plot regular grid data in irregular area

Dear NCL help, The below is an NCL script, which read in data in three columns (lon  lat  values). The data covers an irregular area (e.g. political area) but the data is at regular grids (e.g. all in 0.5/0.5 degree). I want the exact 'raster' plot without any interpolation to the no-data area. Can one kindly supply any help? Thanks very much in advance! Mark ----------------NCL script--------------- begin   fname = "vec.dat"   lines = asciiread(fname,-1,"string")   lon = stringtofloat(str_get_field(lines(1:),1," "))   lat = stringtofloat(str_get_field(lines(1:),2," "))   pwv = stringtofloat(str_get_field(lines(1:),3," "))   delete(lines)   wks = gsn_open_wks("x11","station")   gsn_define_colormap(wks,"gui_default")   res                         = True   res@cnFillOn                = True   res@gsnSpreadColors         = True   res@lbLabelBarOn            = True   res@cnLinesOn               = False   res@cnLineLabelsOn          = False   res@lbLabelAutoStride       = True   res@cnFillMode              = "RasterFill"   res@sfXArray                = lon   res@sfYArray                = lat   res@mpMinLatF               = min(lat)-1   res@mpMinLonF               = min(lon)-1   res@mpMaxLatF               = max(lat)+1   res@mpMaxLonF               = max(lon)+1   map = gsn_csm_contour_map(wks,pwv,res) end

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Received on Thu May 24 04:18:20 2012