GeoJSON | To reduce the file size of GeoJSON files

To reduce the file size of your data.geojson by skipping points (decimation), you can use the following Python script. This script iterates through the geometries (LineStrings, Polygons, etc.) and samples every \(n\)-th point while ensuring that polygons remain closed.

-_$ python reduce_geojson.py data.geojson - $ python reduce_geojson.py data.geojson --step 5 - $ python reduce_geojson.py data.geojson -s 4 -o small_map.geojson

import json
import argparse
import os

def reduce_geometry(geometry, step):
    """
    Reduces the number of points in a geometry by taking every n-th point (step).
    """
    if geometry is None:
        return None
    
    geom_type = geometry.get('type')
    coords = geometry.get('coordinates')
    
    if geom_type == 'Point':
        return geometry
    
    if geom_type == 'MultiPoint':
        geometry['coordinates'] = coords[::step]
        return geometry
    
    if geom_type == 'LineString':
        new_coords = coords[::step]
        if len(new_coords) < 2 and len(coords) >= 2:
            new_coords = [coords[0], coords[-1]]
        geometry['coordinates'] = new_coords
        return geometry
    
    if geom_type == 'Polygon':
        new_coords = []
        for ring in coords:
            sampled = ring[::step]
            if sampled[-1] != ring[-1]:
                sampled.append(ring[-1])
            if len(sampled) < 4 and len(ring) >= 4:
                sampled = [ring[0], ring[len(ring)//3], ring[2*len(ring)//3], ring[-1]]
            new_coords.append(sampled)
        geometry['coordinates'] = new_coords
        return geometry

    if geom_type in ['MultiLineString', 'MultiPolygon']:
        # This handles nested coordinate structures
        def process_recursive(c, depth):
            if depth == 1: # Reached the list of points (LineString/Ring)
                sampled = c[::step]
                if geom_type == 'MultiPolygon' and sampled[-1] != c[-1]:
                    sampled.append(c[-1])
                return sampled
            return [process_recursive(sub, depth - 1) for sub in c]
        
        depth = 2 if geom_type == 'MultiLineString' else 3
        geometry['coordinates'] = process_recursive(coords, depth)
        return geometry

    return geometry

def main():
    parser = argparse.ArgumentParser(description="Reduce GeoJSON file size by skipping coordinate points.")
    
    # Positional argument for the file
    parser.add_argument("input", help="Path to the input data.geojson file")
    
    # Optional arguments
    parser.add_argument("-s", "--step", type=int, default=3, 
                        help="The step size (e.g., 3 to keep every 3rd point). Default is 3.")
    parser.add_argument("-o", "--output", help="Output filename. Defaults to 'reduced_<input>'")

    args = parser.parse_args()

    # Determine output name if not provided
    if not args.output:
        base, ext = os.path.splitext(args.input)
        args.output = f"{base}_reduced{ext}"

    try:
        with open(args.input, 'r') as f:
            data = json.load(f)

        if data.get('type') == 'FeatureCollection':
            for feature in data.get('features', []):
                feature['geometry'] = reduce_geometry(feature.get('geometry'), args.step)
        elif data.get('type') == 'Feature':
            data['geometry'] = reduce_geometry(data.get('geometry'), args.step)
        else:
            data = reduce_geometry(data, args.step)

        with open(args.output, 'w') as f:
            json.dump(data, f)
        
        print(f"Success! Reduced file saved as: {args.output}")
        print(f"Sampling rate: Kept 1 every {args.step} points.")

    except FileNotFoundError:
        print(f"Error: The file '{args.input}' was not found.")
    except json.JSONDecodeError:
        print(f"Error: The file '{args.input}' is not a valid JSON/GeoJSON.")

if __name__ == "__main__":
    main()