Sunday, May 16, 2010

When instruments turn nasty - yield measurement non-linearity

Last week I visited the farm manager at Roseworthy Agricultural College. Most of my work with SAR and crops has been done here. The college farm practices precision agrculture and collects telemetry from harvesters to do yield mapping and produce fertilizer application prescriptions for the next season.

He showed me some yield data from 2009 where there was marked variation in the yield (tonnes/per hectare) depending on the speed at which the harvester was travelling. 2 operators drove the harvester at different speeds and this caused a variation. Fortunately past data I have worked on was obtained by a single operator. Obviously the variation indicates a non-linearity in the flow measurement hardware.

So I went back to my old friend , Gaussian Mixture models and isolated the 2 different working speeds (combines have different optimal speeds). Then plotted the yields obtained at different speeds and sure enough there was trending againsts speed, on top of the standard infield variation the yield mapping excercise is trying to capture. This can be fixed by mathematically recalibrating the flow sensor and detrending the slope so that yield masses show similar variability no matter what speed. To absolutely calibrate the system a value for the total yield obtained in the field is needed.

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