I have a problem with a phenological study. The study area is in temperate zone of Asia. I have been doing phenological research using MOD VI product. I wanted to extract phenological events (onset/offset date, length etc) from NDVI and EVI, based on Greatest increase NDVI value method. After calculating the NDVI ratio, also I built a graph by result of NDVI ratio. But during determination of the average onset date of vegetation green-up and dormancy based on max and min value of NDVI ratio, some unreliable thing get me into trouble. For example: in forest area, max value of NDVI ratio occurred in middle of July corresponding threshold is 0.4 while in grassland area, onset of green-up occurred in middle of May corresponding threshold of 0.09. When I saw the graph, onset of green up in forest already started in beginning of May. So that's why I said it is unreliable.
Therefore, I wanna ask from all of you: 1. Did you have a such kind of problem If you did phenological research using Greatest increase NDVI approach? 2. If so, how did you solve this problem to determine onset date of green-up and dormancy.