Hi Randy,
This projection was added to CF with Trac #72
<
https://cf-pcmdi.llnl.gov/trac/ticket/72>. There's some discussion around
the x,y coordinates. I think comment 12
<
https://cf-pcmdi.llnl.gov/trac/ticket/72#comment:12> from John Caron
addresses the issue of using radians instead of meters:
"Ideally they would be something like "km on the projection plane", but
then (I think) you need scaling factors that depend on the instrument. If
you include the scaling factors into the coordinates, then you have (I
guess) radians."
Cheers,
Ethan
On Tue, Apr 10, 2018 at 8:09 AM, Randy Horne <rhorne at excaliburlabs.com>
wrote:
> Ethan:
>
> What you suggest is fine.
>
> As an aside ?.
> If you look at the CF standard name table, the canonical units for
> standard name ? projection_x_coordinate? and ?projection_y_coordinate? are
> meters (not radians).
>
> The GOES-R designers (specifically me) inadvertently used these two
> standard names, not realizing they should NOT have used them because a
> standard name can not have two different canonical units.
>
> Now, because the GOES-R system is already operational and in use, it would
> be major rework for GOES-R to use a yet to be defined standard name (such
> as projection_x_angilar_coordinate and projection_y_angular_coordinate).
>
>
> Not sure what to d about this ?.
>
>
>
> v/r
>
> randy
>
>
>
> On Apr 9, 2018, at 3:54 PM, Ethan Davis <edavis at ucar.edu> wrote:
>
> Hi all,
>
> The "Geostationary projection" section of Appendix F "Grid Mappings" says
>
> The x (abscissa) and y (ordinate) rectangular coordinates are identified
> by the standard_name attribute values projection_x_coordinate and
> projection_y_coordinate respectively. In the case of this projection, the
> projection coordinates in this projection are directly related to the
> scanning angle of the satellite instrument, and their units are radians.
>
>
> To more explicitly fit CF expectations for units of variables with a
> standard_name attribute, I believe the last bit should read:
>
> ... and their *canonical* units are radians.
>
>
> This came up because the GOES-16 Full Disk data (example below [1]) is
> stored with the projection_{x|y}_coordinate values in microradians and, it
> turns out, the netCDF-Java code didn't like that as it expected radians.
> (Oops!)
>
> Unless anyone disagrees, I will open a CF Trac ticket for this change
> later this week.
>
> Thanks,
>
> Ethan
>
> [1]
> http://thredds-test.unidata.ucar.edu/thredds/dodsC/
> satellite/goes16/GOES16/FullDisk/Channel08/20180406/
> GOES16_FullDisk_20180406_201540_6.19_6km_0.0S_75.0W.nc4.html
>
> netcdf GOES16_FullDisk_20180406_201540_6.19_6km_0.0S_75.0W {
> dimensions:
> x = 1808 ;
> y = 1808 ;
> variables:
> int time ;
> time:units = "seconds since 2017-01-01" ;
> time:standard_name = "time" ;
> time:long_name = "The start date / time that the satellite began capturing
> the scene" ;
> time:axis = "T" ;
> time:calendar = "standard" ;
> short y(y) ;
> y:standard_name = "projection_y_coordinate" ;
> y:units = "microradian" ;
> y:scale_factor = -167.999999999971 ;
> y:add_offset = 151788. ;
> short x(x) ;
> x:standard_name = "projection_x_coordinate" ;
> x:units = "microradian" ;
> x:scale_factor = 167.999999999971 ;
> x:add_offset = -151788. ;
> int fixedgrid_projection ;
> fixedgrid_projection:grid_mapping_name = "geostationary" ;
> fixedgrid_projection:latitude_of_projection_origin = 0. ;
> fixedgrid_projection:longitude_of_projection_origin = -75. ;
> fixedgrid_projection:semi_major = 6378137. ;
> fixedgrid_projection:semi_major_axis = 6378137. ;
> fixedgrid_projection:semi_minor = 6356752.31414 ;
> fixedgrid_projection:semi_minor_axis = 6356752.31414 ;
> fixedgrid_projection:perspective_point_height = 35785831. ;
> fixedgrid_projection:sweep_angle_axis = "x" ;
> short Sectorized_CMI(y, x) ;
> Sectorized_CMI:_FillValue = 0s ;
> Sectorized_CMI:standard_name = "brightness_temperature" ;
> Sectorized_CMI:units = "kelvin" ;
> Sectorized_CMI:grid_mapping = "fixedgrid_projection" ;
> Sectorized_CMI:add_offset = 138.05f ;
> Sectorized_CMI:scale_factor = 0.04224986f ;
> Sectorized_CMI:valid_min = 0s ;
> Sectorized_CMI:valid_max = 4095s ;
> Sectorized_CMI:coordinates = "time y x" ;
>
> // global attributes:
> :title = "Sectorized Cloud and Moisture Imagery for the EFD region." ;
> :ICD_version = "GROUND SEGMENT (GS) TO ADVANCED WEATHER INTERACTIVE
> PROCESSING SYSTEM (AWIPS) INTERFACE CONTROL DOCUMENT (ICD) Revision B" ;
> :Conventions = "CF-1.6" ;
> :channel_id = 8 ;
> :central_wavelength = 6.19f ;
> :abi_mode = 3 ;
> :source_scene = "FullDisk" ;
> :periodicity = 15.f ;
> :production_location = "RBU" ;
> :product_name = "EFD-060-B12-M3C08" ;
> :satellite_id = "GOES-16" ;
> :product_center_latitude = 0. ;
> :product_center_longitude = -75. ;
> :projection = "Fixed Grid" ;
> :bit_depth = 12 ;
> :source_spatial_resolution = 2.f ;
> :request_spatial_resolution = 6.f ;
> :start_date_time = "2018096201540" ;
> :number_product_tiles = 4 ;
> :product_tile_width = 1024 ;
> :product_tile_height = 1024 ;
> :product_rows = 1808 ;
> :product_columns = 1808 ;
> :pixel_x_size = 6. ;
> :pixel_y_size = 6. ;
> :satellite_latitude = 0. ;
> :satellite_longitude = -75. ;
> :satellite_altitude = 35785831. ;
> :created_by = "ldm-alchemy" ;
> :product_tiles_received = 4 ;
> }
>
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>
> _____________________________________
>
> Randy C Horne (rhorne at excaliburlabs.com)
> Principal Engineer, Excalibur Laboratories Inc.
> voice & fax: (321) 952.5100
> cell: (321) 693.1074
> url: http://www.excaliburlabs.com
>
>
>
>
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