2.4.1. Spectral Feature Set

In general, the spectral reflectance of vegetation changes after lodging. To take full advantage of this change, five multispectral image bands were added to the spectral feature set. The VI quantitatively reflects the growing situation of vegetation and the vegetation coverage [18,19]. In this study, three types of VI (see Table 2) were derived to expand the difference between lodging and nonlodging areas. Based on the above image features, a spectral feature set was constructed to identify maize lodging.


**Table 2.** The calculation formula and implication of the vegetable index (VI).

Note: ExG, NDVI, and RENDVI are excess green index, normalized vegetation index, and red-edge normalized difference vegetation index, respectively. r, g, b, nir, and re represent the blue band, green band, red band, red-edge band, and near-infrared band from multispectral images, respectively.
