[ncl-talk] Overlay two grids with different projections

[Brandon Fisel]

Hello,

I am trying to overlay two different grids, which are both on different
projections to see the differences between the grids.

One grid (rectilinear, lat-lon) has attributes:
dimensions:
        lat = 258 ;
        lon = 600 ;
variables:
        double mask(lat, lon) ;
                mask:_FillValue = 1.00000002004088e+20 ;
                mask:long_name = "Land Mask" ;
                mask:missing_value = 1.e+20f ;
                mask:standard_name = "land_binary_mask" ;
                mask:units = "1" ;
        double lat(lat) ;
                lat:long_name = "latitude" ;
                lat:units = "degrees_north" ;
                lat:standard_name = "latitude" ;
        double lon(lon) ;
                lon:long_name = "longitude" ;
                lon:units = "degrees_east" ;
                lon:standard_name = "longitude" ;

The other grid (cylindrical, rotated mercator) has attributes:
dimensions:
        y = 278 ;
        x = 334 ;
variables:
        float mask(y, x) ;
                mask:_FillValue = 1.e+20f ;
                mask:coordinates = "lat lon" ;
                mask:grid_mapping = "rotated_mercator" ;
                mask:long_name = "Land Area Fraction" ;
                mask:missing_value = 1.e+20f ;
                mask:standard_name = "land_area_fraction" ;
                mask:units = "%" ;
        double lat(y, x) ;
                lat:_Coordinate_AxisType = "Lat" ;
                lat:long_name = "Latitude on Cross Points" ;
                lat:standard_name = "latitude" ;
                lat:units = "degrees_north" ;
                lat:grid_mapping = "rcm_map" ;
        double lon(y, x) ;
                lon:_Coordinate_AxisType = "Lon" ;
                lon:long_name = "Longitude on Cross Points" ;
                lon:standard_name = "longitude" ;
                lon:units = "degrees_east" ;
                lon:grid_mapping = "rcm_map" ;
        char oblique_mercator ;
                oblique_mercator:grid_mapping_name = "oblique_mercator" ;
                oblique_mercator:latitude_of_projection_origin = 46. ;
                oblique_mercator:longitude_of_projection_origin = 263. ;
                oblique_mercator:_CoordinateTransformType = "Projection" ;
                oblique_mercator:_CoordinateAxisTypes = "GeoX GeoY" ;
                oblique_mercator:azimuth_of_central_line = 90. ;
                oblique_mercator:scale_factor_at_projection_origin = 1. ;
                oblique_mercator:false_easting = 0. ;
                oblique_mercator:false_northing = 0. ;
        char rotated_mercator ;
                rotated_mercator:grid_mapping_name = "rotated_mercator" ;
                rotated_mercator:azimuth_of_central_line = 90. ;
                rotated_mercator:latitude_of_projection_origin = 46.5 ;
                rotated_mercator:longitude_of_projection_origin = 263. ;
                rotated_mercator:scale_factor_at_projection_origin = 1. ;
                rotated_mercator:false_easting = 0. ;
                rotated_mercator:false_northing = 0. ;
        double x(x) ;
                x:_CoordinateAxisType = "GeoX" ;
                x:axis = "X" ;
                x:long_name = "x-coordinate in Cartesian system" ;
                x:standard_name = "projection_x_coordinate" ;
                x:units = "m" ;
        double y(y) ;
                y:_CoordinateAxisType = "GeoY" ;
                y:axis = "Y" ;
                y:long_name = "y-coordinate in Cartesian system" ;
                y:standard_name = "projection_y_coordinate" ;
                y:units = "m" ;

I have been following something similar to
https://www.ncl.ucar.edu/Applications/Scripts/datagrid_3.ncl from NCL
Graphics: Drawing locations of data values via markers or lines (ucar.edu)
<https://www.ncl.ucar.edu/Applications/datagrid.shtml>, but it seems to
only plot the lat-lon grid and not also the rotated mercator grid.

Is this possible to plot two different projections of a grid as an overlay?

Thanks,

BJF
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