## 1 Introduction  
Digital elevation model (DEM), containing the basic information about the topographic features representing the actual Earth's surface [1]. It has been used for various fields, such as the glacier mass estimation [2], forest inventory [3, 4], geological hazard monitoring [5] and natural hazard mapping [6], and its accuracy determines the reliability of these results.  
In the past decades, the development of Earth observation technologies brought a variety of global or near-global scale digital elevation models (DEMs), including SRTM DEM [7], ASTER GDEM2 [8, 9], AW3D30 DEM [10] and TanDEM-X DEM [11, 12, 13]. ASTER GDEM2 and AW3D30 DEM were obtained by photogrammetric methods using stereo data, but SRTM DEM and TanDEM-X DEM were generated by interferometric synthetic aperture radar (InSAR). Due to different imaging configurations and data processing methods, these DEM products contain various errors [14]. In addition, compared with national DEM products, these global DEMs have lower vertical accuracy. However, for regional scale geographical studies, it is difficult to obtain highly accurate national DEMs covering the entire region, the freely available medium-resolution global DEMs are thus often used instead. Therefore, evaluating the accuracy of these global DEMs is necessary.  
To reveal the vertical accuracy of different DEM datasets, many studies compared the DEM values against highly accurate ground points (e.g., GPS measurement, NASA ICESat/GLAS) [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26] or reference DEMs [27; 28; 29]. Furthermore, independent studies on the global or regional DEMs show that the real vertical accuracy is often higher than the official accuracy [24]. However, comprehensive vertical accuracy evaluations are scarce for the DEMs over the south-central and southwestern regions of China [23; 29], especially Hunan province, where the topography and land cover types are complex. In addition, the ICESat/GLAS data is often selected as the reference data to estimate the DEM accuracy, due to its high accuracy and wide coverage [15; 16; 17]. However, the accuracy of ICESat/GLAS data with larger-footprint size are highly susceptible to the influence of ground slope. Thus, it is not always appropriate to verify the accuracy of DEM, especially in rugged areas. In this paper, we selected the ICESat-2 altimetry data to evaluate the vertical accuracy of different DEM datasets, which include three with 30-m cell resolution (SRTM1, ASTER GDEM2 and AW3D30), one 30-m resampled TanDEM-X DEM (hereafter called TDX30), and two with 90-m resolution (SRTM3 and TanDEM-X 90m DEM (TDX90)).  
The structure of this paper is outlined as follows: Section 2 briefs the study site and data sources. Then, the assessment method is introduced in Section 3. The overall performances of these DEMs are assessed in terms of vertical accuracy using highly accurate ICESat-2 altimetry points in Section 4. In that section, the performances of each DEM with different topographic characteristics (such as the terrain elevation, terrain slope and aspect) and land cover types are tested in the selected study area. Furthermore, the impacts of different reference data on the DEM selection are analyzed. Finally, some conclusions are drawn in Section 6.