Buffering and spatial operations

A buffer is the zone of all points within distance d of a geometry. It's one of the most common spatial operations: "find everything within 500 m of this road", "draw the flood risk zone 100 m from this river". Buffers are also the foundation for proximity analysis: rather than querying with radius, you materialise the zone as a polygon and use polygon containment checks.

Point buffer

A point buffer is a circle. Given centre $(φ, λ)$ and radius $r$ metres, generate $n$ points around it:

function circleBuffer(lat, lon, radiusM, steps = 64) {
  const R = 6_371_008.8;
  const Δlat = (radiusM / R) * (180 / Math.PI);
  const Δlon = Δlat / Math.cos((lat * Math.PI) / 180);

  return Array.from({ length: steps + 1 }, (_, i) => {
    const θ = (i / steps) * 2 * Math.PI;
    return [lon + Δlon * Math.cos(θ), lat + Δlat * Math.sin(θ)];
  });
}

This approximates the circle in planar space, which is accurate for radii under ~50 km. For larger radii, use the destination-point formula (Chapter 9) to generate each vertex from the center with the exact bearing and distance.

Line buffer

A line buffer creates a corridor around a LineString. The approach:

1. For each segment, compute the perpendicular offset lines at distance d
2. Add semicircular end caps at each terminus
3. Union all the pieces into a single polygon

This is significantly more complex to implement from scratch. In practice, use Turf.js:

import buffer from "@turf/buffer";

const road = {
  type: "Feature",
  geometry: { type: "LineString", coordinates: [[74.0, 31.5], [75.0, 31.8]] },
};

const zone = buffer(road, 0.5, { units: "kilometers" });
// → GeoJSON Polygon representing a 500-m corridor

Set operations

Once you have buffered geometries, set operations let you combine them:

Operation	What it produces
Union	All area covered by either A or B
Intersection	Only the area covered by both A and B
Difference	Area of A that doesn't overlap B
Symmetric difference	Area covered by either but not both