TopoJSON (Topology JSON Format)
Apr 28,2026

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Introduction

TopoJSON (Topology JSON Format) is a JSON-based vector data format designed to efficiently represent geospatial data. Proposed by American data visualization researcher Mike Bostock, it extends the standard GeoJSON format and is characterized by explicitly preserving the topology (adjacency relationships and shared boundaries) of geographic data. TopoJSON adopts a structure where boundary lines shared among multiple geographic objects are stored only once and then referenced, enabling significant data size reduction. Therefore, it is widely used in scenarios requiring lightweight, high-speed geographic data delivery, such as web maps and data visualization.

File Structure

The file structure of TopoJSON consists of multiple elements centered around topology information. The main components are as follows:

type (Data type definition): Indicates the basic type of the TopoJSON file. Typically written as "Topology," indicating that the file contains data with a topological structure.
objects (Geometry objects): The part that stores geographic objects (country borders, administrative divisions, roads, etc.). Geometries such as Polygon, MultiPolygon, and LineString are defined, but rather than containing coordinates directly, they are expressed by referencing the arcs described below.
arcs (Shared line segment data): The core structure of TopoJSON, storing boundary lines shared among multiple polygons and lines. Each geometry forms its shape by referencing this arcs array. This eliminates the need to store the same boundary line multiple times.
transform (Coordinate transformation information, optional): Stores scale and offset information for restoring quantized coordinates to original geographic coordinates. This allows for further data size reduction.
bbox (Bounding box, optional): The minimum bounding rectangle showing the geographic extent of the entire dataset. Used for spatial queries and rendering optimization.
properties (Attribute information): Stores attribute data associated with each geometry (name, population, administrative code, etc.). Used for GIS analysis and label display during visualization.

Pros

Significant data size reduction: The topological structure, where shared boundaries are stored only once, allows for substantial data size reduction compared to standard formats like GeoJSON. This is particularly effective for administrative division data.
Preserves topological relationships: Since adjacency relationships and shared boundaries are explicitly preserved, boundary integrity can be maintained during geographic analysis and map rendering.
Optimized for web visualization: The lightweight data structure enables fast loading and rendering in browser-based map visualization and data visualization. Widely used particularly for visualization with D3.js.
Efficient data updates: Because boundary lines are stored in a shared structure, partial geometry modifications can be updated while maintaining consistency with other data.
Compression via quantization: By using coordinate quantization, it is possible to further reduce data size while preserving precision.

Cons

Complex structure: Because TopoJSON has a topological structure, it can be more difficult to understand and edit compared to simple coordinate array formats like GeoJSON.
Limited GIS software support: While many web tools support it, some GIS software cannot read it directly and may require conversion to GeoJSON or other formats.
Difficult editing operations: Because topology is preserved, editing a single geometry may affect other geometries.
Not suitable for real-time updates: Due to its shared reference-based structure, it can be difficult to handle for frequently updated data processing.

Application Scenario

TopoJSON is widely used in the fields of web maps and data visualization, where lightweight geographic data delivery is required. Particularly when displaying world maps or administrative boundary data in browsers, it achieves both data size reduction and improved rendering speed. It is also suitable for interactive visualizations that represent statistical data and socioeconomic data on maps, and is utilized in online dashboards and data journalism. Furthermore, it is effective for projects that require maintaining boundary consistency among multiple geographic objects, making it an important format supporting efficient management and delivery of geographic data.