Use of NDVI and satellite in crop health monitoring

NDVI (Normalized Difference Vegetation Index)

The use of technology in agriculture is vital in modern agriculture, technology can help to have better farm productivity, by monitoring soil, crop and weather data in real time manner. We can use sensors, satellite, AI, IoT etc. to divide farm in to different blocks, and monitor each block, so that we able to focus on the blocks which need improvement. Observing, measuring and building decision system based on these data known as precision agriculture.

What is NDVI?

Normalized Difference Vegetation Index checks the vegetation of plants by measuring the near infrared light reflected by the plants. When the sunlight strikes these objects the spectrum either absorbs the light or reflects the light. NDVI is good indicator of drought.

What is the range of NDVI?

The pigment found in plant leaves chlorophyll strongly absorbs the visible light and the cell structure of the leaf strongly reflects the nearly infrared light. NDVI always ranges from -1 to +1. If it is -1 there probably a problem of water or it can also be a desert, snow or tundra range. If the scale is +1 or (0.8-0.9) it appears to be the darkest shade of green on the NDVI scale. Sometimes it may appear black. The values can also be based on the surface.

Which values represent what surface of land?

Very low values like 0.1 and below represent a barren rocky, sandy or snow. 0.2-0.5 represents shrubs or grasslands. 0.6-0.8 represents a tropical forest.

What is the time when NDVI is measured?

It is measured from 11:00 hours-14:00 hours as the sun shines the brightest during this time.

Which satellites are used to measure NDVI?       

Scientists use satellites like the Landsat-1, Sentinel-2, SPOT and the NOAA AVHRR (National Oceanic and Atmospheric Administration and Advanced Very High Resolution Radiometer) launched by NASA. Use usgs.gov website to view your farm’s NDVI.

How does the Landsat -1 measure NDVI?

The Landsat-1 was originally named the ‘Earth Resources Technology Satellite-1’. It was the first satellite of the Landsat program of NASA.  It was a modified version of the Nimbus-4 which was a meteorological satellite. To perform monitoring of the Earth, it carried two instruments: the Radio Corporation of America (RSA) called the Return Beam Vidicon (RBV) and the Multispectral Scanner (MSS) built by the Hughes Aircraft company. The MSS was found more superior than the RBV. It has 8 generations of its satellites. The most recent was the Landsat-8 launched on February, 11 2013.

How does the SPOT measure NDVI?

The SPOT (Satellite for Observation of the Earth) is a commercial high resolution satellite with optical imaging Earth observing satellite system operating form space. It is run by the Spot image based in Toulouse, France. It was launched by the CNES in the 1970s and was developed in association with the SSTC and the Swedish National Space Board (SNSB). It has got two modes: The Panchromatic or P mode: a single wide band in the visible part of the spectrum and the multispectral mode or XS mode the green, red, and infrared bands of the electromagnetic spectrum. There are 7 generations of the SPOT satellite.   

How does the Sentinel-2 measure NDVI?

The Sentinel-2 is a satellite launched by the European Space Agency. It aims at variability of the land surface every 10 days at the equator, every 5 days under cloud free conditions and every 2-3 days at mid-latitudes supports monitoring of changes to support the changes in vegetation during the growing season. The coverage limits are from between latitudes at 56 degrees south and 84 degrees north. The Sentinel-2 satellite carries a single Multi-Spectral Instrument (MSI) with 13 spectral channels in the visible/near infrared (VNIR) and Short Wave Infrared Spectral Range (SWIR).

How does NOAA AVHRR measure the NDVI?

The NOAA AVHRR is a satellite with five detectors. Two of which can measure a wavelength of 0.55-0.70 micrometers and 0.73-1 micrometer. A NOAA AVHRR pixel is about a square kilometer big.

What is the difference on the NDVI scale of healthy vegetation and sparse vegetation?

Healthy vegetation will absorb most of the visible light and will reflect a large amount of nearly infrared light too. Sparse or Unhealthy vegetation will reflect most of the visible light and will reflect very less amount of nearly infrared light.

What is the formula of NDVI?

The formula of NDVI is: (NIR-VIS)/ (NIR+VIS).

What is the NDVI of Gujarat?

The NDVI range of Gujarat is from 0.2-0.5.