# Extraction of City Roads Using Luojia 1-01 Nighttime Light Data
## 2 Data Source and Preprocessing  
### NPP-VIIRS Nighttime Light Dataset and Preprocessing  
The earliest nighttime light data are the stable Nighttime Light Data on the Defense Meteorological Satellite Program/Operational Linescan System (DMSP/OLS). The DMSP/OLS nighttime light data are free for the public with coarse spatial resolution (15 arc seconds, 2-5 km) and long period (1992-2013) [25]. Several limitations of DMSP/OLS data also restrict its furthering application: The range of DN (digital number) values in DMSP/OLS data is small (0-63), which results in an over-saturation in city centers; the DMSP/OLS data are available just between 1992 and 2003, which also restricts the study of social economy issues after 2013 [26]; and the DMSP/OLS data are obtained through different satellites, which makes the data quality uneven in contiguous years. In October 2011, the Suomi National Polar-Orbiting Partnership (NPP) satellite with the Visible Infrared Imaging Radiometer Suite (VIIRS) was launched by the National Oceanic and Atmospheric Administration (NOAA)/National Geophysical Data Center (NGDC). The NPP-VIIRS nighttime light data are superior in spatial and radiometric resolution (15 arc seconds, 400-700 m), radiometric detection range and onboard calibration [27; 28; 29], compared with DMSP/OLS. However, the spatial resolution of the NPP-VIIRS nighttime light data is still not enough to study microcosmic topics; current research using NPP-VIIRS data mainly focus on the distinguishing of large-scale hot spots [30] and the economic inequality of a country [31]. On 2 June 2018, the Luojia 1-01 satellite produced by Wuhan University, China, was launched with the purpose of acquiring high resolution nighttime light data. The comparisons of Luojia 1-01, DMSP/OLS and NPP-VIIRS data are shown in Table 1, through which the spatial resolution of Luojia 1-01 can be found to be much higher than the previous two data sources. The nighttime light data used in this study are the Luojia 1-01 nighttime light data of Wuhan, China, obtained in July 2018. The data can be accessed from the official website of Hubei data and application center ([[http://www.hbeos.org.cn/](http://www.hbeos.org.cn/)]([http://www.hbeos.org.cn/](http://www.hbeos.org.cn/)), accessed on 29 September 2021). Located in the center of China, Wuhan is the capital city of Hubei Province. As one of the biggest central cities in China, Wuhan is also the national transportation hub and a famous tourism city in China. The rapid development of Wuhan makes it meaningful to find more appropriate ways to extract city roads. In this study, the following preprocessing steps were taken before the analysis of Luojia 1-01: (1) To reduce the influence of background noise and abnormal lights, the maximum radiance value derived city centers, where most commercial facilities and human activities \begin{table}
\begin{tabular}{c c c c} \hline
**Satellite Information** & **DMSP/OLS** & **NPP-VIIRS** & **Luojia 1-01** \\ \hline Institution & U.S. Department of Defense & NASA/NOAA & Wuhan University \\ Available years & 1992–2013 & 2011–Current & 2018,06–Current \\ Spatial resolution & 2–5 km & 400–700 m & 80–130 m \\ Revisit Time & 12 h & 12 h & 15 d \\ Wavelength range & 400–1100 nm & 500–900 nm & 400–800 nm \\ Swath width & 3000 km & 3000 km & 260 km \\ \hline \end{tabular}
\end{table} 
Table 1: The comparisons between Luojia 1-01 satellite and the other two nighttime light satellites DMSP/OLS and NPP-VIIRS. are concentrated, were set as the upper threshold [32; 33]. (2) Most of the faintly lit pixels, which could be urban roads, have very low radiance values as a result of the blooming effect of nearby lit pixels, and the radiance values were normally larger than water, due to their reflectivity difference [34]. Thus, the minimum radiance value obtained from the locations of lakes or rivers was set as the lower threshold to keep all the potential road pixels and eliminate the influence of pixels with 0 radiance value, which are either non-lit areas or noises [35].