Measuring Vegetation

Enhanced Vegetation Index (EVI)

In December 1999, NASA launched the Terra spacecraft, the flagship in the agency’s Earth Observing System (EOS) program. Aboard Terra flies a sensor called the Moderate-resolution Imaging Spectroradiometer, or MODIS, that greatly improves scientists’ ability to measure plant growth on a global scale. Briefly, MODIS provides much higher spatial resolution (up to 250-meter resolution), while also matching AVHRR’s almost-daily global cover and exceeding its spectral resolution. In other words, MODIS will provide images over a given pixel of land just as often as AVHRR, but in much finer detail and with measurements in a greater number of wavelengths using detectors that were specifically designed for measurements of land surface dynamics.

Consequently, the MODIS Science Team is preparing a new data product–called the Enhanced Vegetation Index (EVI)–that will improve upon the quality of the NDVI product. The EVI will take full advantage of MODIS’ new, state-of-the-art measurement capabilities. While the EVI is calculated similarly to NDVI, it corrects for some distortions in the reflected light caused by the particles in the air as well as the ground cover below the vegetation. The EVI data product also does not become saturated as easily as the NDVI when viewing rainforests and other areas of the Earth with large amounts of chlorophyll.

Frequency of Coverage
Neither the NDVI nor the EVI product will eliminate all obstacles. Clouds and aerosols can often block the satellites’ view of the surface entirely, glare from the sun can saturate certain pixels, and temporary malfunctions in the satellite instruments themselves can distort an image. Consequently, many of the pixels in a day’s worth of images are indecipherable, and maps made from the daily Vegetation Indices are patchy at best.

Borneo 10 day vs. 30 day

Longer term averages of vegetation data help remove errors caused by clouds, but removes detail, especially at high spatial resolutions. The above pair of images shows the island of Borneo during September 1999. On the left is a 10-day average from September 21–30. Heavy cloud cover caused some areas to appear as if they had little or no vegetation (light brown). The 30-day average for the whole month of September, however, shows that the entire island is heavily forested.

With the imaging data the MODIS and AVHRR instruments provide, scientists should be able to use these indices to get daily measurements of vegetation density over most of the Earth’s surface. The maps are helpful in monitoring and understanding environmental and climate changes such as deforestation and desertification as well as drought. The maps also play a major role in other types of satellite measurements. For example, they are crucial in helping scientists classify different types of vegetation over the world’s landscapes as well as detecting changes in land surface cover over time.

back: NDVI as an Indicator of Drought

  pullquote

Measuring Vegetation (NDVI & EVI)
Introduction
Normalized Difference Vegetation Index (NDVI)
NDVI as an Indicator of Drought
Enhanced Vegetation Index (EVI)

Enhanced Vegetation Index
The Enhanced Vegetation Index (EVI) improves on the venerable NDVI. Derived from state-of-the-art satellite data provided by the MODIS instrument, EVI improves on NDVI's spatial resolution, is more sensitive to differences in heavily vegetated areas (as seen here in the Yucatan Peninsula), and better corrects for atmospheric haze as well as the land surface beneath the vegetation. Early data from MODIS shows the differences between EVI and NDVI.

Print this entire article