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.
Hi Kaleab,
For any process, a threshold value is determined emperically. Same is true for any model or indices. The characteristics of soils of your study area, exposed area of soil surface, climate and presence of aerosols etc. should be considered. You may read the following article
https://www.researchgate.net/profile/Alfredo_Huete/publication/220040775_A_Soil-Adjusted_Vegetation_Index_(SAVI)/links/0f31753242c82db9df000000.pdf
Good luck
Supriti
Article Huete, A. R. A soil-adjusted vegetation index (SAVI). Remote...
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