I ordered and downloaded several (Surface Reflectance) Landsat 5-8 images from USGS Earth Explorer with the purpose of choosing a few of them to calculate some Vegetation Indices for each (NDVI, SAVI, EVI) and compare the evolution of the vegetation cover from the 1980s to the present.
I need to select the same month for each year I'm going to consider, but I'm not sure which is the best month to ensure the maximum separability between each vegetation type (I need to discriminate between grassland, closed forest, open forest/shrubs and bare soil in a mountainous environment in the Italian Alps).
Among the Landsat images I downloaded, I have a series of images from the same year (5 images for 2005, all falling within the vegetative season), which I want to use to determine the best month for classification.
Does it make sense to perform a semi-automatic classification on each 2005 image separately, and then compare these classifications to a high-res reference image of the same period to see which month gives the most accurate result?
Or is it better if I calculate NDVI for each image first, and perform the classification based on that?
My worry is that, since I will calculate other indices, not just ndvi, basing my choice of month only on NDVI might not be appropriate.
In addition, do you have different suggestions about how to do this?