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Multispectral Imaging For Precision Agriculture



Multispectral imaging is a powerful technology for precision agriculture that allows farmers to capture data on the health and productivity of their crops with unprecedented accuracy and detail. By analyzing data across multiple spectral bands, multispectral imaging sensors can provide insights into a wide range of factors that can impact crop growth, including nutrient levels, water stress, disease, and pest infestations. There are a wide range of multispectral imaging sensors on the market, each with their own strengths and limitations.

Multispectral imaging serves to generate specialized vegetation indices or maps that highlight differences in plant health. A vegetation index is a tool used to assess the health of plants and crops by measuring their reflectance properties at various light wavelengths. Objects, including plants, have a unique spectral signature that is influenced by their chemical composition. By analyzing the spectral signature of a plant or crop, including changes or deviations from the norm, farmers can gain insight into the health and productivity of their crops prior to the appearance of visible signs of stress or deficiency. Using multispectral imaging, a vegetation index is computed by measuring the combined surface light reflectance on at least two wavelengths that highlight a specific plant or crop characteristic, such as growth stage, water stress levels, or nutrient deficiencies.

The Basics of Multispectral Imaging

At its core, multispectral imaging is the process of capturing data across multiple spectral bands. This is typically done using specialized cameras and sensors that are designed to capture data at specific wavelengths. By analyzing this data, farmers and agronomists can gain insights into the health and productivity of their crops. By flying drones equipped with multispectral imaging sensors over their fields, farmers can capture data on the health and productivity of their crops in a matter of hours, rather than days or weeks.

Popular Multispectral Imaging Sensors

There are a wide range of multispectral imaging sensors on the market, each with their own strengths and limitations. Here are a few of the most popular options:


MicaSense RedEdge-MX


The MicaSense RedEdge-MX is a popular multispectral imaging sensor that is commonly used in agriculture. It captures data across five spectral bands, including blue, green, red, red-edge, and near-infrared. This allows for detailed analysis of factors such as plant health, stress, and nutrient levels. The RedEdge-MX is also lightweight and easy to install on most drones.


Parrot Sequoia


The Parrot Sequoia is another popular multispectral imaging sensor that is designed specifically for use in agriculture. It captures data across four spectral bands, including green, red, red-edge, and near-infrared. The Sequoia is also designed to be compact and easy to install on most drones, making it a popular choice for farmers and agronomists.


SlantRange 3P


The SlantRange 3P is a multispectral imaging sensor that is designed for high-precision analysis of crops. It captures data across four spectral bands, including blue, green, red, and near-infrared. One of the key advantages of the 3P is its ability to provide precise data on crop health and yield, which can be used to make targeted management decisions.

DJI Mavic 3 m

The DJI Mavic 3M is the latest commercial drone addition to DJI's Mavic 3 series, featuring advanced multispectral imaging capabilities that surpass those of its predecessors. While maintaining the 20 MP and 4/3-inch CMOS sensor from the regular Mavic 3, the Mavic 3M swaps the telephoto camera for a set of four 5 MP cameras, each tuned to capture light at different wavelengths. These include Near-infrared (860 nm), Red edge (730 nm), Red (650 nm), and Green (560 nm). This multispectral sensor array enables detailed spectral analysis of landscapes, allowing for applications like vegetation health mapping and crop stress detection, making it particularly useful in the field of precision agriculture. Additionally, an upward-facing sensor on the Mavic 3M records sunlight intensity, and this data is utilized during the processing of 2D images to provide accurate and consistent results regardless of varying sunlight conditions.

VEGITATION INDICES

Vegetation indices are calculations made using spectral reflectance measurements captured at different wavelengths. These measurements highlight various characteristics of vegetation, such as growth stage, water stress, and nutrient deficiencies.

The spectral signature of an object, including a plant, is the pattern of light reflected, absorbed, and transmitted across various wavelengths. This signature can change based on the object's condition or changes in its environment. For instance, a healthy plant will reflect more light in the near-infrared and absorb more light in the visible spectrum (particularly in the blue and red wavelengths) because of photosynthesis. Conversely, a stressed or unhealthy plant will reflect less near-infrared light and more light in the visible spectrum.

Multispectral imaging is the primary method used to capture these spectral signatures. It involves capturing image data within specific wavelength ranges across the electromagnetic spectrum. In the context of vegetation health monitoring, this typically involves capturing data in the visible, near-infrared, and sometimes shortwave infrared parts of the spectrum.

RGB Indices


RGB cameras are the most common type of cameras used in drones and other imaging devices. They capture light within the visible spectrum, specifically the red, green, and blue wavelengths. While these cameras can't capture the near-infrared light that many vegetation indices rely on, they can still be used to generate some useful indices.


Two notable indices that can be calculated using RGB imagery include the Visible Atmospherically Resistant Index (VARI) and the Triangular Greenness Index (TGI).


Visible Atmospherically Resistant Index (VARI)

The VARI is a measure of visible light that is resistant to atmospheric effects. This index essentially quantifies the 'greenness' of plants, which can be a useful indicator of overall plant health and vitality. Higher VARI values typically correspond to healthier, more vigorous vegetation. This index can help detect plant stress earlier than what might be visible to the naked eye.


Triangular Greenness Index (TGI)

The TGI is designed to maximize the sensitivity to chlorophyll content in leaves. As chlorophyll is vital for photosynthesis, this index can provide insights into the nutritional status of the plant. Specifically, it can indicate nitrogen levels, as nitrogen is a key component of chlorophyll. This information can help farmers optimize their fertilizer application, applying more where it's needed and less where it's not, leading to cost savings and environmental benefits. These indices are useful for agricultural applications, but they also have their limitations. For instance, they may not be as effective at detecting certain types of plant stress or differentiating between crops and other types of vegetation. Additionally, because they rely on visible light, they can be more affected by atmospheric conditions and lighting variations compared to indices that use other parts of the spectrum.

Normalized Difference Vegetation Index (NDVI)


NDVI Indicates crop health from early to medium stages of growth , so is used earlier in the growing season. It measures the difference between the reflectance of near-infrared light and visible light, and divides by the sum of the two. This index is used to measure the amount and health of vegetation in a given area. The near-infrared light is absorbed by chlorophyll in healthy vegetation, while the visible light is reflected. Therefore, areas with high NDVI values indicate healthy vegetation with high chlorophyll content, while areas with low NDVI values indicate sparse or unhealthy vegetation with low chlorophyll content. NDVI is calculated by taking the difference between the reflectance of near-infrared light and visible light, and dividing by the sum of the two. NDVI values range from -1 to 1, with higher values indicating areas with more vegetation and lower values indicating areas with less vegetation.

NDVI maps can be used to identify areas of the field that may be experiencing stress, such as water stress or nutrient deficiency. For example, if a portion of the field has a lower NDVI value than the rest of the field, it may indicate that the plants in that area are experiencing water stress or nutrient deficiency. This information can be used to make targeted management decisions, such as adjusting irrigation or fertilization practices.

Normalized Difference Red Edge (NDRE)

NDRE is Better for later stages of growth, especially for cereals, high biomass or dense crops, and permanent tree crops. It is similar to NDVI, but instead of using the visible spectrum, it measures the difference between the reflectance of the red-edge band and the near-infrared band, and divides by the sum of the two. This index is used to measure the amount and health of biomass in a given area. The red-edge band is particularly sensitive to changes in chlorophyll content, and therefore, areas with high NDRE values indicate high levels of biomass and healthy vegetation, while areas with low NDRE values indicate low levels of biomass and potentially unhealthy vegetation. This makes NDRE particularly useful for identifying chlorophyll content and plant biomass. NDRE maps can be used to identify areas of the field with high plant biomass, which can be an indication of healthy and productive crops. This information can be used to make targeted management decisions, such as adjusting seeding rates or fertilizer applications.

Multispectral sensing data can also be used to generate other types of maps, such as plant height and canopy cover maps. Plant height maps can be generated by analyzing the differences in reflectance between the ground and the plants themselves, while canopy cover maps can be generated by analyzing the amount of light that is reflected by the plants.

These maps can provide valuable information on the overall structure and health of the crop, and can be used to make targeted management decisions, such as adjusting irrigation or nutrient application rates. Multispectral maps are particularly useful for crops that are grown at a large scale, as they allow farmers to identify and analyze variability across the entire field. For example, NDVI maps can be used to identify areas of the field that may be experiencing water stress, nutrient deficiency, or other factors that may be impacting crop health and productivity. This information can be used to make targeted management decisions, such as adjusting irrigation or fertilization practices.

Multispectral maps can also be useful for crops that are difficult to monitor using traditional methods, such as fruit trees and vineyards. By analyzing multispectral maps of these crops, farmers can gain insights into factors such as canopy cover, biomass, and fruit quality, which can be used to make targeted management decisions to improve crop health and yield.

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