   res at cnFillOn     = True     ; turn on contour fill
>>   res at cnLinesOn    = False    ; turn off contour fill
>>   res at gsnAddCyclic     = False
>> ;   res at mpMinLatF    =  48.0             ; only plot 30S to 30N
>> ;   res at mpMaxLatF    =  60.0
>> ;  res at mpMinLonF    =  -15.0          ; only plot 30S to 30N
>> ;   res at mpMaxLonF    = 6.0
>>  res at mpDataBaseVersion = "MediumRes"
>>    res at mpLandFillColor = "white"
>>
>>   plot = gsn_csm_contour_map(wks,tas(:,:),res)
>>
>> ********************
>> (0) is_valid_latlon2d_attr: Warning: The 'lat2d' attribute must either be
>> (0) the same dimension sizes as the data, or one element larger in both
>> directions.
>> (0) Your data will most likely not be overlaid on the map correctly.
>> (0) check_for_y_lat_coord: Warning: Data either does not contain
>> (0) a valid latitude coordinate array or doesn't contain one at all.
>> (0) A valid latitude coordinate array should have a 'units'
>> (0) attribute equal to one of the following values:
>> (0)    'degrees_north' 'degrees-north' 'degree_north' 'degrees north'
>> 'degrees_N' 'Degrees_north' 'degree_N' 'degreeN' 'degreesN' 'deg north'
>> (0) is_valid_latlon2d_attr: Warning: The 'lon2d' attribute must either be
>> (0) the same dimension sizes as the data, or one element larger in both
>> directions.
>> (0) Your data will most likely not be overlaid on the map correctly.
>> (0) check_for_lon_coord: Warning: Data either does not contain
>> (0) a valid longitude coordinate array or doesn't contain one at all.
>> (0) A valid longitude coordinate array should have a 'units'
>> (0) attribute equal to one of the following values:
>> (0)    'degrees_east' 'degrees-east' 'degree_east' 'degrees east'
>> 'degrees_E' 'Degrees_east' 'degree_E' 'degreeE' 'degreesE' 'deg east'
>>
>> On Thu, Feb 25, 2021 at 4:17 PM Dave Allured - NOAA Affiliate <
>> dave.allured at noaa.gov> wrote:
>>
>>> Does your file contain its own lat and lon coordinate arrays?  You will
>>> need to check this yourself with ncdump or NCL plus print commands.
>>>
>>> If the file has its own coordinate arrays, then do not regrid or
>>> reproject.  Doing that will lose detail and reduce the quality of the
>>> plotted image.  NCL is quite capable of directly displaying any data with
>>> attached coordinate arrays, and it does not need to know anything about the
>>> projection of the data.  Please see documentation for "Plotting data on a
>>> map" for much more information about aligning data on a map plot.
>>>
>>>     https://www.ncl.ucar.edu/Applications/plot_data_on_map.shtml
>>>
>>> If the coordinate arrays in the file are two dimensional, then see
>>> example 3, "Curvilinear grid".  When you have 2-D coordinates, they must be
>>> passed to the NCL plot routine with special attributes *lat2d* and
>>> *lon2d*, or alternatively *sfXArray* and *sfYArray* attached to the
>>> plot resource variable.  Note that 1-D coordinates do not need this special
>>> treatment because they are normally attached directly to the data variable
>>> to plot.
>>>
>>>
>>> On Thu, Feb 25, 2021 at 8:24 AM S Br via ncl-talk <
>>> ncl-talk at mailman.ucar.edu> wrote:
>>>
>>>> Hi All,
>>>> I have a set of regional gridded data in the British National Grid
>>>> (OSGB) spatial coordinate system. I am looking to use this data in NCL but
>>>> the projection is not correctly displayed? Could it be possible to regrid
>>>> this data to Latitude-longitude in rotated pole coordinates or to a regular
>>>> lat/lon grid.
>>>>
>>>> I would like to mention that these data sets are generated using the
>>>> Iris package in Python. The data is correctly displayed in Python but not
>>>> in NCL.
>>>>
>>>> For your convenience I have attached here a sample NetCDF file.
>>>>
>>>> Thank you.
>>>> SBR
>>>>
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