Dear all
Metres and radians are not interconvertible, so projection_[xy]_coordinate
should not be used as a standard name for a quantity in radians. I think that
a defect ticket is needed to change App F for this projection. Possibly we
might need new standard names if there aren't appropriate ones.
Best wishes
Jonathan
----- Forwarded message from Jim Biard <jbiard at cicsnc.org> -----
> Date: Tue, 10 Apr 2018 12:32:00 -0400
> From: Jim Biard <jbiard at cicsnc.org>
> To: CF metadata <cf-metadata at cgd.ucar.edu>
> Subject: Re: [CF-metadata] Units of projection_x_coordinate values in
> "Geospatial projection"
>
> Hi.
>
> It sounds like perhaps we should avoid the word "canonical". Perhaps we
> should change the relevant bit of the definition to read
>
> 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 - typically an angular quantity.
>
>
> Software shouldn't assume the units. Microradians, degrees, grads, etc
> should all be fine. Does anyone think there is a problem with storing an
> angle in a variable with the standard name projection_x_coordinate? Do we
> need different standard names for these?
>
> This may indicate that projections that have specific requirements about
> units in the coordinates need to declare that information in the
> grid_mapping variable attributes. We tend to gloss over that, but there are
> projections that expect the coordinates to be latitude and longitude
> instead of x and y. In addition, spherical or cylindrical coordinate
> systems would expect at least one coordinate to be angular. Thoughts?
>
> Grace and peace,
>
> Jim
>
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>
> On Tue, Apr 10, 2018 at 10:09 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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> > CF-metadata at cgd.ucar.edu
> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >
> >
> > _____________________________________
> >
> > 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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> > CF-metadata at cgd.ucar.edu
> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >
> >
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----- End forwarded message -----
Received on Tue Apr 10 2018 - 11:30:00 BST
