# Retrieval of Nearshore Bathymetry around Ganquan Island from LiDAR Waveform and QuickBird Image
## 1 Introduction  
Water depth is one of the basic elements of seabed topographic mapping and marine environment survey. The key and basic data provided by water depth monitoring are used in coastal engineering construction, ship navigation, and island development and utilization. The traditional bathymetric method of using ship-borne echo sounding achieved remarkable success. However, survey vessels and surveyors cannot easily carry out the traditional methods in shallow water (0-2 m) due to the influence of tide and submerged reef, indicating a restriction of these methods. Compared with traditional onsite acoustic measurements, the remote sensing inversion of water depth has the advantages of synchronous measurement in large coverage areas, high efficiency, and low cost. Remote sensing is an effective complement to traditional bathymetric methods in places where underwater terrains are complicated and unstable, and in sea areas affected by issues with rights and conflicts of interest.  
The remote sensing inversion models of water depth can be divided into three types: Theoretical models [1; 2], semi-empirical and semi-theoretical models [3; 4; 5], and empirical models [6; 7]. Among them, theoretical models need several physical parameters, including water environment, seabed sediment, and atmospheric environment. On the one hand, remote sensing images usually take the form of historical data, and deriving the physical parameters of synchronous images is difficult. On the other hand, morbidity inversion is a serious issue owing to the excessive number of physical parameters. Empirical models use a large amount of water depth data collected onsite and establish functional relations in the gray levels of remote sensing image bands for data inversion, but physical environment parameters, such as atmosphere and water quality, are ignored. Although the empirical models are relatively uncomplicated, their universality is poor, and executing them in inaccessible sea areas is difficult. Semi-empirical and semi-theoretical models combine the advantages of the above two models. These models can simultaneously model the radiation transfer of sunlight in water and use a small amount of field depth data to complete the inversion of water depth. In view of this advantage, the semi-empirical and semi-theoretical models are presently the most mainstream method of water depth inversion. However, the restive nature of acquiring in situ data for islands and reefs far from the mainland, particularly those parts that are difficult or inappropriate to reach, is regarded a crucial bottleneck in the inversion of water depth.  
Airborne light detection and ranging (LiDAR) bathymetry can effectively and efficiently measure shallow waters and oceans, but the advantages are more obvious in areas inaccessible to surveying vessels. LiDAR bathymetry is widely used in the field of modern ocean bathymetry because of its high accuracy, efficiency, and mobility. However, LiDAR bathymetry relies on an aircraft platform, which is expensive. Therefore, the combination of passive remote sensing with a small amount of LiDAR waveform data can not only reduce the cost of depth inversion but also complete the high-precision inversion of depth data in sea areas that are difficult or inappropriate to reach.