Geostationary file support

Added by Miles Sowden almost 2 years ago

Is geostationary projection fully supported in CDO (or NCO for that matter)?
At this point I’m trying to achieve a rescaling into ½ or double grid spacing as per https://stackoverflow.com/questions/55746419/change-grid-size-of-a-netcdf-file. However other operators such as "cdo chiftx,1" also fail. (sidenote I can edit the grid file to gridtype=generic to "fix" the shiftx problem)
I do not want to reproject into latitude/longitude (given as variables) as my files are huge (1.8 Gb every ten minutes). But wish to work with the files as X & Y indices for now.

For this example I cropped a larger file 5500 x 5500 with the command below and attached the file for reference
ncks -h -d x,2700,2800 -d y,4200,4300 AHI_SENSOR_2000.nc AHI.nc

The larger grid description is given in AHI.txt (attached). (obtained with cdo griddes AHI_SENSOR_2000.nc > AHI.txt)
Reading the CF conventions http://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#_geostationary_projection
I think I need to add
scanningMode = 64
grid_mapping = geostationary
grid_mapping_name = geostationary
longitude_of_projection_origin = 140.7
latitude_of_projection_origin = 0
false_easting = 0.
false_northing = 0.
perspective_point_height = 35785863.
sweep_angle_axis="x"
fixed_angle_axis="y"

I think the problem is that both CDO and NCO do not yet fully support the regular grid obtained from geostationary satellites.

Thanks

Download all files

AHI.txt (1.05 KB) AHI.txt
AHI.nc (144 KB) AHI.nc

Replies (4)

RE: Geostationary file support - Added by Karin Meier-Fleischer almost 2 years ago

Hi Miles,

I am not an expert for geostationary data, but I would guess that the following grid description file should work.

gridtype  = projection
gridsize  = 10201
xsize     = 101
ysize     = 101
xname     = x
xlongname = "projection_x_coordinate" 
xunits    = "m" 
yname     = y
ylongname = "projection_y_coordinate" 
yunits    = "m" 
xfirst    = -99000
xinc      = 2000
yfirst    = -2901000
yinc      = -2000
#
grid_mapping = geostationary
grid_mapping_name = geostationary
longitude_of_projection_origin = 140.7
latitude_of_projection_origin = 0
perspective_point_height = 35785863
semi_major_axis = 6378137
semi_minor_axis = 6356752.3
sweep_angle_axis = y
fixed_angle_axis = x
false_northing = 0
false_easting = 0
scanningMode = 64
GeoTransform = -5500000. 2000. 0. 5500000. 0. -2000.
proj_params = "+proj=geos +lon_0=140.7 +h=35785863 +x_0=0 +y_0=0 +a=6378137 +b=6356752.3 +units=m +sweep=y +no_defs"

-Karin

RE: Geostationary file support - Added by Miles Sowden almost 2 years ago

Thanks Karin,

One step forward. Repeating myself for clarity

1: Get the grid information with " cdo griddes in.nc > grid.txt "
2: Amend the grid information as above (note my file was missing the grid mapping information)
3: Update the grid information with " cdo setgrid,grid.txt in.nc out.nc " (note did this on both 1 and 2 km input grids)

Step above worked on the cropped file but I got an error as below on the larger file
cdo remapnn,Generic_1000.nc Generic_2000.nc Remap_1000.nc
Error (gridDefBoundsGeneric) : Allocation of 3872000000 bytes failed. [ line 2985 file grid.c ]

4: Reproject to my double size grid with " cdo remapnn,Crop_1000.nc Crop_2000.nc Remap_1000.nc "
5: Compare the remapped to origional with " cdo sub Remap_1000.nc Crop_1000.nc Delta_1000.nc "

6: Now compare the change on extrapolation to the change between neighbouring pixels with
" cdo sub -shifty,1 -shiftx,1 Crop_1000.nc Crop_1000.nc Near_1000.nc "
This fails with cdo(2) shiftx: Unsupported grid type: projection
I can then fix this by changing the grid type to generic, do the shift function, change back to projection.

I think the grid functions don't yet know how to fully handle geostationary projection mappings.

RE: Geostationary file support - Added by Miles Sowden almost 2 years ago

sorry error message after 3 should actually be after 4

RE: Geostationary file support - Added by Karin Meier-Fleischer almost 2 years ago

First, without the data file it is hard to reproduce the problem, but the error message tells you that there is a memory issue when you use remapnn. If you have CDO compiled with OpenMP you can use the -P option https://code.mpimet.mpg.de/projects/cdo/embedded/cdo.pdf#subsection.1.2.4

cdo -P 8 remapnn,Crop_1000.nc Crop_2000.nc Remap_1000.nc

The operators shiftx and shifty can be used with rectilinear and curvilinear grids only.

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