Re: [CF-metadata] Choice of fill value for unpacked data
On 10/10/2012 6:36 AM, Jim Biard wrote: John, I agree that there is no ambiguity in the original dataset. I was just pointing out a scenario in which the difficulty mentioned by Phil in his original post would manifest itself. The methodology referenced by Russ won't handle it. If you (for reasons beyond your control) have a packed fill value that is inside your valid range, then no guidance is provided by CF to automated client software for how to represent the unpacked fill value, leaving the user with a requirement to dig back through the packed values, find the elements marked as fill, and "manually" mark the unpacked values. Its true CF is silent about this, so "automated client software" has to make their own Convention. The netcdf-JAVA package supplies a method boolean isMissing(double value); for this purpose. Alternately, it will just set missing real values to NaNs, which works well in Java. I think NaNs are the right choice for missing values myself. ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
[CF-metadata] Coordinate systems
Hi. I mentioned I thought/concern I had about coordinate systems in CF as an aside a while back, and I didn't much response, so I'm writing it now as a standalone post. When I look at the way geographic coordinate systems are implemented in CF, I keep getting the feeling that the associations (in a UML sense) don't make good sense. What we have right now is that a variable has coordinates associated with it, and also has a grid_mapping (a definition of a geographic coordinate system) associated with it. Here's a cheap ASCII picture of that. [coordinate]2-->*[variable]*<--1[grid_mapping] * ^ | | 2 [lon/lat] I'm on purpose limiting myself for the sake of this discussion to the case of 2 coordinates (X/Y, lon/lat, etc) for any given variable. I know there can be more, but we are still fuzzy on Z, and other coordinates that might be present aren't geographic. I have also shown the longitude and latitude auxiliary coordinates smashed together into a single element for compactness. I hope it's not overly confusing. Based on this model, the coordinates don't have a coordinate system except by relation through a variable. As a result, the same coordinates could be associated with different geographic coordinate systems in different variables. You could two different grid_mappings defined (UTM zone 8 and Polar Stereographic, for example) associate one with variable 1 and the other with variable 2, yet have the same coordinates associated with both variable 1 and variable 2. Furthermore, the longitude and latitude are assumed to relate (via dimensionality) with the coordinates as defined by the grid_mapping, which could (again) be different for each variable. This model seems more than a bit fragile and unnatural to me. The thing is, coordinates are meaningless without the definition of the coordinate system they belong to. Starting from scratch, the natural approach to this would be to associate coordinate systems with coordinates, which can then be associated with variables. The diagram would be: [grid_mapping]1->*[(aux) coordinate]*-->*[variable] Each coordinate would have a geographic coordinate system associated with it. A variable could have any number of (pairs of) coordinates associated with it. Longitude/latitude or other auxiliary coordinates would fit into this model in just the same way. Written as CDL, what you would see is: netcdf { dimensions: xt = 50; yt = 50; variables: float xt(xt=50); :grid_mapping = "TM"; :units = "m"; float yt(yt=50); :grid_mapping = "TM"; :units = "m"; float temperature(xt=50, yt=50); :coordinates = "lon lat xp yp"; :units = "K"; float lon(xt=50, yt=50); :units = "degrees_east"; :grid_mapping = "LonLat"; float lat(xt=50, yt=50); :units = "degrees_north"; :grid_mapping = "LonLat"; float xp(xt=50, yt=50); :units = "m"; :grid_mapping = "PolarStereo"; float yp(xt=50, yt=50); :coordinates = "xt yt"; :units = "m"; :grid_mapping = "PolarStereo"; char LonLat; :grid_mapping_name = "latitude_longitude"; char PolarStereo; :grid_mapping_name = "polar_stereographic"; char TM; :grid_mapping_name = "transverse_mercator"; } I left out the details of the grid_mapping variables for brevity. So, having said all that, I'm curious to know if there is any particular reason why geographic coordinate systems are being done the way they are right now, and wondering if what I have described makes sense to anyone else. Grace and peace, Jim Jim Biard Research Scholar Cooperative Institute for Climate and Satellites Remote Sensing and Applications Division National Climatic Data Center 151 Patton Ave, Asheville, NC 28801-5001 jim.bi...@noaa.gov 828-271-4900 ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
[CF-metadata] CF-netcdf for Mercator projection - use lat/lon, or xc/yc plus lat/lon
I have data that I'm putting into CF netcdf format (conventions 1.6) that is in the Mercator projection. I'm trying to determine which method is preferred for CF-compliance, in regard to the projection information. The basic question is: Since the Mercator projection is N-S-E-W, is there any need to provide projection coordinates (xc, yc)? I can see the need to provide the projection coordinates for projections such as Polar Stereographic. Example 1: Use of x and y projection coordinates, with separate lat/lon variables. netcdf \1 { dimensions: xc = 287 ; yc = 232 ; time = 1 ; variables: char Mercator ; Mercator:longitude_of_projection_origin = 135.f ; Mercator:long_name = "projection information" ; Mercator:standard_parallel = 0.f ; Mercator:false_northing = 0.f ; Mercator:grid_mapping_name = "mercator" ; Mercator:false_easting = 0.f ; float yc(yc) ; yc:units = "m" ; yc:standard_name = "projection_y_coordinate" ; float xc(xc) ; xc:units = "m" ; xc:standard_name = "projection_x_coordinate" ; int time(time) ; time:units = "hours since 1970-01-01T00:00:00Z" ; time:long_name = "starting time for grids" ; time:standard_name = "time" ; time:calendar = "gregorian" ; float longitude(yc, xc) ; longitude:units = "degrees_east" ; longitude:standard_name = "longitude" ; float latitude(yc, xc) ; latitude:units = "degrees_north" ; latitude:standard_name = "latitude" ; float Wind_SFC(time, yc, xc) ; Wind_Mag_SFC:_FillValue = -3.f ; Wind_Mag_SFC:grid_mapping = "Mercator" ; Wind_Mag_SFC:coordinates = "latitude longitude" ; Wind_Mag_SFC:valid_range = 0.f, 64.30556f ; Wind_Mag_SFC:standard_name = "wind_speed" ; Wind_Mag_SFC:cell_methods = "time: mode" ; Wind_Mag_SFC:units = "m sec**-1" ; Example 2: Just using lat/lon coordinate variables, even though the projection is not latlon. netcdf \1 { dimensions: longitude = 287 ; latitude = 232 ; time = 1 ; variables: char Mercator ; Mercator:longitude_of_projection_origin = 135.f ; Mercator:long_name = "projection information" ; Mercator:standard_parallel = 0.f ; Mercator:false_northing = 0.f ; Mercator:grid_mapping_name = "mercator" ; Mercator:false_easting = 0.f ; int time(time) ; time:units = "hours since 1970-01-01T00:00:00Z" ; time:long_name = "starting time for grids" ; time:standard_name = "time" ; time:calendar = "gregorian" ; float longitude(longitude) ; longitude:units = "degrees_east" ; longitude:standard_name = "longitude" ; float latitude(latitude) ; latitude:units = "degrees_north" ; latitude:standard_name = "latitude" ; float Wind_SFC(time, latitude, longitude) ; Wind_Mag_SFC:_FillValue = -3.f ; Wind_Mag_SFC:grid_mapping = "Mercator" ; Wind_Mag_SFC:valid_range = 0.f, 64.30556f ; Wind_Mag_SFC:standard_name = "wind_speed" ; Wind_Mag_SFC:cell_methods = "time: mode" ; Wind_Mag_SFC:units = "m sec**-1" ; Which example is preferred? If example 2 is used, should the xy coordinates be provided as: float xc(longitude) ; float yc(latitude); and use the coordinate attribute for Wind pointing to xc and yc? Thanks. Regards, Mark -- Mark Mathewson Consulting 535 Swanson Mill Rd. Tonasket, WA 98855 509-429-8635 ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata