How to calculate NDVI from MODIS MOD09GQ?

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5479584.asdf
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How to calculate NDVI from MODIS MOD09GQ?

by 5479584.asdf » Fri Aug 05, 2022 1:41 am America/New_York

I'm a research student who's just getting started in the field of remote sensing and GIS. Therefore I kindly request your valuable support to solve my question.

I downloaded the MODIS band 1 and band 2 files using https://ladsweb.modaps.eosdis.nasa.gov/search/ in tif format.

Then I calculated the NDVI using QGIS. But my NDVI value range is -119 to 141.

Please help me to solve this matter.
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LP DAAC - jwilson
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Re: How to calculate NDVI from MODIS MOD09GQ?

by LP DAAC - jwilson » Tue Aug 16, 2022 1:21 pm America/New_York

Hi @5479584.asdf
We have forwarded your inquiry to our subject matter expert for further assistance. We have not received a reply back from the PI yet on this question.

LP DAAC - afriesz
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Re: How to calculate NDVI from MODIS MOD09GQ?

by LP DAAC - afriesz » Wed Aug 17, 2022 4:04 pm America/New_York

@5479584.asdf,

I was able to replicate what you are seeing in QGIS. I don’t see a problem other than you should be quality filtering these data before you calculate the NDVI. The MODIS granule you downloaded will have a “QC_250m_1” layer in it that you should use to quality filter your data. The QA bit flags for the QC_250m_1 layer is provided in Table 8 of the User Guide (https://lpdaac.usgs.gov/documents/925/MOD09_User_Guide_V61.pdf) on page 19.

To better visualize the data you are working with, open the properties for your NDVI layer (i.e., ndvi in your screenshot), then select “Symbology”. Next…
- Change “Render type” to “Singleband pseudocolor”
- Change “Min” to “0” and “Max” to “1”
- Select a color ramp (I like BrBG for NDVI)
- Under the table, change “Mode” to “Equal Interval” and increase the “Classes” to “11”
- Finally, click “OK”

Hopefully that will give you a better view of the data.

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