GeoJSON

Learning Focus

GeoJSON is the standard for representing geographic features (points, lines, polygons) as JSON. Used by mapping libraries, PostGIS, Mapbox, and web APIs.

What is GeoJSON?

GeoJSON (RFC 7946) is a JSON format for encoding geographic data structures:

point.geojson

{
  "type": "Feature",
  "geometry": {
    "type": "Point",
    "coordinates": [103.8198, 1.3521]
  },
  "properties": {
    "name": "Singapore",
    "population": 5900000
  }
}

Geometry Types

Type	Shape	Coordinates Format
Point	Single location	[longitude, latitude]
LineString	Path / route	[[lon,lat], [lon,lat], ...]
Polygon	Closed area	[[[lon,lat], ...]] (ring)
MultiPoint	Multiple points	Array of Point coordinates
MultiLineString	Multiple paths	Array of LineString coords
MultiPolygon	Multiple areas	Array of Polygon coords
GeometryCollection	Mixed geometries	Array of geometry objects

FeatureCollection

The most common GeoJSON format — wraps multiple features:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "geometry": { "type": "Point", "coordinates": [103.8198, 1.3521] },
      "properties": { "name": "Singapore" }
    },
    {
      "type": "Feature",
      "geometry": { "type": "Point", "coordinates": [100.5018, 13.7563] },
      "properties": { "name": "Bangkok" }
    }
  ]
}

Coordinate Order

GeoJSON uses [longitude, latitude] order — the opposite of most GPS systems that use latitude/longitude. This trips up many developers.

PostgreSQL PostGIS

-- Enable PostGIS
CREATE EXTENSION postgis;

CREATE TABLE locations (
    id   SERIAL PRIMARY KEY,
    name TEXT,
    geom GEOMETRY(Point, 4326)  -- 4326 = WGS 84 (standard GPS)
);

-- Insert a point
INSERT INTO locations (name, geom)
VALUES ('Singapore', ST_SetSRID(ST_MakePoint(103.8198, 1.3521), 4326));

-- Find locations within 100 km of a point
SELECT name FROM locations
WHERE ST_DWithin(
    geom::geography,
    ST_MakePoint(103.8198, 1.3521)::geography,
    100000  -- metres
);

-- Return as GeoJSON
SELECT ST_AsGeoJSON(geom) FROM locations;

Concept Map

Concept Flow

GeoJSON
  ├── Feature
  │     ├── geometry: Point / LineString / Polygon
  │     └── properties: any JSON object
  │           └── coordinates always [longitude, latitude]
  └── FeatureCollection
        └── Used by Mapbox, Leaflet, PostGIS, OpenStreetMap

Common Pitfalls

Pitfall	Consequence	Prevention
Lat/lon reversed in coordinates	Marker appears on wrong continent	Always use [longitude, latitude] (RFC 7946)
Missing type field	Library fails to parse	Every geometry and feature needs type
Integer coordinates for precision	Rounding to wrong location	Use float coordinates with enough decimals

What's Next

• Previous: Binary JSON Formats — Binary serialization alternatives.
• Next: Best Practices — Production JSON conventions.
• Section Overview — Return to the Advanced module index.