Chapter 6

Remote Sensing in Precision Agriculture

Fundamentals of Remote Sensing

Electromagnetic Radiation

The energy that is measured by a remote sensing instrument and that is used to produce an image is called electromagnetic radiation (often abbreviated to EMR). Electromagnetic radiation can vary in strength from low energy to high energy. It includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays.

The Electromagnetic Spectrum

Light emitted by the sun is composed of several different wavelengths, and the full range of wavelengths is called the electromagnetic spectrum. In the electromagnetic spectrum (EM), there are many different types of waves with varying frequencies and wavelengths, as shown in Figure 6.1. The continuous spectrum is subdivided into ultraviolet (UV) radiation, known as near-UV radiation, with wavelengths between 290 and 400 nm, to visible light, with wavelengths between 400 and 700 nm, and near-infrared (NIR) radiation, with wavelengths between 700 and 1,300 nm.

Reflection Spectroscopy

Crops. Knowing the spectral characteristics of the canopies in agricultural fields can help detect normal or abnormal patterns that tend to occur in cropping areas. In remote sensing of plants, chlorophyll absorbs most of the light from the red and blue portions of the visible spectrum but reflects green wavelengths; hence, leaves appear green when chlorophyll content is high.
Soils. In remote sensing of soil, the reflection offers a rapid and non-destructive assessment of soil properties. It estimates nutrient levels, organic carbon content, clay minerals, soil moisture, and salinity by exploiting the unique spectral fingerprints of soil constituents.

Band Ratios Derived from Remote Sensing

One way to distinguish objects with different spectral characteristics is to construct band ratios. For instance, short-wave infrared (SWIR) and near-infrared (NIR) or red/NIR ratios are sensitive to photosynthetically active vegetation. For visual interpretation of the images, it can be useful to calculate these band ratios or combinations of more than two bands. However, it is often preferable to retain as much spectral information as possible for quantitative analysis, which is why band ratios are often used in conjunction with the original bands rather than alone.

Active Versus Passive Sensors

Remote sensors can be classified into two categories: passive and active. Passive remote sensors do not emit radiation themselves but instead measure radiation that is reflected, emitted, or scattered by an object. In passive remote sensing, the source of this radiation is the Sun, thermal energy, or other sources such as anthropogenic light (for sensors collecting at night).

Radiation Interactions with the Atmosphere

There are three forms of interaction that can take place when energy strikes the Earth’s surface. These are absorption (A), transmission (T), and reflection (R). The total incident energy will interact with the surface in one or more of these three ways. The proportions of each will depend on the wavelength of the energy and the material and condition of the feature. Absorption (A) occurs when radiation (energy) is absorbed into the target, while transmission (T) occurs when radiation passes through a target.

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