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 <[email protected]> ----- > Date: Tue, 10 Apr 2018 12:32:00 -0400 > From: Jim Biard <[email protected]> > To: CF metadata <[email protected]> > 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 > > [image: CICS-NC] <http://www.cicsnc.org/>Visit us on > Facebook <http://www.facebook.com/cicsnc> *Jim Biard* > *Research Scholar* > Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/> > North Carolina State University <http://ncsu.edu/> > NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/> > *formerly NOAA’s National Climatic Data Center* > 151 Patton Ave, Asheville, NC 28801 > e: [email protected] > o: +1 828 271 4900 > > *Connect with us on Facebook for climate > <http://www.facebook.com/NOAANCEIclimate> and ocean and geophysics > <http://www.facebook.com/NOAANCEIoceangeo> information, and follow us on > Twitter at @NOAANCEIclimate > <http://www.twitter.com/NOAANCEIclimate>and @NOAANCEIocngeo > <http://www.twitter.com/NOAANCEIocngeo>.* > > > On Tue, Apr 10, 2018 at 10:09 AM, Randy Horne <[email protected]> > 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 <[email protected]> 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 ; > > } > > > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > > > > _____________________________________ > > > > Randy C Horne ([email protected]) > > Principal Engineer, Excalibur Laboratories Inc. > > voice & fax: (321) 952.5100 > > cell: (321) 693.1074 > > url: http://www.excaliburlabs.com > > > > > > > > > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
