Remote Sensing For Agriculture

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Introduction

It is defined as “observing a field or a crop without touching it.”Remote sensing is used to measure reflectance of light energy from crop canopy which is important in detecting plant stress hence provides enough time to correct the issue. Images and pictures created in remote sensing are used in the determination of the characteristics of a crop. Images are also useful in determining scouting plans for direct examination of soil or plants. Images are used in development of site treatment plan. Remote sensed images are used for creative long term management practices. These images provide a visual method for understanding the effects of managed inputs such as fertilizers. They are used in understanding the impact of environmental factors such as drainage or pest infestations. The major advantage of using remote sensed images is that they can be collected several times throughout the growing season. Problems arising in the early stages of growth can be detected and adequately corrected.

PROCESS OF REMOTE SENSING IN AGRICULTURE

Remote sensing in agriculture mostly depends on the reflectance properties of soil, water and plants. Light is absorbed, transmitted and reflected based on the properties of the materials it strikes and the angle of incidence for example light colored soil reflects more light than dark colored soil. Total reflectance of surfaces varies throughout the season and at different times of the day as sun energy and position changes. The most important information is obtained through differences in reflectance along the electromagnetic spectrum. These differences can be used to determine whether a crop is stressed, sick or unhealthy due to pest infestations .This is because various factors such as drought, disease or pests may affect the chlorophyll content in leaves thereby affecting the reflectance of the vegetation.

ELECTROMAGNETIC SPECTRUM

Visible light at the red end of the spectrum has a longer wavelength (lower frequency) than light at the violet end.Visible light is electromagnetic energy, which travels in the form of waves. The colors we see are due to differences in the frequency or wavelength of this electromagnetic energy. The colors of a rainbow are a readily apparent example of light from the visible region of the electromagnetic spectrum; these colors are red, orange, yellow, green, blue and violet. Red light has the longest wavelength of all visible light. Wavelengths are often reported in nanometers (nm), a unit of measure equal to 1 billionth of a meter. The visible portion of the spectrum ranges from about 700 nanometers (the red end of the visible portion of the spectrum) to 400 nanometers (the violet end of the visible portion of the spectrum). 

 

Visible light is only a small portion of the electromagnetic spectrum (Figure 3) that may be useful in analyzing soils and crops. Infrared light, characterized by wavelengths larger than those in the visible spectrum, ranges from 700 nm to 100,000 nm. In particular, reflected infrared light, ranging from 700 nm to 3,000 nm, is useful in remote sensing to detect stress in growing plants. Some adjacent regions of the spectrum also have significance in remote sensing. These include the longer wavelengths of the microwave region from 1 mm to 1 m and the shorter wavelengths of the ultraviolet region, with wavelengths shorter than the violet end of the visible spectrum (400 nm). Smaller yet are X-rays and other forms of radiation. Wavelengths longer than those in the microwave region are used for radio broadcasts.

Visible light is a relatively narrow band in the electromagnetic spectrum.Visible light is a relatively narrow band in the electromagnetic (EM) spectrum. Remote sensing measures reflectance of EM radiation both inside and outside the visible range.

Chlorophyll absorbs the red and blue portions of the electromagnetic spectrum but reflects the green wavelengths. This is why leaves appear green when the chlorophyll content is high.When the chlorophyll content is low, there is less absorption and more reflection of the red portion of the wavelength therefore the leaf appears red or yellow (combination of red and green).

The internal structure of healthy leaves reflects near infrared hence an excellent measure of a healthy crop. 

Filtering techniques are sometimes used to capture light from one or more specific portions of the electromagnetic spectrum which are highly correlated to plant characteristics. Ratios of reflectance of these types of reflected light could act as “fingerprints” for detecting soil, water or crop characteristics important in crop management.

Colors or shades on a map developed from a remote sensed data could represent either true or false colors.

False colors provide a visual representation of specific, directly measured reflectance properties or categories of certain combinations of reflectance properties that represent a high probability of a particular condition, such as stress caused by nutrient deficiencies, diseases or drought.

Current in remote sensing application in agriculture

Photographic film is used in this field. Several companies have specialized in the agricultural field of remote sensing.

The photographic film is of two types that is the panchromatic and color film.

Panchromatic film provides black and white images. It is sensitive to visible light, infrared light up to 900nm and ultraviolet light down to 300nm.

Normal color film consists of three layers sensitive to red, green and blue and produces images that look normal to the human eyes.

Color infrared film consists of three layered film sensitive to red, green and infrared light. It results in a false color image when processed whereby infrared is printed as red, red is printed as green and green vegetation appears blue.

Multiband photography uses multiple lenses and various combinations of films and filters to record simultaneous photographs of the landscape from several small or discrete spectral ranges.