# Separating Built-Up Areas from Bare Land in Mediterranean Cities Using Sentinel-2A Imagery
## 4 Methodology
### Experimental Comparison of the Existing Built-Up Indices  
The main concern for LCU classification in urban areas is the separation of bare land and built-up areas, due to their similar spectral characteristics. Extracting the bare land is a challenging task, due to the complexity of soil components and soil spectra. As [PERSON] et al. (2019) stated, the chemical constituent directly influences the spectral signature of bare lands, which can be strong or weak. In addition, many of these spectral signatures overlap one another, which makes it difficult to determine soil cover. Accordingly, the spectral characteristics of soil cover, with different components and water content, can vary across different environments and seasons, which makes it difficult to differentiate bare land.  
Several indices using different combinations of spectral bands were proposed for mapping built-up areas. Table 2 provides a summary of previously introduced built-up indices used in this research ([PERSON] and [PERSON], 2016; [PERSON] et al., 2019; [PERSON] et al.  
As illustrated in Figure 3, the built-up indices listed in Table 2 can highlight the urban areas and separate them from the water bodies and vegetation cover. However, detailed inspection revealed that these indices could not separate the bare land from the built-up areas, in most cases. The region marked by a blue circle corresponds to the built-up area covered by buildings and impervious surfaces and the region marked by a red circle shows an empty farmland covered by bare soil. There was no major contrast between these two regions in the index images, indicating a low separability between the two land cover classes. The NBI image provided a slight contrast difference between the urban area and the soil cover, compared to the other indices, but not enough for an accurate separation (Figure 3e). The BSI, which is mostly used for determining bare land, in the literature, was not successful in separating the bare land from the built-up areas. Visual interpretation results of the BSI image showed that the urban area and the bare land were highly mixed. The urban areas, composed of buildings with brown roofs, showed a similar spectral response to that of bare land, which made them possible candidates for getting mixed-up (Figure 3i). The initial visual analysis indicated the necessity for an index that could highlight the built-up area and separate it from the soil cover.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Index Name** & **Index ID** & **Bands Used** & **Formula** & **Application** & **Reference** \\ \hline New built-up & NBI & Red, NIR and & SWIR \(\times\) Red & Automating the & & \\ index & NBI & SWIR & NIR & process of mapping & & \\  & & & & residential areas & & \\ \hline Vegetation & & Red, NIR and & NDVI & Segmenting urban & & \\ index & & SWIR & \(\overline{\text{NDVI}+\text{NDBI}}\) & areas & & \\ \hline Index-based & & & & Enhancing the & & \\ built-up index & & Red, Green, & \(\overline{\text{NDBI}-(\text{SAVI}+\text{MNDWI})/2}\) & built-up land feature & & \\  & & & NIR and SWIR & \(\overline{\text{NDBI}+(\text{SAVI}+\text{MNDWI})/2}\) & while effectively & & \\  & & & & suppressing & & \\  & & & & background noise & & \\ \hline Urban index & UI & NIR and SWIR & \(\left(\frac{\text{SWIR}-\text{NIR}}{\text{SWIR}-\text{NIR}}+1.0\right)\times 100\) & Evaluating & & \\  & & & & urbanization & & \\ \hline Bare soil index & BSI & Red, Blue, NIR & \(\overline{\text{SWIR}+\text{R}}-(\text{NIR}+\text{B})\) & Enhancing bare soil & [PERSON] et al. [69] \\  & & and SWIR & \(\overline{\text{SWIR}+\text{R}}+(\text{NIR}+\text{B})\) & areas, fallow lands & & \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 3: Images of (**a**) Sentinel-2A RGB; (**b**) normalized difference built-up index (NDBI); (**c**) built-up index (BUI); (**d**) built-up area extraction index (BAEI); (**e**) new built-up index (NBI); (**f**) vegetation index built-up index (VIBI); (**g**) index-based built-up index (IBI); (**h**) urban index (UI); and (**i**) bare soil index (BSI).