code: def docustom(lon, lat, alt, gh):
      
      model, date, itype = 0, 0, 1
      sv = np.zeros(len(gh))
      colat = 90. - lat
      x, y, z, f = magsyn(gh, sv, model, date, itype, alt, colat, lon)
      return x, y, z, f
docs: Passes the coefficients to the Malin and Barraclough
      routine (function pmag.magsyn) to calculate the field from the coefficients.
  
      Parameters:
      -----------
      lon  = east longitude in degrees (0 to 360 or -180 to 180)
      lat   = latitude in degrees (-90 to 90)
      alt   = height above mean sea level in km (itype = 1 assumed)