# Correlation Analysis between UBD and LST in Hefei, China, Using Luojia1-01 Night-Time Light Imagery
## 1 Introduction  
With rapid development and the intensification of urbanization, the scale of cities has expanded rapidly and urban buildings tend to be denser, which brings more pressure on the population, transportation, and industrial production, and causes severe damage to the ecological environment of the entire city. Urban Heat Islands (UHIs) are attracting more and more attention, especially in summer, as scorching hot temperatures have severely affected people's lives and work [1; 2; 3]. To date, researchers all over the world have carried out much research on UHIs. Among this research, quantitative analysis of the relationship between urban Land Surface Temperature (LST) and the underlying surface has become a research hotspot [4; 5; 6; 7]. Scholars believe that urbanization changes the atmospheric dynamics and heat transfer characteristics of the underlying surface as a large number of natural landscapes are gradually replaced by urban impervious surfaces such as buildings and pavements. This makes the urban landscape fragmented and complicated, which changes the urban ground energy balance [8]. Under the same environmental conditions, impervious surfaces absorb and capture solar radiation faster than green space or other natural underlying surfaces. At the same time, the artificial structureabsorbing heat faster and the specific heat capacity being smaller gives rise to limited heat release. In this way, the average LST in the area gradually increases with the spatial distribution of the UHI area to become more continuous and concentrated, forming a high-temperature area centered on artificial buildings in the city [1; 9; 10]. However, our comprehensive literature research found that past studies focused on the analysis of seasonal, daily, and annual variations of the UHI, which revealed the temporal and spatial variation of the UHI, the distribution characteristics of the horizontal and vertical directions, etc., while there has been less quantitative analysis of the single factors of UHIs, especially discussion of the ubiquitous relationship between impervious surfaces and UHI in the city. As an integral part of the urban impervious surface percentage, the urban building density (UBD) (percentage of built area) can directly reflect the degree of intensification and land-use efficiency in cities to some extent [11]. It is also an essential comprehensive social indicator that can function as an urban planning layout measure [12; 13], resource utilization efficiency measure [14; 15], and assessment of the urban ecological environment and livability [16; 17; 18; 19]. Some researchers have pointed out that UBD is a crucial indicator to reduce the UHI effect [20] and proved a positive correlation between UBD and LST or UHI [21; 22; 23]. Additionally, researchers have tried to use a variety of urban design factors to study the relationships of UHIs, including UBD information. For example, [PERSON] studied the summer heat island intensity of three high-rise residential districts in Shanghai, based on the three indices of building layout, density, and greening [1]. [PERSON] used nine urban design factors to construct the Sky View Factor (SVF), studied high-density urban areas with high UHI and low SVF as the research object, and tested the possibility of UHI by optimizing the urban-form SVF [24]. Remote sensing technology, as an accurate and timely means to obtain macroscopic information on the Earth's surface, has been widely used in many fields [25]. Thermal infrared remote sensing technology is a common means of monitoring LST [26; 27]. Using remote sensing images to extract UBD information is also a fast and effective method to make up for the shortcomings of traditional manual mapping methods. Existing studies on UBD tend to use high-resolution remote sensing satellites for research because they are more accurate [11; 28; 29], but their shortcomings are also obvious, such as complex building analysis, the cumbersome automated extraction process, and a large number of computations.