Normalized Difference Built-up Index (NDBI)
用來分析建成區的指數有很多：
Normalized Difference Built-up Index (NDBI)
Built-up Index (BU)
Urban Index (UI)
Index-based Built-up Index (IBI)
Enhanced Built-up and Bareness Index (EBBI)
這些是最常用的，它們有各自的計算方法. 建成區和裸土在SWIR(短波紅外)波段比在NIR(近紅外)波段反射更多一些. 水體在紅外波段幾乎沒有反射。
For better result, you can use Built-up Index (BU). Build-up Index is the index for analysis of urban pattern using NDBI and NDVI. Built-up index is the binary image with only higher positive value indicates built-up and barren thus, allows BU to map the built-up area automatically.
BU = NDBI - NDVI
NDBI 計算：
NDBI = (SWIR – NIR) / (SWIR + NIR)
For Landsat 7 data, NDBI = (Band 5 – Band 4) / (Band 5 + Band 4)
For Landsat 8 data, NDBI = (Band 6 – Band 5) / (Band 6 + Band 5)
Also, the Normalize Difference Build-up Index value lies between -1 to +1.
NDBI 負值代表水體，高值代表 建成區，植被的NDBI值比較低。
ENVI中Band Math計算公式：float(b5-b4)/float(b5+b4) 或者 (float(b5)-float(b4))/(float(b5)+float(b4))