## 2 Study Area and Data  
Xi'an city is the capital of Shaanxi Province, located in the Guanzhong Plain of the Yellow River Basin. It is an important central city in western China, between 107\({}^{\circ}\)4\({}^{\prime}\)-109\({}^{\circ}\)4\({}^{\prime}\) E and 33\({}^{\circ}\)42\({}^{\prime}\)-34\({}^{\circ}\)45\({}^{\prime}\) N. The main urban area of Xi'an city includes the Weiyang District, Lianhu District, Beilin District, Yanta District, Xincheng District, and Baqiao District (Figure 1), with a total area of 831.87 km\({}^{2}\). By the end of 2021, the total population of the main urban area will be approximately 4.9248 million people. From 1984 to 2016, the construction land in the study area increased by 478.62 km\({}^{2}\), an overall increase of 3.27 times [32], and the sales area of commercial residential buildings in the main urban area will reach 9.5796 million m\({}^{2}\) in 2021 [33].  
The Luojia-1 satellite was developed by Wuhan University and successfully launched in June 2018 [34]. It is the world's first \"one-satellite multipurpose\" low-orbit, micro-nanoscience experimentation satellite, with remote sensing and navigational functions. The Luojia-1 is equipped with a highly sensitive luminous camera that can obtain images of nighttime lights at a high resolution. It has a spatial resolution of 130 m, a width of 250 km, a spectral bandwidth of 0.319 um, a dynamic range of 14 bits at night, and a revisit period of 3-5 days. Compared with traditional DMSP/OLS and NPP/VIIRS nighttime light data (Table 1), Luojia-1 data have obvious advantages in terms of data bits and spatial resolution. Therefore, nighttime light data can more clearly reflect the spatial structure and spatial characteristics of a city [35].  
DMSP/OLS and NPP-VIIRS are the main data sources in current housing vacancy studies. The revisit time of DMSP/OLS and NPP-VIIRS is short, the data are easy to obtain, and it is usually used in studying large regional scales. However, the resolution of DMSP/OLS and NPP-VIIRS cannot meet the needs of small regional scales. In this study, the data of Luojia-1 were used as the main data source. Four time points (19 August 2018, 23 August 2018, 31 October 2018, and 11 March 2019) data that could completely cover the study area were selected to estimate the HVR of Xi'an city.  
POI data is obtained from the Amap Open Platform ([[https://lbs.amap.com/](https://lbs.amap.com/)]([https://lbs.amap.com/](https://lbs.amap.com/)), accessed on 29 October 2021). Amap Open Platform is an open-source and free data interface platform, which only needs to register an account to obtain POI for free. POI has a wide range of application scenarios and strong calculation and expression capabilities. There are 21 first-level classifications, 267 s-level classifications, and 869 third-level classifications of Amap POI. This paper uses a total of 635,364 records at three levels of classification in the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Data Source** & **Country** & \
\begin{tabular}{c} **Number of** \\ **Data Bits** \\ \end{tabular} &
\begin{tabular}{c} **Spatial** \\ **Resolution/m** \\ \end{tabular} & **Time in Orbit** & **Revisit Cycle** & **Width/km** \\ \hline NPP/VIIRS & America & 6 & 500 & 2011–present & 12 h & 3000 \\ DMSP/OLS & America & 14 & 1000 & 1992–2013 & 12 h & 3000 \\ Luojia-1 & China & 15 & 130 & 2018–present & 3–5 d & 250 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of different nighttime light data parameters.  
Figure 1: Location of the study area.  
main urban area of Xi'an city. The POI can reflect a full range of human activity traces and is more representative of secondary data for housing vacancy studies.  
The administrative data were obtained from the National Basic Geographic Information Center ([[http://www.ngcc.cn/](http://www.ngcc.cn/)]([http://www.ngcc.cn/](http://www.ngcc.cn/)), accessed on 21 October 2021).