2.3. Time series MODIS NDVI data
MOD13Q1 products spanning the vegetation growing season from March to November were downloaded from the National Aeronautic sand Space Administration (NASA) of the United States (US) Ware-house Inventory Search Tool (WIST). These data were distributed by the Land Processes Distributed Active Archive Center (LPDAAC), located at the US Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center.MOD13Q1 data were provided every 16 days at a spatial resolution of 250 m in the sinusoidal projection. It included red,NIR,blue and
SWIRreflectance bands, NDVI and Enhanced Vegetation Index (EVI) from the MODIS on board of Terra platform. TheNDVI data was extracted for forest cover classification in thisstudy.The SavitzkyGolay (S-G) filter was used to smooth out noisein the time series MODISNDVI data, specifically that caused primarily by cloud contamination and atmospheric variability . The algorithm made dataapproach the upper NDVI envelope and to reflect the NDVI pat-tern of change. It used a moving window, and noisy values wereapproximated by polynomial regression within the moving win-dows. An original NDVI profile and the S-G filtered NDVI profileat a randomly selected pixel was presented in Fig. 2. It was seenthat S-G filter could effectively eliminate data noise and improvethe quality of time series MODIS NDVI data. Projection of thesmoothed MODIS NDVI data was converted to the same projec-tion with Landsat ETM+ data. The spatial resolution of MODIS NDVIdata was resampled to 30 m, and the same columns and lineswere extracted to keep consist with Landsat ETM+ data for further analysis