faster digitising polygons from colour differentials

[dm769exe]

I am trying to create a digital polygon of the boundaries of exmoor national park, designated in a particular shade of green in the sc, in order to determine the sum of hedgerows inside of it in purple. Is there a way of digitising the park which does not involve spending hours free drawing the boundaries, and also which would have better levels of accuracy which is quite important given the large share of hedgerows near the boundaries of the park.