Quantifying Sources of Error in Ultrasonic Measurements of Citrus Orchards

Ultrasonic sensors can be used to estimate tree canopy volume variability within orchards, which is useful for planning site-specific management practices and estimating crop yield. The objective of this study was to investigate the errors in tree canopy volume measured with a 10-transducer ultrasonic orchard measurement array and Trimble AgGPS 132 DGPS. Sensitivity analysis was used to investigate the magnitude of individual errors in ultrasonically-sensed tree canopy volume measurement (eUCV) caused by several factors including ground speed accuracy measured by DGPS, uncalibrated air temperature, ultrasonic transducers, and deviation in driving path from the centerline between two rows. The height error in the transducer array due to improper tire inflation and uneven ground was also estimated. Canopy volume of a selected tree measured with the ultrasonic system was used as the basis to simulate eUCV caused by each error factor. One hundred data points were simulated within the selected range of each factor to calculate eUCV and the ranges were determined on the basis of measured data and literature. The overall ranking of error sources affecting canopy volume were, from high to low 1) DGPS ground speed (±6.78%), 2) air temperature (+4.83% to -4.69% for the temperature range 5°C to 45°C), 3) ultrasonic transducer performance (±2.29%), and 4) deviations in driving path (±1.56%). The height error due to uneven ground and wheel tracks ranged from 0.025 to 0.12 m. These results could be used to control error in ultrasonically-sensed canopy volumes within orchards.