[GIS] Why is there a difference between NDVI using Sentinel 2, in ArcMap & QGIS

arcgis-10.4arcmapndviremote sensingsentinel-2

I have been trying to calculate the NDVI for a scene using Sentinel 2 bands 4(red) & 8(NIR).
I first did it in ArcMap and then QGIS.
In both softwares, I used the raster calculator and included the basic NDVI formula = (NIR-Red)/(NIR+Red).

ArcMap gave me a range of 0-9221 (this varies with the stretch applied) while QGIS gave me more plausible values from 0.0258-0.5255.

I have read briefly that Sentinel 2 is TOA reflectance and has a scaling factor of 10000, which apparently can be found in the xml file that gets downloaded with the images. I never found this scaling factor mentioned in the xml file I downloaded.

Why is there this difference? How is the calculation being done on QGIS?

Best Answer

Arcmap is notorious for retaining the raster type and not treating values as decimals/float when calculated from 16bit integer rasters.

In order to force Arcmap to treat raster calculation result as a float, you should multiply one of the arguments by 1.0, or simply append a '.0' to a any constant or real number in the formula.

In your case (NIR-Red)*1.0/(NIR+Red) will suffice.

Related Solutions

[GIS] How to smooth NDVI raster (Sentinel 2-A image) in ArcGis

For visualisation purpose, you can select a resampling method from the display properties. right click on layer > properties, then display tab / resampling during display using : cubic convolution (cubic convolution yields the smoothest display, bilinear interpolation also works).

here is an example with a S2 image, with cubic convolution (top) and without (bottom). Note that your "quality" impression is the real pixel size (10 m), which would be 30 m if you display the Landsat-8 image at its true resolution (i.e. using nearest neighbour resampling)

[GIS] Calculating EVI from Sentinel 2 imagery gives difference in results between ArcMap & QGIS

Please stick to one question per post in the future. It makes answering significantly easier.

Question 1 - How to calculate EVI:
The formula that you are using is "correct", in the sense that you are using the constants calculated for MODIS.

Question 2 - Why does QGis and ArcGIS provide different results:
Minuscule differences in results most likely originate from different resampling methods used when combining the spectral bands. It is a difficult discover the reason for the differences, without having the data available.

Question 3 - How to remove extreme values from a raster calculation:
Removing those values is best done by adjusting your raster calculation. One approach would be to simply add a parenthesis around the entire equation and multiply it by a pair of 'greater than' and 'less than' calculations.
Example:

(EVI) * (EVI > -1) * (EVI < 1) + (EVI > 1)*1 + (EVI < -1)*(-1)

The above outlined calculation would set all the values below -1 and above 1 to -1 and 1, respectively.

Question 4 - When to apply the scale factors:
The scale factors should be applied when you are using the bands. If you applied them afterwards, to the EVI, then the effect of the constants in the equation would be changed, and your calculations would become incorrect. As such, you use is currently the right one.

Best Answer

Related Solutions

[GIS] How to smooth NDVI raster (Sentinel 2-A image) in ArcGis

[GIS] Calculating EVI from Sentinel 2 imagery gives difference in results between ArcMap & QGIS

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