GEMI Algorithm Specification |

### GEMI

The Global Environmental Monitoring Index algorithm was developed by Pinty and Verstraete (1991).
They attempt to eliminate the need for a detailed atmospheric correction by constructing a stock atmospheric correction for the vegetation index.
Pinty and Verstraete (1991) provide no detailed reasoning for this index, other than that it meets their requirements of insensitivity to the atmosphere empirically.
A paper by Leprieur et al. (1994) claims to find that GEMI is superior to other indices for satellite measurements.
Qi et al. (1994) shows a violent breakdown of GEMI with respect to soil noise at low vegetation covers.

The GEMI results from the following equation:

GEMI = eta * (1 - 0.25 * eta) - (red_factor * red - 0.125) / (1 - red_factor * red)

, where:
eta = (2 * (IR_factor * near_IR * IR_factor * near_IR - red_factor * red * red_factor * red) + 1.5 * IR_factor * near_IR + 0.5 * red_factor * red) / (IR_factor * near_IR + red_factor * red + 0.5)

The atmosphere is changing all of the time and all remote sensing instruments have to look through it.
The atmosphere both attenuates light passing through it and scatters light from suspended aerosols.
Also, the atmosphere can vary strongly across a single scene, especially in areas with high relief.
This alters the light seen by the instrument and can cause variations in the calculated values of vegetation indices.
This is particularly a problem for comparing vegetation index values for different dates.
The GEMI index is trying to remedy this problem without the requirement of atmospherically corrected data.
WARNING: These indices achieve their reduced sensitivity to the atmosphere by decreasing the dynamic range.
They are generally slightly less sensitive to changes in vegetation cover than NDVI.
At low levels they are very sensitive to the soil background. (See Qi et al. (1994) for comparisons.)

### GEMI-Flags

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

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