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Could anyone provide a site to download NDVI images or raw satellite images of Sri Lanka (Asia) for free for me to calculate the NDVI.

  • Have you tried to download Landsat imagery? Where else are you stuck? – Devdatta Tengshe May 12 '15 at 10:27
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For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).

Primary download site for Sentinel

https://scihub.copernicus.eu/dhus/#/home

alternative solutions can be found on Where / How to download Sentinel 2 Images

If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.

PROBA-V : http://aida.vgt.vito.be/content/products

MODIS (and also Landsat, but for Landsat only you could use Libra as mentioned by @Richard Law): https://earthexplorer.usgs.gov/

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  • @BERA sure, I didn't add it because it is already referenced on gis.stackexchange.com/questions/190681/… Upvote it there if you think that it is better than scihub, because it is currently second. – radouxju Jul 6 '17 at 7:37
  • Both Landsat and Sentinel are considered "medium" spatial resolution. High spatial resolution are Ikonos, WorldView, PlanetLabs, and many other sub 10 meter resolution imaging systems. For moderate spatial resolution VIIRS is a more valid option than MODIS these days as MODIS is 10 years beyond its expected life. TOA means Top of Atmosphere, and it is Level 1. BOA or Bottom of Atmosphere is Level 2. For Landsat conversion of TOA to BOA, Sen2Cor is worthless, you need iCOR from Vito. Both work like a charm from Python. – Emiliano Jul 25 at 12:34
  • Ikonos is often considered as VERY high spatial resolution. There is no strict rule to set the boundary between High and very high – radouxju Jul 26 at 9:51
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Libra provides a really intuitive interface to obtain Landsat imagery according to various spatial, temporal and image-suitability filters. I highly recommend it over EarthExplorer.

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You can try USGS http://www.usgs.gov/. You can create an account for free and then through Earth Explorer http://earthexplorer.usgs.gov/ you can search and download images from different satellites (e.g Landsat).

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Unless you state specific requirements there's a whole number of satellite missions that provide free raw/NDVI imagery. Some examples are:

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