title: Interpolating within a grid in python
body: <p>I have a number of height values across a grid, specified in a series of lists:</p>
  
  <pre><code>[3, 1, -2, -3, -3] # x = 0m 
  [2, -7, -14, -30, -39] # x = 10m
  [46, 22, 5, -2, -8] # x = 20m
  </code></pre>
  
  <p>The example above shows a 40m x 20m grid in the form </p>
  
  <pre><code>[x=0y=0, x=0y=10...][x=10y=0, x=10y=10...] etc.
  </code></pre>
  
  <p>I need to calculate the height at a specific  x,y coordinate. I have looked at various interpolate functions and example but can't make any sense out of them - please help!</p>
  
  <p>An example would be the height at x = 5m, y = 5m in teh grid above, which would be in the middle of the values 3, 1 2 and -7 (-1ish?)</p>
  
  <p>Thanks</p>