Is there any map which has NDVI calculated?
Where can I get highest multispectral satellite image which could be zoom to very large extent?
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There are online tools that can calculate NDVI, but it also depends on which territory you need to analyze and which dataset is the best fit.
You can use this tool to get NDVI from Landsat 8 (5m per pixel; worldwide), Sentinel-2 (10m per pixel; worldwide), CBERS-4 images (multispectral, 10m per pixel; North and South America, Africa). 10 pics are available for free analysis and download daily but images under 5m/pixel resolution are to be paid.
As far as I know, the highest resolution for a weekly NDVI product is the PROBA-V 100m (approximately one image every 5 days). It is provided with a one month delay as a top of canopy NDVI product. Of course, you can also process Sentinel-2 or Landsat 8 images, but for a global extent you would need high performance computing infrastructure.
If you are looking for historical gap filled time series, a smoothed time serie of NDVI is available from the ESA Land cover CCI project. It is global with a spatial resolution of 300 m.