How does TopoJSON make files smaller?

TopoJSON stores boundaries shared between two shapes — like a border between adjacent countries — as a single arc referenced by both. GeoJSON repeats those coordinates in every feature, so eliminating the duplication plus delta encoding can dramatically shrink the file.

What does quantization do?

Quantization snaps every coordinate onto an integer grid of the chosen size, replacing floating-point longitudes and latitudes with small integers. Combined with delta encoding, where each point stores only its offset from the previous one, this produces short repetitive numbers that compress well.

Which geometry types are supported?

Point, MultiPoint, LineString, MultiLineString, Polygon, and MultiPolygon are supported. Points are stored directly in the topology; line and polygon rings are converted into the shared arc array.

Will the output match the d3 topojson library?

It follows the TopoJSON 1.0 specification — a topology object with arcs, transform, and named objects — so it round-trips through standard TopoJSON clients. Exact arc ordering may differ from topojson-server, but the decoded geometry is equivalent.

No. The entire topology build, including arc extraction and quantization, happens in your browser. Nothing is sent to any server.

GeoJSON to TopoJSON Converter

GeoJSON to TopoJSON Converter turns a GeoJSON file into the more compact TopoJSON format. TopoJSON encodes geometry as topology — shared boundaries are stored once as arcs and referenced by every shape that touches them — which removes the coordinate duplication that bloats GeoJSON. Paste your data, pick a quantization level, and get back a valid TopoJSON object with a reported size reduction, all without leaving your browser.

How it works

The converter runs three stages that mirror the TopoJSON specification:

Quantization. Every coordinate is mapped from its geographic range onto an integer grid of quantization × quantization cells. The mapping is stored in a transform object with scale and translate so the original values can be reconstructed.
Arc extraction. Each ring and line is broken into arcs. The tool deduplicates identical arc coordinate sequences so a border shared by two polygons becomes one entry in the arcs array, referenced by index from both features.
Delta encoding. Within each arc, only the first point is absolute; every following point stores the difference from the previous one. After quantization these deltas are small integers, which is what makes TopoJSON compress so well.

Tips and notes

Higher quantization (for example 1e5) preserves fine detail at the cost of larger numbers; lower values (like 1e3) compress harder but visibly coarsen boundaries. For maps that share long borders — administrative regions, country outlines — the savings are largest because those boundaries collapse to single arcs. The output is a standard TopoJSON topology, so you can decode it later with the companion TopoJSON to GeoJSON tool.