The NDISIr operator algorithm computes the Normalized Difference Impervious Surface Index Red from a given product.

The NDISIr results from the following equation:

NDISIr = (T - (R + N + S1) / 3.0) / (T + (R + N + S1) / 3.0)

, where:

The bands are:

- R is the Red band
- N is the NIR band
- S1 is the SWIR 1 band
- T is the Thermal band

Also the processor computes an additional flags band called 'ndisir_flags' with the following bit coding:

Bit Position	Description
Bit 0	The computed value for NDISIr is NAN or is Infinite
Bit 1	The computed value for NDISIr is less than -1 (minus one)
Bit 2	The computed value for NDISIr is greater than 1 (one)