I think you should do spatial downscaling before you validate it. Bring them both to the same resolution. There are various methods of spatial downscaling (Support Vector Machine, Random Forest, Cubist, etc).

Hi Nabila,

You can do this in ArcGIS if you have limited timeframes or size using different likelihood methods. However, it may be easier for you in case you do in Google Earth Engine. Here are links which may help you:

https://developers.google.com/earth-engine/tutorial_api_06

https://developers.google.com/earth-engine/reducers_reduce_neighborhood

Cheers

I hope this article may give you a brief idea about your doubt on validating vegetation density.

The links are provided below.

https://dx.doi.org/10.1016%2Fj.annepidem.2011.09.001

Article Stratification of Forest Density and its validation by NDVI ...