MKGeodesicPolyline
We knew that the Earth was not flat long before 1492. Early navigators observed the way ships would dip out of view over the horizon many centuries before the Age of Discovery.
For many iOS developers, though, a flatMKMap was a necessary conceit until recently.
What changed? The discovery ofMKGeodesic, which is the subject of this week’s article.
MKGeodesic was introduced to the Map Kit framework in iOS 7. As its name implies, it creates ageodesic—essentially a straight line over a curved surface.
On the surface of asphereoblate spheroidgeoid, the shortest distance between two points appears as an arc on a flat projection. Over large distances, this takes apronounced, circular shape.
AnMKGeodesic is created with an array of 2MKMaps orCLLocations:
Creating anMKGeodesic
letLAX=CLLocation(latitude:33.9424955,longitude:-118.4080684)letJFK=CLLocation(latitude:40.6397511,longitude:-73.7789256)varcoordinates=[LAX.coordinate,JFK.coordinate]letgeodesicCLLocation*LAX=[[CLLocationalloc]initAlthough the overlay looks like a smooth curve, it is actually comprised of thousands of tiny line segments (true to itsMKPolyline lineage):
print(geodesicNSLog(@"%d",geodesicLike any object conforming to theMKOverlay protocol, anMKGeodesic instance is displayed by adding it to anMKMap withadd and implementingmap:
RenderingMKGeodesic on anMKMap
// MARK: MKMap#pragma mark - MKMap
For comparison, here’s the same geodesic overlaid with a route created from
MKDirections:
As the crow flies, it’s 3,983 km.
As the wolf runs, it’s 4,559 km—nearly 15% longer.
…and that’s just distance; taking into account average travel speed, the total time is ~5 hours by air and 40+ hours by land.
Animating anMKAnnotation on aMKGeodesic
Since geodesics make reasonable approximations for flight paths, a common use case would be to animate the trajectory of a flight over time.
To do this, we’ll make properties for our map view and geodesic polyline between LAX and JFK, and add new properties for theplane andplane (the index of the current map point for the polyline):
// MARK: Flight Path Propertiesvarmap@interfaceMapNext, right below the initialization of our map view and polyline, we create anMKPoint for our plane:
letannotation=MKPointself.planeThat call toupdate in the last line ticks the animation and updates the position of the plane:
funcupdate-(void)updateWe’ll perform this method roughly 30 times a second, until the plane has arrived at its final destination.
Finally, we implementmap to have the annotation render on the map view:
funcmap-(MKAnnotation
Hmm… close but noSkyMall Personalized Cigar Case Flask.
Let’s update the rotation of the plane as it moves across its flightpath.
Rotating anMKAnnotation along a Path
To calculate the plane’s direction, we’ll take the slope from the previous and next points:
letpreviousMKMapdirection is a function that returns aCLLocation (0 – 360 degrees, where North = 0) given twoMKMaps.
We calculate from
MKMaps rather than converted coordinates, because we’re interested in the slope of the line on the flat projection.Point
privatefuncdirectionstaticCLLocationThat convenience functionradians (and its partner,degrees) are simply:
privatefuncradiansstaticinlinedoubleXXRadiansThat direction is stored in a new property,var plane, calculated fromself.plane inupdate (ideally renamed toupdate with this addition). To make the annotation rotate, we apply atransform onannotation:
annotationself.annotation
Ah much better! At last, we have mastered the skies with a fancy visualization, worthy of any travel-related app.
Perhaps more than any other system framework, MapKit has managed to get incrementally better, little by little with every iOS release[1][2]. For anyone with a touch-and-go relationship to the framework, returning after a few releases is a delightful experience of discovery and rediscovery.
I look forward to seeing what lies on the horizon with iOS 8 and beyond.