Limitations of NDVI
While NDVI is a recognised measure of plant health and growth, and is very widely applied, it does have limitations that need to be taken into account when using this technique.
NDVI outputs are directly related to the raw values within an individual image, and these values are affected by several factors, such as atmospheric conditions, which alter the raw image values, such that different values can be recorded over the same scene, even if nothing has changed on the ground. Therefore, NDVI values acquired at different times are not directly comparable, unless measures such as atmospheric corrections have been first applied to the imagery, prior to NDVI being calculated.
NDVI also tends to become “saturated”, i.e. the ratio values peak at a certain point and can no longer record differences in the ratio values. This occurs when vegetation growth is very heavy or dense and so NDVI tends to be less useful under these conditions, i.e. it cannot detect variation in growth patterns between very dense or strongly growing vegetation.