GeoJason

GeoJSON Simplifier

Simplify complex GeoJSON geometries to reduce file size while preserving shape accuracy.

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Census tract files, OSM extracts, and country-boundary GeoJSONs routinely arrive at 20–80 MB — far too heavy for a browser map to render at 60 fps. A simplifier cuts the vertex count by an order of magnitude with imperceptible visual loss at zoom levels above ~z6, which is the single biggest win available for web-map performance.

Paste a FeatureCollection on the left and drag the tolerance slider. The tool runs the Ramer–Douglas–Peucker algorithm in your browser, reports the new vertex count and file size on every step, and renders the simplified geometry over the original so you can spot the zoom level where the simplification starts to bite.

Below the widget you'll find the tolerance/quality trade-off table, notes on when to prefer Visvalingam over Douglas–Peucker (it preserves area better for choropleths), and the Turf.js / Mapshaper recipes for doing this from a script. For the deeper picture, see the file-size optimisation guide.

Watch out for: shared boundaries between adjacent polygons go out of sync when each polygon is simplified independently — the gap between two countries' borders that used to share a vertex now opens up into a sliver. The fix is to either topology-aware simplify (Mapshaper's -simplify with visvalingam) or to merge to TopoJSON before simplifying and convert back. This tool simplifies geometries independently, which is fine for a single Polygon but a known gotcha for full-country choropleths.

How GeoJSON Simplification Works

This tool uses the Douglas-Peucker algorithm (also known as the Ramer-Douglas-Peucker algorithm) to reduce the number of vertices in GeoJSON geometries while preserving the overall shape. The algorithm works by recursively finding the point farthest from a line segment between two endpoints. If that distance is below the tolerance threshold, all intermediate points are removed. If it exceeds the threshold, the farthest point is kept and the process recurses on both sub-segments.

Simplification is applied to LineString and Polygon geometries. Point features pass through unchanged since they have no vertices to remove. The tool uses the "high quality" variant of the algorithm, which processes each ring and segment independently for better shape preservation.

Choosing the Right Tolerance

The tolerance value represents the maximum distance (in degrees) that a simplified edge can deviate from the original geometry. As a rough guide:

  • 0.001 — minimal simplification, good for city-level detail. A polygon with 10,000 vertices might reduce to 5,000.
  • 0.01 — moderate simplification, suitable for country-level maps. File sizes typically shrink by 50-70%.
  • 0.05–0.1 — aggressive simplification, best for world-overview maps. Complex coastlines become smooth curves.

Watch the map preview as you adjust the slider to find the right balance between file size and visual fidelity for your use case. The percentage reduction is shown in the status bar after each adjustment.

When to Simplify GeoJSON

  • Reducing file size for faster loading in web mapping applications
  • Improving rendering performance on mobile devices
  • Creating overview tiles or thumbnails from detailed boundary data
  • Preparing data for APIs with payload size limits
  • Building zoom-dependent layers where lower zoom levels use simpler geometry

Simplification Pitfalls

Over-simplification can introduce geometry errors. Watch for these common issues:

  • Self-intersecting polygons — aggressive simplification can cause polygon edges to cross each other, creating invalid geometry
  • Topology breaks — adjacent polygons that shared a boundary before simplification may develop gaps or overlaps afterward, since each polygon is simplified independently
  • Small feature loss — very small islands, enclaves, or narrow peninsulas may collapse to zero area and disappear entirely

After simplifying, validate the result with the validator and visually check the map for artifacts. For topology-preserving simplification, consider using TopoJSON, which simplifies shared boundaries together.

Programmatic Equivalent

In JavaScript with Turf.js:

JavaScriptconst simplified = turf.simplify(featureCollection, {
  tolerance: 0.01,
  highQuality: true
});

For additional file size reduction, combine simplification with coordinate precision reduction (turf.truncate()) or whitespace stripping (JSON.stringify(geojson)). For a comprehensive approach, see the file size optimization guide.

Frequently asked questions

What does the tolerance number actually represent?
The tolerance is in decimal degrees of longitude/latitude — the same units as your input coordinates. At the equator, 0.01° works out to roughly 1.1 km; at 45° latitude, 0.01° of longitude is about 0.78 km. To start from a metres-based target, divide by ~111,000 to get a starting tolerance.
Will simplification change feature properties?
No. Only geometry coordinates are touched. Every Feature's properties object passes through unchanged, and feature IDs are preserved. The FeatureCollection's bbox is also left as you pasted it — recompute it with the bounding box calculator if you need a fresh value.
Can I simplify a Point feature?
No. Points have no edges or vertices to remove and pass through the simplifier unchanged. Simplification only affects LineString, Polygon, MultiLineString, and MultiPolygon geometries. A FeatureCollection containing both Points and Polygons will simplify the polygons and pass the points through.
What is the difference between Douglas–Peucker and Visvalingam?
Douglas–Peucker (this tool's algorithm) removes vertices that fall close to a straight line between their neighbours. Visvalingam removes the vertex whose triangle with its neighbours contributes least to the polygon's area, which preserves area better for choropleth maps. For most use cases the visual difference is small at moderate tolerance.
How do I simplify large files from the command line?
For files above ~50 MB or batch jobs, use Mapshaper: mapshaper input.geojson -simplify 5% keep-shapes -o output.geojson. The keep-shapes flag prevents tiny features from disappearing, and Mapshaper's percentage-based simplification removes the need to convert decimal-degree tolerances. Mapshaper is also the standard tool for topology-aware simplification across shared boundaries.
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