How to draw an arc on Google Maps in iOS?

Question

How to draw an arc between two coordinate points in Google Maps like in this image and same like facebook post in iOS ?

Answer 1

None of the answers mentioned is a full proof solution. For a few locations, it draws a circle instead of a polyline. To resolve this we will calculate bearing(degrees clockwise from true north) and if it is less than zero, swap the start and end location.

func createArc(
    startLocation: CLLocationCoordinate2D,
    endLocation: CLLocationCoordinate2D) -> GMSPolyline {

    var start = startLocation
    var end = endLocation

    if start.bearing(to: end) < 0.0 {
        start = endLocation
        end = startLocation
    }

    let angle = start.bearing(to: end) * Double.pi / 180.0
    let k = abs(0.3 * sin(angle))

    let path = GMSMutablePath()
    let d = GMSGeometryDistance(start, end)
    let h = GMSGeometryHeading(start, end)
    let p = GMSGeometryOffset(start, d * 0.5, h)
    let x = (1 - k * k) * d * 0.5 / (2 * k)
    let r = (1 + k * k) * d * 0.5 / (2 * k)
    let c = GMSGeometryOffset(p, x, h + 90.0)
    var h1 =  GMSGeometryHeading(c, start)
    var h2 = GMSGeometryHeading(c, end)

    if (h1 > 180) {
      h1 = h1 - 360
    }

    if (h2 > 180) {
      h2 = h2 - 360
    }

    let numpoints = 100.0
    let step = ((h2 - h1) / Double(numpoints))
    for i in stride(from: 0.0, to: numpoints, by: 1) {
      let pi = GMSGeometryOffset(c, r, h1 + i * step)
      path.add(pi)
    }
    path.add(end)

    let polyline = GMSPolyline(path: path)
    polyline.strokeWidth = 3.0
    polyline.spans = GMSStyleSpans(
      polyline.path!,
      [GMSStrokeStyle.solidColor(UIColor(hex: "#2962ff"))],
      [20, 20], .rhumb
    )
    return polyline
  }

The bearing is the direction in which a vertical line on the map points, measured in degrees clockwise from north.

func bearing(to point: CLLocationCoordinate2D) -> Double {
    func degreesToRadians(_ degrees: Double) -> Double { return degrees * Double.pi / 180.0 }
    func radiansToDegrees(_ radians: Double) -> Double { return radians * 180.0 / Double.pi }

    let lat1 = degreesToRadians(latitude)
    let lon1 = degreesToRadians(longitude)

    let lat2 = degreesToRadians(point.latitude);
    let lon2 = degreesToRadians(point.longitude);

    let dLon = lon2 - lon1;

    let y = sin(dLon) * cos(lat2);
    let x = cos(lat1) * sin(lat2) - sin(lat1) * cos(lat2) * cos(dLon);
    let radiansBearing = atan2(y, x);

    return radiansToDegrees(radiansBearing)
  }