EFFECT OF HEIGHT, TILT AND TWIST ANGLES OF AN ACTIVE REFLECTANCE SENSOR ON NDVI MEASUREMENTS

Vinícius Stocker,Franciléia De O E Silva,Humberto M Beneduzzi,Jerry A Johann,Eduardo G De Souza

doi:10.1590/1809-4430-eng.agric.v39nep96-108/2019

Abstract

The application of nitrogen (N) fertilizer is complex and expensive, so its correct management has financial and environmental benefits. The use of optical proximity sensors is a promising technique. However, the movement of the agricultural machinery or of the person carrying the sensor will result in height differences and/or different tilt and twist angles with respect to the canopy. We considered whether these variations would affect the reflectance measurement. In this study, we took normalized difference vegetation index (NDVI) readings of a wheat canopy, to which 90 kg ha-1 of urea had been applied in stage 5, and observed the NDVI in stages 6, 8 and 10.5. We also tested soybeans, to which 90 kg ha-1 of urea had been applied in stage R1, and took NDVI readings in stages R2 and R5. Our goal was to study the effects of the position of an active reflectance sensor (GreenSeeker) on the NDVI index at different heights and at different angles to the canopy. We observed that the height of the sensor affected the NDVI depending on the stage of the plant and that angles up to 15° of the sensor did not directly affect the readings.

Highlights

The correct application of nutrients to the soil may contribute to a reduction of the possible environmental impacts of agricultural activity
The coefficient of variation (CV) of the normalized difference vegetation index (NDVI) indices (Table 1) for the functions of the crop and the GreenSeeker sensor heights were considered low, it was observed that the data was homoscedastic
The mean NDVI for wheat in stage 10.5 was lower than the other readings, which could be explained by the advanced development cycle of the crop at the time the reading was taken

Summary

Introduction

The correct application of nutrients to the soil may contribute to a reduction of the possible environmental impacts of agricultural activity. Both the reduction of pollution and the leaching of applied nutrients results in minor impacts (Schieffer & Dillon, 2015). According to Singh (2015), the chlorophyll in the leaves absorbs part of the radiation at wavelengths in the red range and the leaf structure reflects part of the radiation in the infrared range, so there is a correlation between the NDVI response and the N present in the soil. The stress level of the plant can be estimated by its N content derived from the ratio of the absorbed and reflected radiation (Schwerz et al, 2016)

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