By Richard Price
Remote sensing is a precision agriculture tool that provides a bird’s-eye view of crop conditions across our fields. Interpreting imagery is not always straightforward because crop growth is influenced by weather, soils and management practices. However, remotely sensed imagery can reveal spatial patterns in crop health and productivity, helping identify where and how field conditions are changing.
Remote sensing 101
Current remote sensing technologies use specialized sensors to measure sunlight reflected by plants. These sensors capture visible light, called red, green and blue (RGB) wavelengths that we can see with our eyes, as well as wavelengths that we can’t see called near-infrared (NIR) (Table 1). These different wavelengths provide valuable information about plant health because plants absorb and reflect light differently depending on their overall condition. Chlorophyll, a pigment in the leaves, strongly absorbs visible light, particularly in the blue and red wavelengths, while a healthy leaf structure strongly reflects NIR. By comparing the reflectance across different portions of the spectrum, vegetation indices (VIs) can be calculated to highlight differences in crop growth, vigor and stress across the field.
Why do plants appear green?
Of the primary colors that make up visible imagery, plants prefer blue and red to make chlorophyll in the leaves, therefore, they reflect more green light, which makes them appear green. This relationship can be seen by the slight bump in the amount of light reflected by both healthy and stressed plant lines in Figure 1. It’s important to highlight the difference in this relationship as the blue (representing healthy plants) and black (representing stressed plants) lines move into the non-visible NIR spectrum. Healthy plants reflect more NIR light, which is why VIs that use NIR light, like Normalized Difference Vegetation Index (NDVI), are common in precision agriculture. Other common indices and their benefits are listed in Table 2.
Source : msu.edu