How does decoding work?

Each arc in the topology is stored delta-encoded: the first point is absolute and the rest are offsets. The decoder runs a cumulative sum to rebuild absolute integer coordinates, then applies the topology transform (scale and translate) to recover real longitude and latitude.

What are negative arc indices?

TopoJSON references an arc in reverse with the index ~i, which equals -i - 1. When a geometry uses ~i, the decoder takes that arc and reverses its point order before stitching, which is how shared boundaries are reused in opposite directions.

Can it handle topologies with multiple objects?

Yes. Each top-level entry under objects becomes a selectable layer. Pick one from the dropdown and the tool converts just that object into a GeoJSON FeatureCollection, preserving each geometry's properties.

Does it support topologies without a transform?

Yes. If no transform is present the arc coordinates are already absolute, so the decoder skips dequantization and uses the values as-is, which is valid per the TopoJSON specification.

Is my data sent anywhere?

No. Decoding runs entirely in your browser. The topology and the resulting GeoJSON never leave your device.

TopoJSON to GeoJSON Converter

TopoJSON to GeoJSON Converter reverses TopoJSON encoding so you can use the data in the many tools that only read plain GeoJSON. It rebuilds each geometry by stitching together the topology’s shared arcs, undoing the delta encoding, and applying the transform to restore real-world coordinates — entirely in your browser with nothing uploaded.

How it works

A TopoJSON topology stores all line segments once in a global arcs array and references them by index from each geometry. Decoding reverses three encoding steps:

Un-delta. Each arc lists its first point absolutely and every subsequent point as an offset from the previous one. The decoder accumulates these offsets to recover absolute integer coordinates.
Transform. If the topology has a transform, each integer point p becomes point = p * scale + translate, converting the quantized grid back to longitude and latitude.
Arc stitching. A geometry’s arcs are arrays of arc indices. A positive index i uses arc i forwards; a negative index uses arc ~i (which is -i - 1) reversed. The decoder concatenates the arcs, dropping the duplicated shared vertex where one arc ends and the next begins.

Tips and notes

Polygons in TopoJSON are arrays of rings, and MultiPolygons are arrays of those — the decoder rebuilds the full nesting so the output matches the GeoJSON spec. Properties attached to each topology geometry are copied onto the resulting GeoJSON feature. Use the companion GeoJSON to TopoJSON tool to compress data, and this one to decode it back whenever a downstream tool needs GeoJSON.