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1. Introduction
The ocean is not only a treasure trove of abundant resources but is also an important strategic space to support future development (Li et al., 2015). In recent years, the rising ocean economy has become a blue engine for the economic development of coastal states. Global maritime transport services account for 70% of total transport services trade, and 90% of China's foreign trade is completed by sea (Zhang, 2018). However, the rapid development of maritime traffic has created great pressure and challenges to the safety of marine navigation. The navigational chart is an important guide to the safety of navigation. Due to the wide range of maritime traffic waters and the complex and changeable environment of coastal ports and fairways, it is necessary to obtain data through hydrographic survey and update navigational charts. If the cycle of navigational chart updating is long, the changes of coastal topography, fairway depth and navigation obstacles cannot be displayed on the chart in time, the effectiveness of the chart cannot be guaranteed, and there is a great potential safety hazard for ship navigation. If the cycle of navigational chart updating is short, it is necessary to carry out frequent hydrographic survey work, which consumes a lot of manpower and material resources. Therefore, a reasonable survey cycle is essential to ensure the effectiveness of the navigational chart.
To solve this problem, coastal states have developed their own hydrographic survey plans for navigational waters. Based on various factors, such as nature, economy and policy, they determined the order and cycle of survey of each navigational water area through different evaluation methods. The National Oceanic and Atmospheric Administration (NOAA) of the United States determined that approximately 500,000 square nautical miles of the Exclusive Economic Zone (EEZ) are ‘navigationally significant’, and divided the areas into two types, namely critical areas and prioritised areas. The critical areas possess the highest priority, and the prioritised areas have five priority levels, from 1 to 5. The division is based on several factors: shipping tonnage and trends; vintage of surveys in the area (year, equipment and processes utilised) under-keel clearance of vessels; potential for unknown dangers to navigation due to dynamic bottom or human influence; and requests for surveys from pilot associations (NOAA, 2012). In...