Normalized Difference Vegetation Index (NDVI) was designed to understand the status of vegetation using remotely sensed data applied to an easy formula. It represents quantity and activity level of vegetation. Vegetation is usually seen in green. This is because of the characteristics of light, as shown on the right figure, that vegetation especially reflects green light well (high reflectance) and does not reflect red light so much (low reflectance) among the lights from the sun. Therefore, a lot of green light can be reached to the eyes of humankind, which consequently makes us see the vegetation green.
Besides the green light, vegetation has a characteristic to show a high reflectance on part of infrared (near infrared of band 800 nm to 1300 nm). Vegetation index was calculated making the best use of these characteristics.
Vegetation index data is calculated based on NOAA AVHRR data.
In this way, normalized difference vegetation indices (NDVI) has been prepared using the values of channel 1 and 2 of NOAA AVHRR sensor. The more the vegetation grows, the lower the value of channel 1 becomes, but the higher the value of channel 2 becomes. To make the best us of these characteristics, calculation is made using the above formula. Values of vegetation indices obtained from the calculation ranges in integer from 0 to 200. The result shows that the higher number the area has, the more the vegetation flourishes.
As found from the above figure, NOAA observes the status how vegetation reflects the light from the sun. However, actual vegetation status cannot be observed when cloud is in the sky. As the Japanese islands are often covered with cloud, it is very hard to obtain cloud-free data at once for the entire islands. Consequently, monthly NDVI data with little influence of cloud are being produced by extracting the maximum value of NDVI data from the NDVI data obtained in one month at each longitudinal and latitudinal point. Therefore, value of NDVI can become low when cloud or ice covers the vegetation for the whole month. As for water body, it is difficult to remove cloud over the water body, masking is given for convenience assuming that there are no vegetation in water body and a value of 00 is stored.