Point in polygon
Testing whether a point lies inside a shape, often by counting how many edges a ray from the point crosses: odd means inside.
Deciding whether a delivery address is inside a zone is the point-in-polygon test. The classic method casts a ray: draw a line from the point straight east, and count how many of the polygon's edges it crosses. An odd count means inside (you crossed in and never out), even means outside. For small zones you can treat longitude and latitude as plain x and y, because the shape distortion is the same for the point and the edges.
Try it now
Most location questions are one of four: how far, what's nearest, which zone, or which way around.
Learn it properly
Location data · Haversine distances, nearest places, GeoJSON, point in polygon and simple maps. About 40 minutes.
Other terms from the same lesson
- GeoJSON: A JSON format for points, lines and polygons on a map, which writes every position as [longitude, latitude].
- great circle: A circle around a sphere's middle, like the equator; the shortest route between two points on Earth runs along one.
- haversine formula: The formula for the great-circle distance between two points on a sphere from their latitudes and longitudes.
- spatial index: A data structure that finds nearby points or shapes without measuring every one, so location queries stay fast at scale.
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