A method of estimating the instantaneous crop yield y2(t) at time t in a field, during harvesting with a harvesting machine (10) that operates by cutting and/or controlling the crop and has a crop mass flow sensor that measures the mass flow rate of crop processed by the machine at a location remote from that at which the harvesting machine cuts the crop and generates a signal mout(t) indicative thereof, comprises the steps of: (i) generating a said mass flow rate signal mout(t) using said sensor; (ii) generating an area rate signal s(t) indicative of time domain variations in the area of the field harvested per time unit; (iii) filtering said area rate signal s(t) using a function P(s) representative of the dynamics of the harvesting machine to generate a filtered area rate signal s2(t); and (iv) using said filtered area rate signal s2(t) for deriving said mass flow rate signal mout(t) a yield per area unit y2(t).
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