## 2 Materials and Methods
### Research Data
#### 2.2.2 Characteristic Variables for Permafrost Mapping  
As remote sensing technology has advanced and a growing number of remote sensing products have been produced, the characteristics variables employed in permafrost mapping methods have seen significant growth in recent years [37]. The practice of permafrost mapping currently fully utilizes the permafrost environmental elements gathered by remote sensing observation, such as surface temperature, vegetation indices, snow cover indices, and soil characteristics [37]. Permafrost distribution mapping can be considered from the perspective of the formation mechanism of permafrost and the performance characteristics of permafrost [6]. In addition, the resolution of the available characteristic variable data is also a key consideration. Especially in high-resolution permafrost mapping research, the resolution of the characteristic variable should be close to the result resolution. In this study, the environmental variables used and their relationship with the characteristics of permafrost were as follows:  
* Land surface temperature (LST). For permafrost connected with contemporary climate, i.e., in contact with AL, the difference in the temperature field between permafrost areas and non-permafrost areas is reflected in the difference in the surface thermal state. Limited by the observation technology and the observation cost, the resolution of existing ground (land) temperature data is always 1 km or lower. In this study, we hoped to obtain the observed temperature data at a resolution of 30 m. Based on Landsat data, LST with 30 m resolution can be obtained. The satellite image data used to retrieve the surface temperature should be cloud-free or possess only a small amount of clouds. ALT in the study area reaches the minimum in spring and the maximum in autumn. The satellite image data of these two seasons can better distinguish the permafrost area from the non-permafrost area [38]. At the same time, the interference of deciduous plants on the surface heat radiation is weakened in spring and autumn. Referring to the period of data selected by a commonly used method, the surface frost number model, in permafrost mapping [39], is suitable to add winter surface temperature data. Based on the above principles, in this study, three Landsat-8 OLI image data covering the study area were selected to retrieve the surface temperature (PATH: 122; ROW: 027), and the acquisition times were 25 April 2021, 16 September 2021, and 23 February 2022. The method used for the inversion of the surface temperature was the radiative transfer equation (RTE) algorithm, which is a commonly used algorithm for the inversion of the surface temperature [40]. The research of [PERSON] and [PERSON]. [41; 42] indicated that the results of the inversion of the surface temperature by the RTE algorithm have good reliability in Northeast China. Therefore, three surface temperature data, LST1 (25 April 2021), LST2 (16 September 2021), and LST3 (23 February 2022), in the study area were calculated using the RTE algorithm. The spatial resolution of LST1, LST2, and LST3 was 30 m.
* Deviation from mean elevation (DEV). In Arxan, extensive vegetation coverage and abundant groundwater resources make the occurrence of permafrost be closely related to the spatial distribution of soil moisture [43]. At present, the remote sensing monitoring of soil moisture is not comparable to LST in terms of spatial and temporal resolution. Some scholars use high-resolution DEM to calculate topographic indexes, such as TPI [44], TWI [45], and DEV [46], to study the relative distribution of water in a certain area. According to the pretest of selecting characteristic variables, in the study area, DEV has a good correlation with the borehole data of permafrost. DEV calculates the elevation standard deviation and the central point's topographic position [47]. The topographic position is measured by DEV as a percentage of the local relief normalized to the local surface roughness.