title: Issues with scipy interpolate griddata polar coordinates
body: <p>I have used scipy griddata to interpolate with polar data. I have a file with azimuth, altitude, and a value. </p>
  
  <p>First, I converted azimuth and altitude to Cartesian coordinates to use with griddata. I then create steps from 0 to 90 for altitude and 0 to 360 for azimuth. </p>
  
  <pre><code>alt = np.array(alt)
  x,y = alt * np.cos(az), alt*np.sin(az)
  xi = np.linspace(0, 90, 100)
  yi = np.linspace(np.radians(10.), np.radians(350.), 100)
  zi = griddata((x, y), value, (xi[None,:], yi[:,None]), method='cubic')
  ax = plt.subplot(111, projection='polar')
  cm = plt.cm.get_cmap('jet_r')
  levels = np.linspace(min(value),max(value), 100)
  cf = plt.contourf(yi,90-xi,zi.T,levels=levels,cmap=cm)
  
  cbar = plt.colorbar()
  cbar.set_ticks(np.arange(15.5,22, .5))
  cbar.set_label(r'mag/arcsec$^2$')
  ax.set_theta_zero_location('N')
  ax.set_theta_direction(-1)
  ax.yaxis.grid(False)
  #ax.set_rticks([])
  ax.scatter(az,90-alt, marker='.', edgecolors='k', facecolors='none')
  ax.xaxis.grid(False)
  plt.ylim(0,90)
  
  plt.show()
  </code></pre>
  
  <p>I cannot agree with what was returned for griddata. For example, the data point (the black dots) at 225 at the lowest altitude is 16.5 mags/arcsec^2. Griddata returned a value around 18.5 mag/arcsec^2 for that area. My other issue is how bad the interpolation is at azimuth 350-10. It does not blend well. See below:</p>
  
  <p><a href="https://i.stack.imgur.com/bY5ch.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bY5ch.png" alt="enter image description here"></a></p>
  
  <p>I changed the range from 10 to 350 to get:</p>
  
  <p><a href="https://i.stack.imgur.com/GJqeg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GJqeg.png" alt="enter image description here"></a></p>
  
  <p>Which I am happier with, but I would like to fill in the keyhole in the plot. </p>
  
  <p>Any help filling in the keyhole is greatly appreciated. As well as how to improve the griddata results. </p>
  
  <p>I also do not agree with the polar plot rotation and data mapping. I believe there is some errors there, but that is for another post.</p>