Normalized Difference Built-up Index (NDBI)
用来分析建成区的指数有很多：
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)
这些是最常用的，它们有各自的计算方法. 建成区和裸土在SWIR(短波红外)波段比在NIR(近红外)波段反射更多一些. 水体在红外波段几乎没有反射。
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 计算：
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 负值代表水体，高值代表 建成区，植被的NDBI值比较低。
ENVI中Band Math计算公式：float(b5-b4)/float(b5+b4) 或者 (float(b5)-float(b4))/(float(b5)+float(b4))