GeoJSON and geographic coordinates: longitude first, WGS 84 and the right-hand rule

What it is

GeoJSON (RFC 7946, cited) is a JSON format for geographic data. A position is an array of numbers whose first two elements are longitude and latitude, precisely in that order, in decimal degrees on the WGS 84 datum (the coordinate reference system urn:ogc:def:crs:OGC::CRS84); an optional third element is elevation in metres above or below the reference ellipsoid, and positions should not carry further elements. Geometry objects are Point, MultiPoint, LineString, MultiLineString, Polygon, MultiPolygon and GeometryCollection. A Feature has a geometry (or null when unlocated), a properties object and optionally an id; a FeatureCollection holds features. An optional bbox is [west, south, east, north]. Polygon rings are closed (first position equals the last) and must follow the right-hand rule: exterior rings counterclockwise, holes clockwise, although parsers should not reject the older orientation.

Why it matters

People say "latitude, longitude", several map libraries take that order, and swapping the two places a point on another continent without any error message. The datum matters as well: coordinates from another datum pasted in unchanged land in the wrong place by an amount that depends on the datum, invisible on a world map and fatal for a delivery route; a national grid in metres is not degrees at all.

How to apply

• Emit [lon, lat] and convert at the boundary from any lat/lon source; keep a test with a known place whose longitude and latitude differ clearly in sign or magnitude so that a swap fails the test.
• Keep coordinates as JSON numbers, not strings, with only the digits the measurement justifies; the RFC states that the digit count must not be interpreted as uncertainty, so record accuracy in properties when it matters.
• Cut geometries that cross the antimeridian into Multi* parts, as the RFC recommends, so that renderers do not draw a line around the world.
• Put application data in properties, keep id stable, and remember that a line between two positions is straight in longitude/latitude space, not a great-circle path; compute distances with a geodesic function, never with Pythagoras on degrees.
• In a database, store geometry in a spatial type with a declared reference system so that indexes and distance functions apply.

Pitfalls

Foreign members are allowed on GeoJSON objects, but the GeoJSON types themselves are not extensible, and the crs member of the 2008 draft is gone. A polygon whose exterior ring is listed second is a hole without an outside. Large collections should be produced and consumed feature by feature rather than parsed as one document.