DKRZ PyNGL example shapefile use

Example script:

#
#  File:
#    TRANS_shapefile.py
#
#  Synopsis:
#    Illustrates how to use shapefiles
#
#  Categories:
#    map plot
#    contour plot
#    shapefiles
#
#  Author:
#    Karin Meier-Fleischer, based on NCL example
#
#  Date of initial publication:
#    September 2018
#
#  Description:
#    This example shows how to use shapefiles.
#
#  Effects illustrated:
#    o  Reading netCDF data
#    o  Drawing contours on a map
#    o  Using manual levels
#    o  Reading shapefile content
#    o  Drawing shapefile polylines
#
#  Output:
#    A single visualization is produced.
#
#  Notes: The data for this example can be downloaded from
#    http://www.ncl.ucar.edu/Document/Manuals/NCL_User_Guide/Data/
#
'''
  Transition Guide Python Example: TRANS_shapefile.py

  -  Reading netCDF data
  -  Drawing contours on a map
  -  Using manual levels
  -  Reading shapefile content
  -  Drawing shapefile polylines

  18-09-11  kmf
'''
from __future__ import print_function
import numpy as np
import Ngl, Nio

#-----------------------------------------------------------------------
#-- Function: main
#-----------------------------------------------------------------------
def main():

    #--  open file and read variables
    f    = Nio.open_file("tas.nc", "r")
    var  = f.variables["tas"][0,0,:,:]
    lat  = f.variables["rlat"][:]
    lon  = f.variables["rlon"][:]

    #-- start the graphics
    wks = Ngl.open_wks("png","plot_TRANS_shapefile_py")

    #-- resource settings
    res                 =  Ngl.Resources()
    res.nglFrame        =  False                #-- don't advance frame
    res.nglDraw         =  False                #-- don't draw plot

    res.cnFillOn        =  True
    res.cnFillPalette   = "NCL_BYR-03"          #-- choose color map
    res.cnLinesOn       =  False
    res.cnLineLabelsOn  =  False
    res.cnLevelSelectionMode = "ManualLevels"   #-- set levels
    res.cnMinLevelValF  =  240.0                #-- minimum contour level
    res.cnMaxLevelValF  =  310.0                #-- maximum contour level
    res.cnLevelSpacingF =  0.5                  #-- contour level spacing
    res.cnFillMode      = "RasterFill"          #-- turn on contour fill

    res.lbBoxLinesOn    =  False                #-- turn off labelbar box lines
    res.lbLabelStride   =  10                   #-- skip every other label
    res.lbBoxMinorExtentF =  0.24               #-- decrease height of labelbar box
    res.pmLabelBarOrthogonalPosF = -0.05        #-- move labelbar upward

    res.mpLimitMode     = "LatLon"
    res.mpMinLatF       = -36.0
    res.mpMaxLatF       =  42.6
    res.mpMinLonF       = -23.0
    res.mpMaxLonF       =  60.3
    res.mpGridAndLimbOn =  False                #-- don't draw grid lines

    res.sfXArray        =  lon
    res.sfYArray        =  lat

    #-- create the contour plot
    plot = Ngl.contour_map(wks,var,res)

    #-- read shapefile contents
    shpf     = Nio.open_file("act4567/country.shp", "r") #-- open shapefile
    lon      = np.ravel(shpf.variables["x"][:])
    lat      = np.ravel(shpf.variables["y"][:])
    segments = shpf.variables["segments"][:,0]

    #-- polyline resource settings
    plres             =  Ngl.Resources()        #-- resources for polylines
    plres.gsLineColor = "black"
    plres.gsSegments  =  segments

    #-- add shapefile polylines to the plot
    id = Ngl.add_polyline(wks, plot, lon, lat, plres)

    #-- write variable long_name and units to the plot
    txres               = Ngl.Resources()
    txres.txFontHeightF = 0.022

    Ngl.text_ndc(wks,f.variables["tas"].attributes['long_name'],0.30,0.88,txres)
    Ngl.text_ndc(wks,f.variables["tas"].attributes['units'],    0.78,0.88,txres)

    #-- advance the frame
    Ngl.draw(plot)
    Ngl.frame(wks)

    Ngl.end()


#-- call  main function
if __name__ == '__main__':
    main()