# Analysis of Coastal Zone Data of Northern Yantai Collected by Remote Sensing from 1990 to 2018
## 2 Materials and Methods
### Coastline Extraction  
The coastline is the demarcation line between ocean and land, and it is the link of the average high tide trace [19]. There are many methods to extract coastline data by remote sensing, such as man-machine interactive visual interpretation [20], the normalized water body index [21], and the improved water boundary method [22]. However, considering the large time span, different resolution of image data, and the complex and diverse types of coastline, in this study, we use a visual interpretation method to draw and extract the coastline every 10 years from 1990 to 2018. Due to the decrease of coastal zone variation in recent years, we also extracted the coastline data of the research area from 2015 to 2018, which provides us with high resolution image data for the past four years. The extraction of different types of coastlines is based on defined principles and methods. In order to minimize the human errors in the interpretation of the coastline by different researchers, all of the years of coastline data extraction were completed by the same person. In order to ensure its accuracy, field survey and UAV photography were used to verify the data. First, the coastline types which are difficult to distinguish in remote sensing images are investigated and validated on the spot. Second, a random selection of several areas, using UAV data and remote sensing image data comparative analysis, to verify the accuracy of coastline extraction. Yantai's coastline can be divided into three categories: natural, artificial, and estuarine. On the basis of the first class, the second-class types were classified. The interpretation signs and basic norms of different types of coastlines mainly refer to the contents and indicators of basic geographic national conditions monitoring in 2018. Detailed classification information is shown in Table 1.  
Different types of coastlines are extracted from standard pseudo-color synthetic images taken by satellite remote sensing [23]. Among them, the natural boundary line is the trace line of the average high tide line formed naturally by natural land-sea interaction without human disturbance [24], including the gravel-sandy boundary line, silty boundary line, and bedrock boundary line. The gravel-sandy boundary line is relatively straight, and the boundary line between land and sea is generally located on the top of the ridge to the seaside. Images show a distinct difference between wet and dry due to the influence of tidal water, and the boundary line is located at the dry-wet boundary. The silty boundary line is mainly located on the land-facing side of the broadband where the growth of salt-tolerant plants has changed significantly over the intertidal zone. The acquisition position of the bedrock boundary line is defined at the base of a cliff or at the junction of land and water. The definition of an artificial boundary line is relatively easy, it mainly refers to the sea-land boundary line divided by the periphery of artificial facilities, such as damp-proof dikes, slope protection, wharfs, roads,