[JCharlton]

While I can't say I've tried it with the actual field, this is a solution you might try. For my example I used a screen capture from the field tour video.

After you acquire the image the algorithm works like this:
- Crop the image You probably will capture more than you want. Get rid of as much of the background as you can.
- Search for dark parts of the image with colour threshold Use HSL and filter with luminance. You don't need the full colour threshold function, but it works for other things too.
- Filter out small bits with Particle Filter 3 This should clean up a lot of junk.
- Dilate the image to merge particles into one The intention of this step is to blur all the particle blobs into one big particle.
- Use particle analysis You'll want to a) check there's only one particle, and b) get the x position.

Not saying it'll be easy, but it might give you something to experiment with, though the suggestions to look for the upper vision target are valid too.