TopoJSON
An extension of GeoJSON that encodes topology
...For example, the shared boundary between California and Nevada is represented only once, rather than being duplicated for both states. A single TopoJSON file can contain multiple feature collections without duplication, such as states and counties. Or, a TopoJSON file can efficiently represent both polygons (for fill) and boundaries (for stroke) as two feature collections that share the same arc mesh. See How To Infer Topology for a visual explanation of how TopoJSON works. See Command-Line Cartography for an introduction to TopoJSON and related tools. ...