This paper describes two different solutions for the estimation of weed coverage. Both measuring systems discriminate the weed from the ground by means of the colour difference between the weed and ground and can be used to on-line control tractor sprayers in order to reduce weedkiller use. The solutions differ with respect to the sensor type: one solution is based on a digital camera and a computer that analyses the images and determines the weed amount, while the other simpler solution makes use of two photo detectors and an analogue processing system. The camera-based solution provides an uncertainty of a few percent, while the photo detector-based one, though extremely cheap, has an uncertainty of about 5% and suffers from changes in light conditions, which can alter the estimations
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