*2.1. Study Site*

A 1500 m<sup>2</sup> (30 m ×50 m) paddy rice field (Figure 1a) (variety: '*Meixiang zhan*') in the Ningxi Teaching and Research Bases (NTRB) at the South China Agricultural University, Guangzhou, China (23◦1402200N, 113◦3803100E), was selected for the survey. Plant spacing and row spacing in the field were both 0.2 m. The rice field was divided into five comparison zones: F1, F2, F3, F4 and F5. Each zone consisted of three fields, F1: A1, B1 and C1; F2: A2, B2 and C2; F3: A3, B3 and C3; F4: A4, B4 and C4; F5: A5, B5 and C5 (Figure 1b). The size of each field was 10 m × 10 m. Additionally, five zones were fertilized with different nitrogen application rates (Figure 2). It should be noted that the fields of each comparison zone were treated with the same fertilization strategy. This strategy involved applying different amounts of base fertilizer, tillering fertilizer and panicle fertilizer to create differences in crop growth.

**Figure 1.** Study site. (**a**) The study site was located at a paddy-rice field, marked by the pink rectangle in Ningxi Teaching and Research Bases (NTRB) at the South China Agricultural University, Guangzhou, China; (**b**) five field zones were present: F1, F2, F3, F4 and F5, marked by red rectangles; blue flight route is a schematic diagram that represents the 4 different routes at different flight altitudes: 40 m, 60 m, 80 m and 100 m; (**c**) unmanned aerial vehicle (UAV)-based multispectral camera acquiring raw multispectral images of the study site at a low flight altitude (40–100 m).

*Remote Sens.* **2020**, *12*, x FOR PEER REVIEW 4 of 15

**Figure 2.** Fertilization program of the test field. **Figure 2.** Fertilization program of the test field.

#### *2.2. UAV Data Collection 2.2. UAV Data Collection*

A Micasense RedEdge‐M camera (MicaSense, Inc., Seattle, WA, USA) was attached to the Matrice‐600 Pro UAV (DJI, Inc., Shenzhen, China), a six‐rotor UAV, for the aerial imagery collection. This camera has five independent imagers, each with a custom narrowband filter that enables the imager to receive a precise spectrum. The RedEdge‐M camera also includes a downwelling light sensor (DLS) and a GPS module. The DLS is a light sensor with five bands, which need to connect directly to the host for radiation calibration. The specifications of the RedEdge‐M camera are given A Micasense RedEdge-M camera (MicaSense, Inc., Seattle, WA, USA) was attached to the Matrice-600 Pro UAV (DJI, Inc., Shenzhen, China), a six-rotor UAV, for the aerial imagery collection. This camera has five independent imagers, each with a custom narrowband filter that enables the imager to receive a precise spectrum. The RedEdge-M camera also includes a downwelling light sensor (DLS) and a GPS module. The DLS is a light sensor with five bands, which need to connect directly to the host for radiation calibration. The specifications of the RedEdge-M camera are given in Table 1.


in Table 1. The flight plans were designed using the Atlas Flight application (Micasense, Inc., Fremont, CA, **Table 1.** The specifications of the RedEdge-M camera. Acronyms: DLS—downwelling light sensor; FOV—field of view; FWHM—full width at half maximum; NIR—near infrared.

**Table 1.** The specifications of the RedEdge‐M camera. Acronyms: DLS—downwelling light sensor; FOV—field of view; FWHM—full width at half maximum; NIR—near infrared. **Specification Parameters** Weight 170 g (including DLS) Dimensions 9.4 cm × 6.3 cm × 4.6 cm Power 4.2–15.8 V DC, 4 W nominal, 8 W peak Spectral bands Blue (475 nm, FWHM: 20 nm); green (560 nm, FWHM: 20 nm); red (668 nm, FWHM: 10 nm); red edge (717 nm, FWHM: 10 nm); NIR (840 nm, FWHM: 40 nm) The flight plans were designed using the Atlas Flight application (Micasense, Inc., Fremont, CA, USA). The flight altitudes were set to 40 m, 60 m, 80 m and 100 m and the flight speed was set to 5 m/s. In addition. the side overlap, and forward overlap of the camera were set to 80% to ensure effective orthorectification and mosaicking of the resulting images. Following the instructions of the Micasense manual (https://support.micasense.com), a calibrated Teflon panel was imaged by the RedEdge-M camera for radiometric calibration before and after each flight. Using these settings, the image acquisition was conducted above the study area from 6 am to 6 pm on 11 June 2018. A total of 48 flights were thus conducted and 31,980 images were collected by performing each flight mission at an interval of 15 min. Detailed flight parameters are given in Table 2.

#### Maximum capture speed <sup>1</sup> capture per second (all bands) *2.3. Ground Data Collection*

Storage format 16 bits TIFF Field of view (FOV) 46° The ground NDVI data were obtained using the GreenSeeker handheld crop sensor (Trimble, Inc., Sunnyvale, CA, USA), which has two independent LEDs that can emit 774 nm near-infrared (NIR) light and 656 nm red light. Due to its own light source, the sensor is relatively unaffected by solar conditions. In this study, the GreenSeeker sensor was used to acquire NDVI ground data (called GS–NDVIs from

**Table 2.** Flight parameters of the study.

here forward), to determine the different growth levels of the five zones (F1–F5) described above. The GreenSeeker sensor calculates the NDVI values using the following formula:


**Table 2.** Flight parameters of the study.

$$(\text{GS} - \text{NDVI}(656 \text{ nm}, 774 \text{ nm}) = \frac{R\_{774} - R\_{656}}{R\_{774} + R\_{656}} \tag{1}$$

where *R*<sup>774</sup> is the reflectance of near-infrared light (774 nm) and *R*<sup>656</sup> is the reflectance of red light (656 nm).

In practice, the GS–NDVI values were acquired by positioning the sensor at a 0.8-m height above the canopy. These data were acquired simultaneously with the UAV data to minimize potential discrepancies caused by asynchronous data acquisition. A total of 60 randomly distributed GS–NDVI values were collected from each zone, resulting in a total of 300 measurements across the study area.
