Agriculture Drones with multispectral imaging sensors allow the farmer to manage crops and soil more effectively. Multispectral remote sensing technology use Green, Red, Red-Edge and Near Infrared wavebands to capture both visible and invisible images of crops.
The multispectral images integrate with specialized software applications which output the information into meaningful data. This land telemetry, soil and crop data allow the farmer to monitor, plan and manage the farm.
In this article, we look at the basics of multispectral imaging technology, reflectance, wavebands and vegetation indices including how all this knowledge coming together will give the farmer a full picture of the health of his soil and plants. We also take a look at some of the latest multispectral sensors and drones for farming with a few videos along the way.
Multispectral Imaging Drones On Farms
Benefits Of Multispectral Imaging
Multispectral images are a very effective tool for evaluating soil productivity and analyzing plant health. Viewing the health of soil and crops with the naked eye is very limited and is reactionary. Multispectral sensor technology allows the farmer to see further than the naked eye.
Data from multispectral imaging has the following benefits;
- Identify pests, disease and weeds. Optimize pesticide usage and crop sprays through early detection.
- Provide data on soil fertility and refine fertilization by detecting nutrient deficiencies. Help with land management and whether to take ground in or out of production or rotate crops etc.
- Count plants and determine population or spacing issues. Estimate crop yield.
- Measure irrigation. Control crop irrigation by identifying areas where water stress is suspected. Make land improvements such as install drainage systems and waterways based on multispectral data.
- View damage to crops from farm machinery and make necessary repairs or replace problematic machinery.
- Survey fencing and farm buildings.
- Monitor livestock.
Multispectral Imaging For Agriculture
Basics Of Multispectral Imagery
Every surface reflects back some of the light that it receives. Objects having different surface features reflect or absorb the sun’s radiation in different ways. The ratio of reflected light to incident light is known as reflectance and is expressed as a percentage.
Vegetation reflectance properties are used to derive vegetation indices (VIs). The indices are used to analyze various ecologies. Vegetation Indices are constructed from reflectance measurements in two or more wavelengths to analyze specific characteristics of vegetation, such as total leaf area and water content.
Vegetation interacts with solar radiation differently from other natural materials, such as soils and water bodies. The absorption and reflection of solar radiation is the result of many interactions with different plant materials, which varies considerably by wavelength.
Water, pigments, nutrients, and carbon are each expressed in the reflected optical spectrum from 400 nm to 2500 nm, with often overlapping, but spectrally distinct, reflectance behaviors. These known signatures allow scientists to combine reflectance measurements at different wavelengths to enhance specific vegetation characteristics by defining VIs.
More than 150 vegetation indexes have been published in scientific literature, but only a small subset have substantial biophysical basis or have been systematically tested. Here is a list of the
Multispectral Software Applications
Many precision farming and agricultural crop stress tools and applications are built around VIs to give a complete solution which include processing, storage, presentation, and analysis of multispectral data. More on the multispectral software applications below.
The reflectance properties of an object depend on the particular material and its physical and chemical state (e.g. moisture), the surface roughness as well as the geometric circumstances (e.g. incidence angle of the sunlight). The most important surface features are color, structure and surface texture. The perceived color of an object corresponds to the wavelength of the visible spectrum with the greatest reflectance.
These differences make it possible to identify different earth surface features or materials by analyzing their spectral reflectance patterns or spectral signatures. These signatures can be visualized in so called spectral reflectance curves as a function of wavelengths.
The below diagram show typical spectral reflectance curves of three basic types of Earth features: green vegetation, dry bare soil and clear water. Green, Red, and Infrared are the main ones used in agriculture. The Red Edge (short band corresponding to the Near Infrared entry point) is also sometimes used for obtaining additional indices.
The vegetation spectrum image is from Markelowitz with further details and explanations regarding the reflectance and vegetation wavebands below.
The spectral reflectance curve of healthy green vegetation has a significant minimum of reflectance in the visible portion of the electromagnetic spectrum resulting from the pigments in plant leaves. Healthy vegetation will absorb in both the blue and red bands, giving rise to what is called the “green bump of healthy vegetation”.
Reflectance increases dramatically in the near infrared. Stressed vegetation can also be detected because stressed vegetation has a significantly lower reflectance in the infrared.
The spectral reflectance curve of bare soil is considerably less variable. The reflectance curve is affected by moisture content, soil texture, surface roughness, presence of iron oxide and organic matter. These factors are less dominant than the absorbance features observed in vegetation reflectance spectra.
The water curve is characterized by a high absorption at near infrared wavelengths range and beyond. Because of this absorption property, water bodies as well as features containing water can easily be detected, located and delineated with remote sensing data. Turbid water has a higher reflectance in the visible region than clear water. This is also true for waters containing high chlorophyll concentrations. These reflectance patterns are used to detect algae colonies.
Multispectral Vegetation Bands
The Green corresponds to the reflected energy in the 500–600 nm spectral band and has the greatest reflectance of a plant in this band. The reflectance peak is at around 550 nm. It has been proven that this spectral band is strongly correlated with the amount of chlorophyll contained in the plant.
In this visible portion of the vegetation spectrum, the reflectance curve of a healthy plant exhibits the greatest reflectance in a green waveband (in the range of 550 nm). This is why plants appear green to us.
A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths. Leaves appear “greenest” to us in the summer, when chlorophyll content is at its maximum. In autumn, there is less chlorophyll in the leaves, so there is less absorption and proportionately more reflection of the red wavelengths, making the leaves appear red or yellow (yellow is a combination of red and green wavelengths).
The internal structure of healthy crops act as excellent diffuse reflectors of near-infrared wavelengths. Measuring and monitoring the near-IR reflectance is one way to determine how healthy (or unhealthy) vegetation may be.
Still most of the light in the visible spectrum reflected by a plant under stress is in the green range. Hence, to the naked eye, a plant under stress is indistinguishable from a healthy one. On the other hand, the difference can be seen in the reflectance of light in the infrared range, which is far less.
Corresponds to the reflected energy in the 600–700 nm spectral band. The strong chlorophyll absorption in this band results in a low reflectance. Reflectance varies significantly in relation to factors such as biomass, LAI, soil history, crop type, humidity and plant stress. For most crops this band gives an excellent contrast between the plants and the soil and it is extensively used for compiling most of the vegetation indices in agriculture.
This a very narrow band (700–730 nm) that corresponds to the entry point of Near Infrared. It is the point of sudden change in reflectance, from strong absorption of Red to substantial reflection of Near Infrared. This band is very sensitive to plant stress and provides information on the chlorophy.
- Crop health analysis
- Plant counting
- Water management
Corresponds to the wavelengths in the 700 nm to 1.3 µm range, has the strongest reflectance of the bands studied. There is a very strong correlation between this reflectance and the level of chlorophyll in the plant. A highly significant variation of the reflectance in this band is produced when a plant is under stress. Along with the Red spectral band, infrared is extensively used for compiling most of the vegetation indices in agriculture
NIR is sensitive to the leaf cellular structure and provides critical data to monitor changes in crop health.
- Soil property and moisture analysis
- Crop health and stress analysis
- Water management
- Erosion analysis
- Plant counting
Healthy vegetation absorbs blue and red-light energy to fuel photosynthesis and create chlorophyll. A plant with more chlorophyll will reflect more near-infrared energy than an unhealthy plant. Thus, analyzing a plants spectrum of both absorption and reflection in visible and in infrared wavelengths can provide information about the plants’ health and productivity.
Thermal infrared radiation is the part of electromagnetic spectrum which has a wavelength of between 3.0 and 20 micrometers. Most remote sensing applications make use of the 8 to 13 micrometer range. The main difference between thermal infrared and the infrared (color infrared – CIR) is that thermal infrared is emitted energy that is sensed digitally, whereas the near infrared (also called the photographic infrared) is reflected energy.
Thermal imaging has been growing fast and playing an important role in various fields of agriculture such as;
- Nursery monitoring
- Plant physiology analysis
- Irrigation scheduling,
- Soil salinity stress detection
- Plant disease detection
- Maturity evaluation
- Bruise detection of fruits
- Yield forecasting
Visible light is defined as having wavelengths in the range of 400 to 700 nm. In agriculture, a quality drone with with an excellent gimbal and camera can be used for visual farm inspections, elevation modeling and even plant counting.
(Source – http://www.dronezon.com/learn-about-drones-quadcopters/multispectral-sensor-drones-in-farming-yield-big-benefits/)