How to calculate NDVI difference for two different time periods in QGIS

Question

I have calculated the NDVI of an area of interest for 2015 (Landsat 8) and 2023 (Landsat 9) in QGIS. I want to detect NDVI change between the two rasters by subtracting NDVI values in Raster Calculator. I am using a formula from a similar post which is:

"(-1) * ("S2_NDVI_2015@1" < 0) * (("S2_NDVI_2018@1" - "S2_NDVI_2015@1" ) / "S2_NDVI_2015@1") * 100 + ("S2_NDVI_2015@1" >= 0) * (("S2_NDVI_2018@1" - "S2_NDVI_2015@1" ) / "S2_NDVI_2015@1") * 100".

My resulting map is having an extremely large positive value and an extremely low negative value which produces an all white raster output.

I am new to QGIS.

Answer 1

While I'm not sure exactly why QGIS outputs those extreme limits (would love to hear the underlying explanation), this can be easily remediated by tweaking the min/max values in the raster's properties (The properties are accessible by right-clicking the concerned Raster and choosing Properties..).

If I try a raw NDVI difference with two rasters, I get these large values as well:

Changing my min/max to -1 and 1 in the properties:

After the change, I get a legible map:

As you seem to be working with Percent Change, I suggest setting your limits to -100 and 100 to get the proper range.