**5.2 Multi-source topographic information fusion**

Obtaining high accuracy, high resolution, and global coverage, Digital Elevation Model (DEM) is the goal of aerospace remote sensing mapping [76]. The existing open-source DEM mainly includes SRTM (Shuttle Radar Topography Mission), GDEM (Global Digital Elevation Model), etc. GDEM is an elevation model acquired by Terra satellite optical sensor ASTER with 30 m resolution, covering about 99% of the global land surface. The SRTM is a Space Shuttle radar topographic mapping product with the latest release being the 30 m resolution SRTM-1. GDEM and SRTM-1 have nominal elevation accuracies of 17 m (95% confidence) and 16 m (90% confidence), respectively [77, 78]. In comparison, GDEM has noise and data artifacts due to optical imaging limitations, and SRTM has stable accuracy but data gaps at large slope locations in mountainous areas. ICESat-2 elevation accuracy is much higher than these two open DEMs, while the point density along the track direction is extremely high, and the ICESat-2 point cloud can be used as a control point to correct the open-source DEM elevation deviation and ensure the quality of DEM blank filling [79–82].

Specific processing should pay attention to the unification of coordinate system and elevation datum, ICESat-2 ATL03 point cloud coordinate system is WGS84, the elevation value is ellipsoidal height, the file provides the elevation aberrant value under EGM08 (Earth Gravitational Model 2008) (field/gtx/geophys\_corr/geo), which can be read and converted to normal elevation. Corrected DEMs are evaluated with ICESat-2, which allows the mapping of laser points to open-source DEM values to be established and corrections carried out. Vacancy filling is achieved through steps such as rasterization, interpolation (e.g., inverse distance weights, kriging, etc.) [83, 84] and triangulated mesh surface filling. The method that SRTM-1 and GDEM fusion filling were supported by ICESat data has been shown to be effective, and the evaluation shows that the accuracy of GDEM, SRTM-1, and ICESat after fusion processing is 10.9 20.2, 5.8 16.2, and 5.7 14.9 m, respectively, and ICESat-2 is theoretically better for fusion processing.

As shown in **Figure 14**, the results of the fusion processing of multi-source terrain information supported by the strip control datum are used. For the differences of shift and rotation between multiple topographic data in the same area, a fusion processing

method consisting of ICP topographic matching, spatial geometric transformation overall adjustment, and topographic reconstruction is proposed, and the experimental results show that the method can effectively eliminate the systematic geometric errors between different topographic data and improve the fusion accuracy of multi-source topographic data. At the same time, because the terrain matching depends on the terrain features, the feature matching accuracy is better when the terrain is highly rolling, and the overall adjustment accuracy is also better in mountainous or hilly areas compared with flat areas.
