TopoJSON extends GeoJSON by encoding topology, that is, relationships between individual shapes. Instead of storing the coordinates of each shape, TopoJSON stores line segments (called arcs) that are used to build the shapes, removing duplicates that occur when two shapes share a common border. This eliminates redundancy and results in a file that is much smaller and more efficient. TopoJSON makes it easier to compute neighboring shapes, merge all shapes into one and create a mesh from shape outlines. This can considerably reduce the amount of objects in a Web page, making it more memory-efficient.
TopoJSON files aren't as popular as GeoJSON, but you can easily generate one using command-line or online tools. D3 supports TopoJSON through a JavaScript library with methods that generate GeoJSON data from TopoJSON.
The following code reveals the structure of a simple...
