I am trying to use a logic ( rule -based classification ) to extract built-up area, similar to as suggested by Xu (2007).He uses, three spectral indices, SAVI( Soil Adjusted Vegetation Index), NDBI ( Normalized Difference Built-up Index ) and MNDWI ( Modified Normalized Difference Water Index) to help extract built-up area. However, I am not able to know the method he chose a threshold of SAVI=-0.344 in the following statement:( In other words, how am i able to extract the right threshold value for SAVI to avoid spectral confusion?)
Given, Band 1=SAVI , Band 2= NDBI and Band 3=MNDWI
If Band 1 < -0.344 and Band 2 > Band 3 then 1 Else 0.
The maximum of built-up land class in band1 (SAVIband) of Fuzhou image is -0.343, whereas the minimum of vegetation class in that band is - 0.182. Therefore, using -0.344 as a threshold value can help avoid the confusion between vegetation and built-up land classes and greatly increase the extraction accuracy (see sections below).
Xu, H., 2007. Extraction of Urban Built-up Land Features from Landsat Imagery Using a Thematic oriented Index Combination Technique. Photogrammetric Engineering & Remote Sensing 73 12: 1381–1391.