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Variability of Reflectance Measurements with Sensor Altitude and Canopy Type¹

Use of portable, ground-based sensors for measuring crop reflectance has created a need for comparable and reliable measurement procedures capable of providing calibrated and reproducible canopy reflectance data. In this field experiment we determined how canopy reflectance varies as a function of sensor altitude above the crop and what minimum altitude is needed to acquire repeatable reflectance measurements with a 15 degree field of view instrument. Data were acquired in 1979 on three canopies grown on a Typic Argiaquoll; mature corn (Zea mays L.) planted in 76 cm rows, mature soybeans [Glycine mays (L.) Merr.] planted in 96 cm rows with 71% soil cover, and mature soybeans planted in 76 cm rows with 100% soil cover. Spectral data were acquired using a Landsat band radiometer (Exotech 100) at 10 altitudes ranging from 0.2 to 10 m above the canopy. At each altitude, measurements were taken at 15-cm intervals along a 2.0 m transect perpendicular to the crop row direction.

The variance of reflectance factor measurements at low altitudes was attributable to row effects which disappeared at higher altitudes where the sensor integrated across several rows. The coefficient of variation of reflectance factor in both visible and near infrared bands decreased exponentially as the sensor was elevated. Systematic sampling (at odd multiples of 0.5 times the row spacing interval) required fewer measurements than simple random sampling over row crop canopies. Extreme care must be exercised in analyzing and interpreting data acquired at sensor altitudes where the diameter of the sensor's FOV at the top of the canopy is smaller than several multiples of the row spacing.

Key Words: Remote sensing • Radiometer • Sampling errors • Zea mays L. Merr. • Glycine max (L.) Merr.

² Research agronomist, research engineer, and graduate student, respectively.

Received for publication November 4, 1981.

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