How to Write Expressions in RAVI

This guide explains how to create mathematical expressions to calculate custom spectral indices in the RAVI plugin. It also provides practical examples and important tips to avoid common mistakes.

Available Bands

Sentinel-2 provides the following bands that you can use in your expressions:

Band (use it in the expression)	Name	Description	Wavelength	Spatial Resolution
B1	Coastal aerosol	Coastal aerosol	443 nm	60 m
B2	Blue	Blue	490 nm	10 m
B3	Green	Green	560 nm	10 m
B4	Red	Red	665 nm	10 m
B5	Red Edge 1	Red Edge 1	705 nm	20 m
B6	Red Edge 2	Red Edge 2	740 nm	20 m
B7	Red Edge 3	Red Edge 3	783 nm	20 m
B8	NIR	Near Infrared	842 nm	10 m
B8A	Narrow NIR	Narrow Near Infrared	865 nm	20 m
B9	Water vapor	Water vapor	940 nm	60 m
B11	SWIR 1	Shortwave Infrared 1	1610 nm	20 m
B12	SWIR 2	Shortwave Infrared 2	2190 nm	20 m

Supported Operators

Arithmetic Operators

+ (addition)
- (subtraction)
* (multiplication)
/ (division)
** (exponentiation)

Relational Operators

> (greater than)
< (less than)
>= (greater than or equal to)
<= (less than or equal to)
== (equal to)
!= (not equal to)

Logical Operators

&& (logical AND)
|| (logical OR)
! (logical NOT)

Available Mathematical Functions

sqrt(x) - Square root of x
abs(x) - Absolute value of x
min(x, y) - Minimum value between x and y
max(x, y) - Maximum value between x and y
log(x) - Natural logarithm of x
exp(x) - Exponential function \( e^x \)
sin(x), cos(x), tan(x) - Trigonometric functions
round(x) - Round x to the nearest integer
floor(x) - Round x down to the nearest integer
ceil(x) - Round x up to the nearest integer
pow(x, y) - Raise x to the power of y

Expression Examples

NDVI (Normalized Difference Vegetation Index)

(B8 - B4) / (B8 + B4)

EVI (Enhanced Vegetation Index)

2.5 * ((B8 - B4) / (B8 + 6 * B4 - 7.5 * B2 + 1))

SAVI (Soil Adjusted Vegetation Index) with L=0.5

(1 + 0.5) * ((B8 - B4) / (B8 + B4 + 0.5))

NDRE (Normalized Difference Red Edge)

(B8 - B5) / (B8 + B5)

NDMI (Normalized Difference Moisture Index)

(B8 - B11) / (B8 + B11)

Conditional Expression

This expression calculates NDVI only when the red band (B4) is greater than 0.2. Otherwise, it returns 0:

(B4 > 0.2) ? ((B8 - B4) / (B8 + B4)) : 0

This expression calculates NDMI only when the SWIR1 band (B11) is greater than 0.1. Otherwise, it returns -1:

(B11 > 0.1) ? ((B8 - B11) / (B8 + B11)) : -1

Conditional Expression with Logical Operators

This expression calculates NDVI only when both the red band (B4) is greater than 0.2 and the NIR band (B8) is greater than 0.3. Otherwise, it returns 0:

(B4 > 0.2 && B8 > 0.3) ? ((B8 - B4) / (B8 + B4)) : 0

Important Tips

Automatic normalization: All bands are already divided by 10000 to convert from digital values to reflectance (0-1).
Avoid division by zero: Ensure that the denominator in your expressions cannot be zero.
Use parentheses: To ensure operations are executed in the correct order.
Test your expression: Start with known indices and modify them to understand how they work.
Follow Python conventions: Expressions must follow Python syntax, including the use of operators like ** for exponentiation.

Attention

When creating complex expressions, test them incrementally to ensure they work as expected. Extreme values or division by zero can cause unexpected results.