**5.5 Shallow Sea topographic mapping**

Shallow bathymetry is the basic geographic data required for many applications such as marine engineering construction and marine environmental research, and it is

**Figure 18.**

*Schematic diagram of ICESat-2 shallow bathymetry. (a) Geographical location of Xisha Coral Island; (b) ICESat-2 photon profile point cloud and refraction correction effect; (c) shallow water deep pseudo-color map.*

a difficult and hot spot in the field of marine mapping to obtain shallow bathymetry accurately and efficiently [91–93].

Laser propagation has to go through a complex process of atmosphere-water bodyatmosphere, and the returned laser energy is significantly weakened due to the attenuation of the water body, it is difficult for satellite platforms to detect shallow water depth in the linear detection laser stage, and the ultra-high sensitivity of photoncounting LiDAR makes it a reality. The on-board experimental results of MABEL (ICESat-2 principle prototype) show that the laser bathymetry depth reaches 8 m with a root mean square error 0.7 m [94]; after the release of ICESat-2 data, analysis showed a maximum bathymetric depth of up to 40 m with a root mean square error in the range of 0.43–0.6 m [53]. **Figure 18** shows the joint bathymetry test with World-View-2 optical images using ICESat-2 at the Xisha coral reef in China, with an accuracy of 0.23 0.98 m. It should be noted that the standard product released by ICESat-2 does not have shallow bathymetry, i.e., the elevation value of ATL03 point cloud does not take into account the refraction of the water body, so the practical application needs to carry out targeted denoising, refraction correction, surface/submerged separation, and bathymetry calculation; at the same time, due to the large spacing of ICESat-2 beam vertical tracks, many distant islands and reefs are not passed by the survey line, so the joint processing of laser and image in a certain range to carry out water body inversion parameter transposition is a more feasible method for the area lacking direct active bathymetry values.
