Developing a Spatial-ID–BasedWeb API for Serving 3-D City-Model Attributes and Reviewing the Spatial ID Specification

Three-dimensional city models are essential for urban planning and smart city applications, yet accessing specific attributes from these models remains challenging when integrating diverse spatial datasets. This paper bridges Japan’s Spatial ID specification—a universal voxel-based 3D indexing system—with Project PLATEAU’s comprehensive 3D city models covering over 250 Japanese cities. We present a Web API framework that enables efficient retrieval of CityGML data through Spatial ID coordinates, achieving median response times of 1 second for spatial queries. The API provides three complementary endpoints for file discovery, feature search, and attribute retrieval, supported by an interactive visual interface that makes abstract Spatial ID strings accessible to users without extensive GIS expertise. Our implementation demonstrates practical applications in real-time navigation, indoor-outdoor data integration, and sensor data fusion for urban analytics. Performance evaluation shows the system scales effectively to national-level deployment whilst identifying opportunities to enhance the Spatial ID specification, particularly regarding vertical reference system consistency. This work establishes critical infrastructure for Japan’s digital twin ecosystem, enabling seamless integration of heterogeneous urban datasets through universal spatial referencing as envisioned under the Society 5.0 initiative.