The need for facility maintenance is gradually increasing owing to the aging of domestic infrastructure such as port facilities. Accordingly, various measures are being taken for maintenance, but more efficient and productive measures are needed. Recently, digital transformation has emerged as a trend in the era of the Fourth Industrial Revolution, and the importance of 3D-data-based maintenance is increasing. Aging port facilities require more structured and interoperable digital data for efficient maintenance. In this study, we propose a City Geography Markup Language (CityGML)-to-land Extensible Markup Language conversion workflow for terrain data supporting port facility maintenance. The method derives terrain-related required classes and elements from both standards, matches semantically equivalent items, and transfers the extracted attribute values into a maintenance-oriented target structure. Validation was conducted using CityGML-based terrain data extracted from the 2019 Hague city model provided by Delft University of Technology for educational and research purposes. The extracted dataset consisted of one terrain object represented by 5595 surface elements in a triangulated irregular network structure. The results confirmed the successful generation of all required target items without omission, along with file-size reduction from approximately 1.71 to 1.07 MB and a conversion time of approximately 0.112 s. Because terrain serves as the spatial reference for facility objects, the proposed workflow provides a foundational step toward future facility-level maintenance data integration.