title: Cartopy projection transformations - transforming from integer values to map coordinates
body: <p>I have a 2d image along with its projection information.</p>
  
  <p>I can't figure out how to transform from pixel numbers into the correct projection.</p>
  
  <p>For example</p>
  
  <pre><code>In [23]: image.shape
  Out[23]: (1351, 1143)
  
  In [24]: projection
  Out[24]: _EPSGProjection(32611)
  </code></pre>
  
  <p>I want to now plot a circle at indices (500, 700) with radius 10 </p>
  
  <p>I think I can do something like this to get the point in the correct projection: </p>
  
  <pre><code>projection.transform_point(500, 700, ???) 
  </code></pre>
  
  <p>but I don't know what the pixel-based coordinate reference system should be.</p>
  
  <p>Am I completely missing something obvious with projections?</p>
  
  <p>Here's a basic example:</p>
  
  <pre><code>image=np.random.rand(500, 500)
  
  crs_latlon = ccrs.PlateCarree()
  
  subplot_kw = dict(projection=crs_latlon)
  
  fig, ax= plt.subplots(1, 1, figsize=(10, 10), subplot_kw=subplot_kw)
  
  ax.imshow(image, extent=(-20, 20, -15, 15))
  ax.set_extent((-30, 30, -25, 25))
  
  ax.coastlines()
  plt.show()
  </code></pre>
  
  <p>This plots an image of random data over the coast of Africa.</p>
  
  <p>How do I now reference something at the index of the original image? </p>
  
  <p>e.g. Draw a circle centred on image(200, 400)?</p>