问题
I've been playing around with the Google Maps/Google directions API. Does anyone have an inkling on how I might be able to find the mid-point along a route, not the geographic midpoint.
Ideally I'd like to find the lat and long values of this midpoint.
Any thoughts? I'm a bit stumped and hoping I may be able to find a suggestion without going crazy trying to find the answer myself.
回答1:
You can use the GetPointAtDistance prototype from Gisgraphy. The prototype returns a LatLng for a specified distance along a polyline. The following code:
- Define a polyline
- Determine the half length of this polyline
- Use the prototype to return the midpoint LatLng
- Extract Lat and Lng from the midpoint LatLng
var polyline = new google.maps.Polyline({
path: [ new google.maps.LatLng(..., ...),
new google.maps.LatLng(..., ...),
... ];
}), //1.
midDistanceLength = polyline.getPath().getLength() / 2, //2.
midDistanceLatLng = polyline.GetPointAtDistance(midDistanceLength),//3.
midDistanceLat = midDistanceLatLng.lat(), //4.
midDistanceLng = midDistanceLatLng.lng(); //4.
//The prototype from Gisgraphy:
google.maps.Polygon.prototype.GetPointAtDistance = function(metres) {
// some awkward special cases
if (metres == 0) return this.getPath().getAt(0);
if (metres < 0) return null;
if (this.getPath().getLength() < 2) return null;
var dist=0;
var olddist=0;
for (var i=1; (i < this.getPath().getLength() && dist < metres); i++) {
olddist = dist;
dist += google.maps.geometry.spherical.computeDistanceBetween (
this.getPath().getAt(i),
this.getPath().getAt(i-1)
);
}
if (dist < metres) return null;
var p1= this.getPath().getAt(i-2);
var p2= this.getPath().getAt(i-1);
var m = (metres-olddist)/(dist-olddist);
return new google.maps.LatLng( p1.lat() + (p2.lat()-p1.lat())*m, p1.lng() + (p2.lng()-p1.lng())*m);
}
google.maps.Polyline.prototype.GetPointAtDistance = google.maps.Polygon.prototype.GetPointAtDistance;
回答2:
An easiest way is that you can first:
1) Calculate total distance of the path using GMSGeometryDistance, by calculating the distance between every two subsequent points by GMSGeometryDistance function, then summing all the distance.
2) Then you calculate again, and summing in each step. When the sum is about half of the total distance, then you are at the middle point. Sample code is as following:
func findTotalDistanceOfPath(path: GMSPath) -> Double {
let numberOfCoords = path.count()
var totalDistance = 0.0
if numberOfCoords > 1 {
var index = 0 as UInt
while index < numberOfCoords{
//1.1 cal the next distance
var currentCoord = path.coordinateAtIndex(index)
var nextCoord = path.coordinateAtIndex(index + 1)
var newDistance = GMSGeometryDistance(currentCoord, nextCoord)
totalDistance = totalDistance + newDistance
index = index + 1
}
}
return totalDistance
}
func findMiddlePointInPath(path: GMSPath ,totalDistance distance:Double) -> CLLocationCoordinate2D? {
let numberOfCoords = path.count()
let halfDistance = distance/2
let threadhold = 10 //10 meters
var midDistance = 0.0
if numberOfCoords > 1 {
var index = 0 as UInt
while index < numberOfCoords{
//1.1 cal the next distance
var currentCoord = path.coordinateAtIndex(index)
var nextCoord = path.coordinateAtIndex(index + 1)
var newDistance = GMSGeometryDistance(currentCoord, nextCoord)
midDistance = midDistance + newDistance
if fabs(midDistance - halfDistance) < threadhold { //Found the middle point in route
return nextCoord
}
index = index + 1
}
}
return nil //Return nil if we cannot find middle point in path for some reason
}
There is more to optimize the function. I wrote a detail answer in Swift in here
来源：https://stackoverflow.com/questions/10694472/google-maps-api-midpoint-along-route