QX/T 540-2020 in English

Analysis of Standard Technical Framework

Core algorithm principle

NDWI index application

Normalized difference water index (NDWI) is calculated by the formula:
$$NDWI=\frac{R_{green}-R_{nir}}{R_{green}+R_{nir}}$$
When NDWI≥0, it is judged as a water body, which is suitable for identifying large areas of water bodies in plains.

SWI Index Innovation

Shadow Water Index (SWI) solves the problem of shadow interference in mountainous areas:
$$SWI=R_{blue}'+R_{green}'-R_{nir}'$$
Empirical thresholdC₂=0.015, which needs to be combined with the solar altitude angle correction formula:
$$R'=R/sinZ$$

Standardization Implementation Process

1. Data preparation phase: Select clear sky data in the wet/dry season
2. Preprocessing: Complete 6 steps including atmospheric correction and geometric precision correction
3. Terrain adaptation: Use NDWI for plains and SWI for mountains
4. Change analysis: Calculate the absolute change (ΔS) and relative change (P₁) of area

Technology Evolution Analysis

Compared with GB/T31163-2014, this standard:

• Added a correction formula for the solar altitude angle
• Clarified the parameter requirements for domestic satellites
• Established a standard process for identifying water bodies in mountainous areas

Implementation Suggestions