TopoJSON 介绍
TopoJSON 是 GeoJSON 的扩展，增加了拓扑逻辑的编码。
Rather than representing geometries discretely, geometries in TopoJSON files
are stitched together from shared line segments called arcs. TopoJSON
eliminates redundancy, offering much more compact representations of geometry
than with GeoJSON; typical TopoJSON files are 80% smaller than their GeoJSON
equivalents. In addition, TopoJSON facilitates applications that use topology,
such as topology-preserving shape
simplification, automatic map
coloring, and
cartograms.