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NETCDF:menuNETCDF:
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  1. CF standard: add a dummy variable crs and add all

...

  1. grid mapping related meta-data as attributes.
  2. The ADAGUC WMS/WCS software requires some additional attributes that contain the same information as the CF grid_mapping attributes: proj4_params and epsg_code. In the Dutch National Model and Data Centre NMDC.eu ADAGUC is used as a central component, so for NMDC these additional attributes are highly recommended.
    Code Block
    
netcdf test_map {
variables:
        double var(n);
                var:proj4_params = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs" ;
                var:EPSG_code = "EPSG:4326";
}
  • Proposal for multiple coordinate systems: https://cf-pcmdi.llnl.gov/trac/wiki/MultipleProjections
  • proposed extension: add a WKT (well-known text) string of all grid mapping related meta-data (the WKT quite laborious for parsing though). Note that the plethora of brackets and slashes can ruin opendap access to a netCDF file, see the following slash*.nc examples.
  • proposed extension:
    For files where local coordinates are important we propose the following in addition to CF make sure that the dummy variable crs it an integer and that its value is the EPSG code of the coordinate systemadd a WKT (well-known text) string of all projection related meta-data., e.g:
    • 28992 for Dutch Rijksdriehoek (projection_x_coordinate,projection_y_coordinate)
    • 4326 for WGS84 global ellipsoid (latitude, longitude)
    • 32631 for WGS 84 / UTM zone 31N (projection_x_coordinate,projection_y_coordinate)

Examples

Table of Content Zone
locationtop
typelist

D-Flow FM map file on local xy-grid (e.g., Amersfoort / RD New).

  • Currently lacks lon/lat for x/y-grids.
  • The RD projection (oblique stereographic) is not recognized in CF-1.4 (and many others), in PROJ.4 'sterea' is used: http://www.spatialreference.org/ref/epsg/28992/
  • More details are here: unstructured grid format.
    Code Block
    
netcdf test_map {
// Only relevant parts for crs are shown!
dimensions:
        nNetCell = 665 ;
        time = UNLIMITED ; // (1 currently)
variables:
        double time(time) ;
                time:units = "seconds since 2010-01-01 00:00:00" ;
        double NetCell_xc(nNetCell) ;
                NetCell_xc:units = "m" ;
                NetCell_xc:standard_name = "projection_x_coordinate" ;
                NetCell_xc:long_name = "Net cell circumcenter x" ;
                NetCell_xc:bounds = "NetCellContour_x" ;
        double NetCell_yc(nNetCell) ;
                NetCell_yc:units = "m" ;
                NetCell_yc:standard_name = "projection_y_coordinate" ;
                NetCell_yc:long_name = "Net cell circumcenter y" ;
                NetCell_yc:bounds = "NetCellContour_y" ;
        double s1(time, nNetCell) ;
                s1:cell_methods = "area: mean" ;
                s1:coordinates = "NetCell_xc Netcell_yc" ;
                s1:grid_mapping = "crs" ;
        int crs ;
                crs:grid_mapping_name = "oblique_stereographic" ;
                crs:projection_name = "Amersfoort / RD New" ;
                crs:semi_major_axis = 6377397.f ;
                crs:inverse_flattening = 299.1528f ;
                crs:latitude_of_projection_origin = 52.15616f ;
                crs:longitude_of_central_meridian = 5.387639f ;
                crs:scale_factor_at_projection_origin = 0.9999079f ;
                crs:false_easting = 155000 ;
                crs:false_northing = 463000 ;
                crs:proj4_params = "+proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +towgs84=565.4174,50.3319,465.5542,-0.398957388243134,0.343987817378283,-1.87740163998045,4.0725 +no_defs" // ADAGUC extension
                crs:EPSG_code = "EPSG:28992" // ADAGUC extension

// note that the default proj4 (epsg) string for the Dutch RD system (EPSG:28992 & EPSG:7415) is wrong, it contains an erroneous ellipse reference, hence the full ellipse values need to be supplied.

//...
crs = 28992 ;

D-Flow FM map file on lon/lat-grid (e.g., WGS 1984)

  • More details are here: unstructured grid format.
    Code Block
    
netcdf test_map {
// Only relevant parts for crs are shown!
dimensions:
        nNetCell = 184324 ;
        time = UNLIMITED ; // (1 currently)
variables:
        double time(time) ;
                time:units = "seconds since 2010-01-01 00:00:00" ;
        double NetCell_xc(nNetCell) ;
                NetCell_xc:units = "degrees_east" ;
                NetCell_xc:standard_name = "longitude" ;
                NetCell_xc:long_name = "Net cell circumcenter x" ;
                NetCell_xc:bounds = "NetCellContour_x" ;
        double NetCell_yc(nNetCell) ;
                NetCell_yc:units = "degrees_north" ;
                NetCell_yc:standard_name = "latitude" ;
                NetCell_yc:long_name = "Net cell circumcenter y" ;
                NetCell_yc:bounds = "NetCellContour_y" ;
        double s1(time, nNetCell) ;
                s1:cell_methods = "area: mean" ;
                s1:coordinates = "NetCell_xc Netcell_yc" ;
                s1:grid_mapping = "crs" ;
        int crs ;
                crs:grid_mapping_name = "latitude_longitude" ;
                crs:longitude_of_prime_meridian = 0.f ;
                crs:semi_major_axis = 6378137.f ;
                crs:inverse_flattening = 298.2572f ;
                NetCell_xc:proj4_params = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs"  // ADAGUC extension
                NetCell_xc:EPSG_code = "EPSG:4326";  // ADAGUC extension
//...
crs = 4326 ;