Hi Shabnam,

Yurii has already given a perfect answer.

Just in case you are looking for MODIS NDVI data for an entire month, you can download the daily data product instead (https://developers.google.com/earth-engine/datasets/catalog/MODIS_MOD09GA_006_NDVI). The monthly product is the monthly average of daily NDVI images (the same goes for the 16-day average). :)

Hi Shabnam Shadloo , I agree with Yurii Moskalenko and Saheba Bhatnagar .

When you download the MODIS data or any other MODIS product, there is usually a User Guide associated with it. Here is the NASA GSFC webpage https://modis.gsfc.nasa.gov/data/dataprod/mod13.php that provides an overview of all the VI products of MODIS.

There are several sources for MODIS data such as USGS EarthExplorer, LPDAAC, Google Earth Engine, NASA Earth Data Search etc. however, the algorithm/method for deriving these products are the same as documented in these Product/User Guides. So, for example, the monthly MODIS Vegetation Indices (MYD13A3) 1 km product is derived using the "algorithm that ingests all the MYD13A2 products that overlap the month and employs a weighted temporal average." In turn, the MYD13A2 (16 Day 1Km VI ) "chooses the best available pixel value from all the acquisitions from the 16 day period."

https://lpdaac.usgs.gov/products/myd13a3v006/

https://lpdaac.usgs.gov/products/myd13a2v006/

Let us know if you have more questions.

Thanks everyone your answers are very helpful. :)))

I have another question that has arised from dear Saheba's answer.

If the product of monthly NDVI is an average of each day, can we safely assume that to creat a yearly NDVI we need to average the NdVIs for 12 months?

yes Shabnam Shadloo

I think that You have to do Radiometric correction, before that check MDL file there you will found datas for that correcton

Regards

Tomislav

fruitful discussion

Very informative discussion.

Hi.. I need with NDVI index remote sensing data with spatial resolution 1 KM and temporal resolution Daily.

Are you can help mi??