I have a RGB aerial picture of an agricultural crop. The drainages (hundreds of them) can be barely seen because they a very slightly greener then the surrounding crop area (picture is from April). How is it possible to make these drainages more visible in order to be able to vectorize them?
I would probably start by reclassifying your raster on the various bands and seeing how best you could isolate that specific green. I find the identify tool and checking out multiband scatterplots or histograms to be most helpful here. Once you get those depressions on one band and classify the rest as "NoData" running a Vectorize raster function should be the easy part.