How to find tennis courts in aerial imagery
You have some very strong color and geometry cues you can leverage. I would try the following:


Extract the Green channel &amp; apply watershed type algorithm on it, followed by connected components. Subsequently compute component statistics (area &amp; bounding box) for each component. Retain only the components with area ~= bounding box size. This will be true only for rectangular objects and will eliminate forests/wooded areas etc.
Isolate the white channel (R=G=B) and apply hough transform on the output. This will give you the lines. 
Combine 1 &amp; 2 to get your tennis courts.