The internet is full of various contradictory advice on corrections one can make to get NDVI computed from Landsat 8 to fall between -1 and 1.
This is for instructional purposes, so I hope to use the Analysis-Ready Data products (surface and TOA reflectance), which have already been atmosphere- and sun-angle-corrected.
I assume the surface reflectance product would be the best for NDVI, but the values I get
(raster calculator: 1.0*("NIR"-"red")/("NIR"+"red")) range from -1.14 to -0.6, which seems unrealistic. The TOA product gives slightly better numbers, -1.1 to 1.38, but I still don't know why we're exceeding the -1 to 1 range. Any ideas?
The values on the SR product range from -522 to 10133 (red) and -47 to 10558 (NIR) and on the TOA product they are more positive: 216 to 9775 (red)and 66 to 10600 (NIR).