Today, we will learn about the different indices, like the , to understand and analyze various satellite images. Before getting started, let’s first understand why it is essential to know about different landscapes like vegetation, water, and the built-up area of our environment.
Landscapes around us are changing every day. Natural surfaces and forests give way to houses, office buildings, roads, croplands, highways, and parking lots. To know the extent, rate, and pattern of change, we need an easy way to measure the extent. Remote sensing provides an easy way to map different landscapes of our environment using the bands of satellite images through employing some algorithms. The outputs of employing the methods to measure distinctive landscapes are often called indices. So, what are these indices?
Indices are measures that help determine a vegetation’s health, the city’s growth, the conversion of a wetland to a forest, etc. Image indices are made by computing the number of bands of satellite images which is used to measure changes that occur on landscapes.
We know that satellite remote sensing is based on sensors that capture images of the earth’s surface. These satellite images have a number of bands. Each band produces a record of energy reflected in a specific wavelength, which is used to identify the object from which the energy is reflected. Based on this idea, various algorithms and methods have been provided to measure the distinctive landscape types we call indices. These images emphasize a specific phenomenon that presently makes the target’s features and the degree of those features more visible and distinguishable. For instance, a vegetation index will show healthy vegetation as bright, unhealthy vegetation darker, and barren terrain as entirely dark. So, why do we need indices?
Different indices help us with the acquisition of land cover, landscapes, and ecological and aquatic information from satellite and drone data, through the analysis of multi or hyperspectral imagery bands. So, we can easily determine our land cover distribution in a large area. For example, you want to know the cropping pattern distribution of an area of 2500sq miles. Doing that by the survey will cost lots of money and time. However, using satellite images and vegetation indices, you can easily measure the cropping pattern and distribution with minimal time and money. Now, we will focus on the Modified Soil Adjusted Vegetation Index.
Global Vegetation Monitoring
The whole world is concerned with the topic of climate change and global warming. Emissions, carbon dioxide, methane, permafrost melting, carbon farms, and “green” fuel began to be discussed not only by scientific communities but also by the first people of countries. The impact of climate change has been felt by almost all the inhabitants of the planet: some countries suffer from abnormal heat and fires, others from too cold winters, and others from catastrophic floods.
One of the biggest environmental issues is the greenhouse effect – a natural phenomenon that raises the temperature on the planet as a result of the fact that the gases (carbon dioxide, methane, etc) retain thermal energy. Due to human activities and rapid technological progress, the greenhouse effect is intensified, which means it contributes to an increase in average temperatures on the planet.
Excess gases from the atmosphere are absorbed by vegetation. Carbon dioxide is converted into organic matter (glucose) through photosynthesis. Plants take in CO2 and release oxygen. Thus, they “tame” the climate and maintain the temperature balance on the planet.
Thus, the best absorbers of greenhouse gases are forests: Siberian, Amazonian, Brazilian, deciduous, coniferous. Whatever categories and geographical references forest ecosystems belong to, they will absorb carbon dioxide. Same goes for any other vegetation that has a green biomass like plants, crops, grass, etc. That is why it’s critical to perform global vegetation monitoring. Usually, the most effective here are satellite pictures and their advanced analysis based on vegetation indices with NDVI being the most used one.
Global Vegetation and Carbon Emissions
All plants (both wild and domestic) absorb carbon dioxide. Plant residues are decomposed by animals and microorganisms, and CO2 is released during decomposition. In ecosystems, the processes of growth and development of plants (and, accordingly, the absorption of carbon dioxide) and the processes of decomposition of plant residues (leaves, branches) always go on simultaneously. The balance between these processes is important. If decomposition processes, which also include combustion processes as a result of forest fires, prevail, the ecosystem begins to work not as a sink, but as a source of greenhouse gases.
Forest ecosystems can best serve as a store (or bank) for carbon when human impact on them is minimized throughout their life. First of all, it concerns uncontrolled logging and fires. Forest is one of the most complex open ecosystems, which has the variability of landscape and geographical location, and long life of trees. Its openness means that it has an input (energy from the sun and rainfall) and an output (animals, plants, wood, a place for human rest).
Modified Soil Adjusted Vegetation Index (MSAVI)
It is used to lift the limit on applying NDVI and NDRE to land with a high degree of bare soil. So, it is best used when the NDVI and NDRE produce invalid data primarily due to a small amount of vegetation or lack of chlorophyll in the leaves. Thus, it is used to minimize soil background influence and increase the dynamic range of vegetation signals.
Experts can also detect uneven seed germination using this index. The seeds could have been destroyed by bad weather or other external factors. This index can identify regions on the field with seeds that are not growing. With this information, farmers can decide what to do next. So, they can choose to resow in these areas, taking into consideration external factors. The faster it is done, the better chances of getting good crop yields.
The NDVI and NDRE values will indicate poor vegetation during the early stages of plant growth because bare soil is very visible. This type of picture will be read as poor vegetation by these indices. This is where you will need MSAVI. It considers the presence of soil and adjusts its value to reflect the state of the crops accurately.
According to the MSAVI values, -1 to 0.2 indicate bare soil, 0.2 to 0.4 represent the seed germination stage, 0.4 to 0.6 reflect the leaf development stage. Once the value gets past 0.6, it is high time to use NDVI. At this point, bare soil is not visible.
Satellite-Based Drought Indicators
The most popular are Vegetation Drought Response Index (VegDRI) and Vegetation Health Index (VHI)
The Vegetation Health Index has different indices provided by polar-orbiting satellites. The pictures and data have worldwide covers and indicate the average conditions over one week. This index monitors the health of crops regardless of the reason.
The National Drought Mitigation Center (NDMC) created the VegDRI product with other partners. This map is generated once every two weeks and shows the level of drought stress on vegetation throughout the U.S.
These different indices help farmers monitor their crops’ growth and health. Ultimately, they can pick out problem areas in their fields. With this information, they can make swift changes and interventions to increase their crop yield.