A CLLocation extension that adds a method to calculate the shortest distance between a location and an imaginary line between two other locations.

CLLocation.swift

extension CLLocation {
	
	/// Returns the shortest distance (measured in meters) from the current object's location to the imaginary line running between the two specified locations.
	///
	/// - Parameters:
	///   - start: The first location that makes up the imaginary line.
	///   - end: The second location that makes up the imaginary line.
	/// - Returns: The shortest distance (in meters) between the current object's location and the imaginary line.
	func distanceToLine(from start: CLLocation, to end: CLLocation) -> CLLocationDistance {
		let s0lat = degreesToRadians(coordinate.latitude)
		let s0lng = degreesToRadians(coordinate.longitude)
		let s1lat = degreesToRadians(start.coordinate.latitude)
		let s1lng = degreesToRadians(start.coordinate.longitude)
		let s2lat = degreesToRadians(end.coordinate.latitude)
		let s2lng = degreesToRadians(end.coordinate.longitude)
		let s2s1lat = s2lat - s1lat
		let s2s1lng = s2lng - s1lng
		let u = ((s0lat - s1lat) * s2s1lat + (s0lng - s1lng) * s2s1lng) / (s2s1lat * s2s1lat + s2s1lng * s2s1lng)
		if u <= 0.0 {
			return distance(from: start)
		} else if u >= 1.0 {
			return distance(from: end)
		} else {
			let sa = CLLocation(latitude: coordinate.latitude - start.coordinate.latitude,
								longitude: coordinate.longitude - start.coordinate.longitude)
			let sb = CLLocation(latitude: u * (end.coordinate.latitude - start.coordinate.latitude),
								longitude: u * (end.coordinate.longitude - start.coordinate.longitude))
			return sa.distance(from: sb)
		}
	}
	
	private func degreesToRadians(_ degrees: Double) -> Double { return degrees * .pi / 180.0 }
	
}