GeoJason

GeoJSON Union Tool

Compute the union of multiple GeoJSON polygons into a single geometry.

What Is a Geometric Union?

The union operation combines two or more overlapping polygons into a single polygon that covers the total area of all inputs. Where polygons overlap, the shared boundary is dissolved and the combined shape is returned as one geometry.

How to Use the Union Tool

Provide GeoJSON polygons and the tool computes their union. Non-overlapping polygons are returned as a MultiPolygon. Overlapping polygons are merged into a single continuous shape. The result preserves the outer boundary while removing internal boundaries.

Common Use Cases

  • Combining adjacent administrative regions into a single boundary
  • Merging overlapping buffer zones into a unified coverage area
  • Creating a single outline from multiple overlapping parcels
  • Simplifying complex multi-part boundaries for display purposes

Compare with merge (concatenates features without spatial combination), dissolve (merges features sharing a property value), and intersection (returns only the shared area).

Try It

Union two overlapping rectangles — one around Times Square and one around Central Park. The rectangles share an overlapping strip in between:

GeoJSON{
  "type": "Feature",
  "properties": { "name": "Times Square Area" },
  "geometry": {
    "type": "Polygon",
    "coordinates": [[[-73.99, 40.75], [-73.98, 40.75], [-73.98, 40.77], [-73.99, 40.77], [-73.99, 40.75]]]
  }
}
GeoJSON{
  "type": "Feature",
  "properties": { "name": "Central Park Area" },
  "geometry": {
    "type": "Polygon",
    "coordinates": [[[-73.985, 40.76], [-73.975, 40.76], [-73.975, 40.79], [-73.985, 40.79], [-73.985, 40.76]]]
  }
}

The result is a single Polygon covering the combined area of both rectangles. The overlapping strip between 40.76 and 40.77 latitude is merged seamlessly — no internal boundary remains. On the map, you see one L-shaped polygon instead of two overlapping rectangles.

How It Works

The union tool uses Turf.js (@turf/union), which computes the boolean union of two polygon geometries. The algorithm finds all intersection points between the polygon edges, then traces the outermost boundary to produce the combined shape. When polygons do not overlap, the result is a MultiPolygon containing both shapes as separate parts. For background on GeoJSON polygon structure, see the GeoJSON specification guide.