TopoJSON to GeoJSON Converter

About this tool

TopoJSON to GeoJSON Converter expands a compact TopoJSON topology back into the standard GeoJSON that mapping libraries, GIS desktops and spatial databases actually read. Leaflet, Turf.js, PostGIS, QGIS and geopandas all consume GeoJSON; TopoJSON is a distribution format, and this is the step that turns one into the other.

TopoJSON is small because it never stores a boundary twice. Every line segment lives once in a shared arc list, and each shape refers to arcs by index — a negative index like -1 means "arc 0, walked backwards". On top of that, a quantized topology stores integer grid coordinates plus a transform (a scale and a translate), and each arc's positions are delta-encoded: the first position is absolute and every later one is an offset from the position before it.

Expanding all three layers is what this tool does: decode each arc once, accumulate the deltas, apply the transform, then stitch the referenced arcs back together — dropping the duplicated position where two arcs meet, and reversing the ones referenced negatively — until every ring closes.

Worked example

Two unit squares that share their middle edge. The shared edge is stored once, as arc 0; the east square reuses it backwards as -1:

{
  "type": "Topology",
  "transform": { "scale": [0.5, 0.5], "translate": [10, 20] },
  "objects": {
    "blocks": {
      "type": "GeometryCollection",
      "geometries": [
        { "type": "Polygon", "id": "a", "properties": { "name": "West" }, "arcs": [[0, 1]] },
        { "type": "Polygon", "id": "b", "properties": { "name": "East" }, "arcs": [[2, -1]] }
      ]
    }
  },
  "arcs": [
    [[2, 0], [0, 2]],
    [[2, 2], [-2, 0], [0, -2], [2, 0]],
    [[2, 0], [2, 0], [0, 2], [-2, 0]]
  ]
}

With Indent = 0 (minified), the output is:

{"type":"FeatureCollection","features":[{"type":"Feature","id":"a","properties":{"name":"West"},"geometry":{"type":"Polygon","coordinates":[[[11.0,20.0],[11.0,21.0],[10.0,21.0],[10.0,20.0],[11.0,20.0]]]}},{"type":"Feature","id":"b","properties":{"name":"East"},"geometry":{"type":"Polygon","coordinates":[[[11.0,20.0],[12.0,20.0],[12.0,21.0],[11.0,21.0],[11.0,20.0]]]}}]}

Both rings are now fully written out and closed, the quantized [2, 0] grid values have become real coordinates [11.0, 20.0] via the transform, and id and properties came through untouched. Leave Indent at its default of 2 to get the same thing pretty-printed.

Options

• Object to expand — the name of one entry in the topology's objects map, e.g. countries. Leave it blank (the default) to expand every object and merge them into a single collection, in document order. A name that isn't there is rejected with the list of names the file does have.
• Output shape — FeatureCollection (default) is standard RFC 7946: each geometry becomes a Feature that keeps its properties, id and bbox. GeometryCollection emits the bare geometries instead, which is what you want when the consumer only cares about shapes.
• Add a bounding box — appends "bbox": [west, south, east, north] to the result. It's computed from the coordinates actually emitted, so selecting a single object gives that object's extent rather than the whole topology's.
• Coordinate decimals — rounds every coordinate to N places. Quantized grids decode to values like -179.99999999999997; rounding to 6 or 7 decimals removes that floating-point noise (7 decimals is roughly 1 cm of longitude) and shrinks the file. -1, the default, keeps full precision.
• Indent spaces — 1–8 spaces per level (default 2), or 0 to minify onto one line.

Limits and behavior

• The output is bigger than the input — that's the point. GeoJSON has no topology, so every shared border gets written out once per shape that touches it. Expect roughly 2–5× the byte count, more for dense administrative boundaries.
• Un-quantized topologies (no transform member) are handled too: their arc positions are read as absolute coordinates rather than deltas.
• Point and MultiPoint coordinates are quantized but not delta-encoded, per the TopoJSON spec, so the transform is applied to each position on its own.
• A third dimension (elevation) or any further ordinate is carried through unchanged — the spec only quantizes x and y.
• The computed bbox is 2D, [west, south, east, north], even for 3D coordinates. A bbox already present on an individual geometry is copied to its feature as-is.
• Degenerate input is padded to stay valid GeoJSON: a ring stitched from a two-position arc is padded to the 4 positions RFC 7946 requires.
• A geometry whose "type" is null becomes a feature with a null geometry, which GeoJSON allows. An unrecognized type name is an error, not a silent drop.
• Everything runs locally in WebAssembly, so file size is bounded only by your browser's memory. Multi-megabyte topologies work but take a moment to render.

FAQ