TopoJSON is a format for encoding geographic data structures, particularly for large data sets, that uses topology to reduce duplication and increAse efficiency. It is similar to GeoJSON but hAs the advantage of smaller file sizes and fAster rendering times....TopoJSON is a format for encoding geographic data structures, particularly for large data sets, that uses topology to reduce duplication and increase efficiency. It is similar to GeoJSON but has the advantage of smaller file sizes and faster rendering times.