REIP Algorithm Specification |

### REIP

The Red-Edge Inflection Point Index algorithm was developed for applications in biomass and nitrogen (N) uptake measurement/management in heterogeneous fields.- Guyot et al., (1988)
Red edge, as the inflection point of the strong red absorption to near infrared reflectance, includes the information of both crop N and growth status.
The reflectance around red edge is sensitive to wide range of crop chlorophyll content, N content, LAI and biomass (Hatfield et al., 2008; Mutanga and Skidmore, 2007; Steele et al., 2008b)
The REIP general formula is based on linear four-point interpolation technique and it uses four wavebands (670, 700, 740 and 780 nm) - Guyot and Baret, (1988)

The REIP results from the following (Sensor-dependent) equation:

REIP = 700 + 40 * ((r670 + r780)/2 - r700) / (r740 - r700) - as general formula
or:
REIP = 700 + 40 * ( (red1_factor * red1 + IR_factor * near_IR)/2) - red2_factor * red2 ) / (red3_factor * red3 - red2_factor * red2) )

For Sentinel-2 (Sentinel-2 bands are at 705 nm and 740 nm) the formula is:
705 + 35 * ((B4 + B7)/2 - B5) / (B6 - B5), where (Central wavelength/Bandwidth): B7 = 783 nm (15 nm), B6 = 740 nm (15 nm), B5 = 705 nm (15 nm), B4 = 665 nm (30 nm)

### REIP-Flags

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

Bit Position | Description
--- | ---
Bit 0 | The computed value for REIP is NAN or is Infinite
Bit 1 | The computed value for REIP is less than 700 (seven hundred)
Bit 2 | The computed value for REIP is greater than 760 (seven hundred and sixty)