Hello!

Depending on the size of your study area and the ecological variation within it, you may or may not actually have NDVI values of -1 or 1 contained within the dataset. If you have a very large study area, ArcGIS will only sample a limited number of pixels (~10,000 to 30,000) to generate estimates minimums and maximums.

To verify if -0.17 is the minimum of your raster, you can use the 'Summary Statistics' function in ArcGIS. Alternatively, with software like R or GDAL you can also compute the true minimums or maximums.

-1 to +1 is value of vegetation index, it will change depending in the study area. To recheck your results you can download the imagery and perform NDVI analysis using R, QGis or in ArcGIS. if the values are changed then you have to reconsider your results

I would suggest to run an NDVI analysts in QGIS, using enmap-box plugin. You just need to install the plugin and put there the raster, then applications->imagemath and then follow the steps of the attached picture. There you will get your NDVI.

Hello Harsha Jade Puttaswamy,

As mentioned in the answers above NDVI values depend on the vegetation within your study site. NDVI values will also change during a year, or between years, due to changes in vegetation. NDVI values can range between -1 and +1. However, this does not mean, that these values always have to range between -1 and +1. To understand NDVI values better, think about the formula used for calculating these values.

NDVI = (nIR - red)/(nIR + red)

nIR...near infrared band (reflectance)

red...red band (reflectance)

Living vegetation absorbs very strongly in the visible range, especially in the blue and red range, and reflects very strongly in the near infrared range. Chlorophyll is responsible for this typical reflectance properties of living vegetation. The higher your NDVI value the higher the amount of chlorophyll, the higher the overall plant biomass.

The check if your downloaded values are correct, you can calculate NDVI values by yourself, but be aware that vegetation changes over time. For calculating NDVI values you can use geographic information systems like ArcGIS or QGIS. If you use the latter you can use the raster calculator in the menu bar "raster" and the following formula: NDVI = ((NIR - R)/(NIR + R)). NIR = pixel values from the near infrared band. R = pixel values from the red band. Just figure out the respective bands of your satellite data first.

If you want to use ArcGIS, the following link might be of interest for you:

https://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/ndvi-function.htm

Short summary: NDVI values smaller than 0.1 correspond to areas without any vegetation, like rocks, sand or snow. From 0.1 to 0.2 you have almost no vegetation. Spare vegetation like senescing crops, shrubs and also some types of grassland have moderate NDVI values from approximately 0.2 to 0.5. High NDVI values (higher than 0.6) indicate dense vegetation structures, like forests or crops at their peak growth rate. Negative NDVI values usually indicate water bodies.

I hope this helps!

Best regards

Paul

https://www.mdpi.com/1999-4907/12/1/76/pdf

check this out.