Hi.

Is it possible that we just need to loosen the definition of
projection_x_coordinate and projection_y_coordinate?

Jim

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On Tue, Apr 10, 2018 at 1:33 PM, Ethan Davis <eda...@ucar.edu> wrote:

> 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 <rho...@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 <eda...@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 (rho...@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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