[gradsusr] Use of basemap and country shape file to maskout land features

[See Hai Ooi]

Dear all,
 
        As shown in the attached figure, I used basemap and country shape file consecutively to maskout the land feature so as to display only the chlorophyll-a concentration portion over the sea.  Unfortunately, the available global map resolution in basemap is only medium.  Is there any alternate solution to clip the land portion perfectly for my stated purpose ?  I use window version of OpenGrADS version 2.0.1.oga.1.
 
       Your help will be greatly appreciated.
 
      Best regards.
 
Ooi 
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