Band maths

I have backscatter images and I want to classify each image into three classes using thresholding (<-20db as waterbody, -15db to -20db as non-forest and >-15 as forest). Please tell me how to write a condition in band maths for three classes?

Check out page 12 on this tutorial. Is that what you’re looking for?

You can use following band math expression in SNAP

if Sigma0_IW3_VV<-20 then 1 else if -20<Sigma0_IW3_VV<-15 then 2 else 3

Here 1, 2,3 are the values for the different class. After generating the band using above expression, save the product in .tiff format. Open it in envi classic and go to density slicing.
Apply different color for the classes. For eg. 1- blue, 2 - brown, 3- green.