User:Dr Azad Rasul/sandbox

Dry Built-up Index (DBI) and Dry Bare-soil Index (DBSI)
Arid and semi-arid regions have different spectral characteristics from other climatic regions. Therefore, appropriate remotely sensed indicators of land use and land cover types need to be defined for arid and semi-arid lands, as indices developed for other climatic regions may not give plausible results in arid and semi-arid regions. For instance, the normalized difference built-up index (NDBI) and normalized difference bareness index (NDBaI) are unable to distinguish between built-up areas and bare and dry soil that surrounds many cities in dry climates. Therefore, the dry built-up index (DBI) and dry bare-soil index (DBSI) introduced to map built-up and bare areas in a dry climate from Landsat 8.

1. Dry Built-Up Index
Spectral disparity of built-up and bareness classes is low in the Blue band; however, it is large in the TIR1 band. The urban areas had higher blue reflectance than bare soil as a result of the type of building materials, mainly concrete, used for roof surfaces and walls. After testing Landsat 8 bands, we found that these classes can be better distinguished in Blue and TIR1 bands in the study area, thus the proposed equation for built-up area in dry climate is:

$$DBI = \frac{Blue-TIR1}{Blue + TIR1}- NDVI $$ where  Blue = band 2,  TIR1 = band 10. A mask was applied to water surface pixels using water index NDWI.

$$NDVI = \frac{NIR - Red}{NIR + Red} $$ where NDVI is normalized difference vegetation index, NIR is the surface reflectance of band 5 and  Red is the surface reflectance of band 4 in Landsat 8. The reason for subtracting the DBI layer from the NDVI values is that built-up areas can be identified better. Through this step, DBI values of green vegetation will decrease and the features with higher DBI values but lower NDVI values will be enhanced. The DBI values can be between −2 to +2 and the higher numbers represent more built-up areas. An appropriate threshold of the built-up class from the output of the equation can be used for mapping built-up and non-built-up areas. A test was conducted on the number of built-up pixels by selecting values 0.72 and higher as built-up for the study area while lower than this threshold as non-built up class. Note that the threshold of DBI and DBSI to extract built-up and bare-soil may vary based on the study site.

2. Dry Bareness Index
The result of testing common bare soil index (NDBaI) indicated that bare soil has lower mean NDBaI than mean NDBaI of built-up areas, meaning that this index was not able to accurately detect bare soil in the study area. DN of bareness and built-up classes is low in band 6 and high in band 3. Inspection of the Landsat 8 bands suggested that differentiation of these classes could be done based on spectral values in the SWIR1 and Green bands. In these bands, generally the DN of bare land is slightly higher than the DN of the built-up class. Thus, the proposed equation for bareness area in dry climate is the inverse of the Modified Normalized Difference Water Index (MNDWI):

$$DBSI = \frac{SWIR1 - Green}{SWIR1 + Green} - NDVI $$

where  SWIR1 = the surface reflectance of band 6,  Green = the surface reflectance of band 3 in Landsat 8.

The DBSI values can be between −2 to +2, and higher numbers represent more bare soil. An appropriate threshold for the bare soil class can be used for mapping bare soil and non-bare soil areas. Based on a test carried out with a sample of bare soil pixels, a DBSI value 0.26 and higher was delineated as bare soil for the study area, and areas with lower values were delineated as other classes.