## 2 Study Area and Data
### Study Datasets
#### ICESat-2 Altimetry Data  
The Ice, Cloud, and land Elevation Satellite (ICESat) altimetry dataset has been widely used to validate the quality of DEM, due to its very high altimetry accuracy [14; 15; 16; 41]. The Geoscience Laser Altimeter System (GLAS) is the first space-based laser altimetry instrument onboard ICESat, which was launched on 12 January 2003. During its seven of years in-orbit operation, it completed 18 laser-operations campaigns. GLAS provided invaluable global data for estimating the Earth's elevation with a footprint size of approximately 70 m and the along-track sampling interval of about 170 m [42; 43]. More information regarding the ICESat mission can be found at the NASA website [43; 44].  
However, a single beam laser cannot separate surface slope effects from true elevation changes orbit by orbit bias and thus many years of data were needed to separate these two effects. In addition, the local surface slope has stronger effect on the estimation accuracy, as the accuracy drops quickly in steep areas [45]. In order to solve this problem, a new NASA space-based laser altimetry mission, ICESat-2 was launched on 15 September 2018. As a more advanced space-based Light Detection and Ranging (LiDAR), ICESat-2 has multi-beam instrument design (six beams arranged as three pairs of beams), small footprint size, and the ability to resolve rougher terrains, which can realize higher accuracy mountain measurements. Each beam pair of ICESat-2 consists of a strong and weak energy beam (4:1 ratio) which allows for local slope determination between each beam pair, as well as to compensate for varying surface reflectance. Furthermore, the strong energy beams represent the best option for detecting ground and canopy photons [46].  
Based on the distribution of signal photons, the ATL08 algorithm can estimate the ground surface and top of canopy surface elevations reported on the ATL08 data product [46]. As a result, unlike ICESat/GLAS data, ICESat-2 data record ground surface elevation and the canopy height information, providing more intensive and sufficient reference data for the quality analysis of different DEMs. In the previous research, [PERSON] et al. presented the quantitative assessment of the terrain height of ICESat-2 ATL-08 product as compared to airborne lidar data [47]. The initial result of the best fit terrain height validation over Finland indicates that the terrain residual has a mean absolute error of 0.5 m, while the Root Mean Square Error (RMSE) is 0.82 m. Thus, it is suitable to be used to assess the DEM accuracy. In this study, the altimetry data acquired by ICESat-2 in the first few months were selected to further assess the vertical accuracies of different DEMs, to help us understand the DEM accuracy and the corresponding error characteristics. In this study, the ICESat-2 ATL08 data from the NASA Earthquake Search website ([[https://search.earthdata.nasa.gov/search](https://search.earthdata.nasa.gov/search)]([https://search.earthdata.nasa.gov/search](https://search.earthdata.nasa.gov/search))) were used for the analysis. In addition, to solidify the analysis result, ICESat-1 GLA14 version 34 was also selected as the reference data.