Bellow there are 2 different NDVI values, before and after applying scale/offset on bands 4 and 5.
Which is correct, the one with scale/offset being applied or the other?
I used a Landsat 8 L2SP image, with scale factor 0.0000275 and additive offset -0.2 on each band.
B4, B5 are the bands without scale and offset being applied.
B4_SCALE_OFFSET, B5_SCALE_OFFSET are the bands with scale and offset being applied.
Best Answer
NDVI is computed with the following equation:
NDVI = (reflectance NIR - reflectance RED)/(reflectance NIR + reflectance RED)
The raw satellite image is distributed with DN (digital number / digital level) values. This is because an integer value image weights a lot less than a float value image. In the case of L8 Col 2, DN values have a valid range of 1-65455. The scale and offset of this product is 0.0000275 and -0.2, so given an example of:
DN NIR: 25000
DN red: 10000
Using scale and offset, reflectance is:
Reflectance NIR: 0.4875
Reflectance RED: 0.075
Then:
NDVI using DN values: 0.429
NDVI using reflectance values: 0.73
When a satellite product has scale and offset values, you should always convert it to reflectance first, then compute NDVI or other vegetation indices. The old Landsat products (and the old version of Sentinel-2) had only scale value, so applying NDVI directly over DN values wasn't a problem.