# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Underwater Ocean  
RS has several applications for underwater ocean environments. In this study, three important applications of bathymetry, Aquatic Vegetation (AV), and Coral Reef (CR) using different types of RS systems are discussed. Bathymetric data provide the depth of water bodies as a function of geographical coordinates [273]. Bathymetric mapping is important in planning near-shore activities (e.g., pipeline laying, port management, dredging operations, oil drilling, and fishing), navigation, oceanography, marine geology, and paleoclimate research [273; 274; 275]. Additionally,accurate bathymetry maps are required for marine power plant planning, storm surge modeling, and coastal management [2]. AVs, including all underwater flowering plants, play a critical role in aquatic ecosystems. For example, AV provides food and habitat to wildlife [274; 276; 277], and is important for biogeochemical cycling [278], sediment stabilization and trapping [279], nutrient uptake [280], as well as wave attenuation and hydraulic resistance [281]. Despite various ecological, physical, and economic benefits of AVs, they are disappearing mainly due to climate change and anthropogenic disturbance, especially in coastal areas [282; 283]. Given the importance of studying AV, several national and international protocols and frameworks, such as hyperspectral RS protocol development for AV mapping and monitoring in shallow waters [284], have been proposed. CR is home to at least 25% of all marine species [285]. CRs provide different ecosystem services, such as flood protection [286], tourism [287], coastline protection [288], fisheries, and food sources to millions of people [289]. For example, it was estimated that nearly 500 million people worldwide depend on CRs [290]. CR is very sensitive to changes in hydrodynamic settings (e.g., water quality, light, and temperature), and can only thrive in a restricted domain of environmental conditions [291; 292]. Based on the most recent report from the global CR monitoring network in 2008, 54% of the world's CRs are suffering from regional or global threats [293]. This is mainly related to the side effects of climate change and anthropogenic disturbance [288; 293]. The increasing global awareness of the importance of CR, on the one hand, and the necessity of detailed mapping and classification of CRs based on actions 15 and 17 of the reef 2050 long-term sustainability plan [294], on the other hand, have been leading to conducting a wide range of CR programs worldwide. Despite the importance of underwater ocean studies, there are still challenges in generating adequate data on global oceans due to the large extent and limited accessibility in the ocean environment [273]. For instance, it is estimated that 70% of the world's littoral zone has not yet been mapped based on modern standards [279]. In recent decades, several methods have been investigated to obtain accurate information about ocean topography, AV and CR, ranging from field surveys to RS methods (see Figure 11). Field surveys are time-consuming, expensive and have accessibility issues [277; 296]. On the other hand, a pre-measured cable or rope located on ships, which was historically  
Figure 11: Summary of approaches applied to underwater studies [295]. used to collect bathymetric data, is sometimes inefficient because it can only provide point-based information [273]. The shipborne methods, such as echo sounding, are relatively efficient for underwater studies [274]. However, these methods are also limited over large remote ocean areas mainly due to the high cost [297; 298]. Additionally, the applicability of this technique is negatively affected by technological restraints and environmental conditions in coastal regions [299]. Airborne and spaceborne RS methods, however, can effectively address several limitations of field surveys and shipborne methods. RS methods are safer, cost-effective, and faster than field surveys. For example, [PERSON] et al.