S57 Map Data Decoding: Data Decompression, Parsing, and Processing Workflow

# 1. Overview of S57 Chart Data

## 1.1 What is S57 Chart Data

S57 chart data is an internationally recognized nautical chart data format, established by the International Hydrographic Organization (IHO). It is primarily used in the fields of navigation and marine navigation, encompassing various elements in nautical charts such as buoys, lighthouses, shipping lanes, and water depths.

S57 chart data employs an object-based method to store chart information, providing detailed and accurate chart data for safe navigation and voyage planning.

## 1.2 Applications of S57 Chart Data

S57 chart data is widely applied in the fields of maritime navigation, marine navigation, marine resource development and utilization, and marine environmental protection. During voyages, ships use S57 chart data for route planning, obstacle avoidance, and determination of water depths to ensure safe navigation.

Beyond maritime applications, S57 chart data is also utilized in GIS systems for marine scientific research and marine resource management, providing convenient and accurate map information support for related industries.

## 1.3 Introduction to S57 Chart Data Structure

S57 chart data uses vector data format for storage, with the data structure including features (Feature), attributes (Attribute), and geometry (Geometry). Features represent specific objects in nautical charts, such as lighthouses and shipping lanes; attributes include descriptive information about features, such as name and color; geometry describes the spatial location information of features, such as coordinates, lines, and areas.

The S57 chart data structure is rigorous and standardized, facilitating parsing and application. Understanding the structure of S57 chart data can enhance the comprehension of nautical chart information, enabling data analysis and applications.

# 2. Data Decompression

Data decompression is an essential step in the processing of S57 chart data. This chapter introduces the compression format of S57 chart data, common data decompression algorithms, and how to decompress S57 chart data.

### 2.1 Compression Format of S57 Chart Data

S57 chart data is typically stored in ENC (Electronic Navigational Chart) format, utilizing a binary-based compression algorithm to reduce the size of data files, thereby saving storage space and lowering transmission costs.

### 2.2 Common Data Decompression Algorithms

Common data decompression algorithms include:
- DEFLATE: A universal data compression algorithm widely used in the compression and decompression of various file formats.
- LZMA: A high compression ratio algorithm often used in scenarios requiring higher compression ratios.
- BZIP2: An efficient and high compression ratio algorithm suitable for compressing large files.

### 2.3 How to Decompress S57 Chart Data

Decompressing S57 chart data can be achieved using corresponding decompression tools or libraries. Below is an example using Python's zipfile library to decompress S57 chart data files:

import zipfile

def unzip_s57_data(file_path, output_path):
     with zipfile.ZipFile(file_path, 'r') as zip_ref:
         zip_ref.extractall(output_path)

# Example call
s57_file = 'example.s57'
output_folder = 'unzipped_data'
unzip_s57_data(s57_file, output_folder)

This Python code demonstrates how to decompress an S57 chart data file named `example.s57` into the `unzipped_data` directory.

In this chapter, we briefly introduced the compression format of S57 chart data, common data decompression algorithms, and how to decompress S57 chart data using Python. In practical applications, choosing the appropriate decompression algorithm and tool based on specific situations can help improve data processing efficiency and accuracy.

# 3. Data Parsing

Data parsing is the process of decoding the encoding rules of S57 chart data, converting them into a usable format, and is a crucial step in the entire data processing workflow. During data parsing, we need to use corresponding tools and libraries to help us effectively parse S57 chart data, and we also need to address some common problems and challenges.

#### 3.1 Encoding