Android 地理围栏(多边形)

问题描述

如何从多个地理位置(经纬度值)创建多边形地理围栏.此外，如何跟踪用户是进入这个地理围栏区域还是在 android 上退出这个区域.

How to create Polygon Geofence from multiple geo locations(long,lat values) . Also how to track user is entering into this geofence region or exiting from this region on android.

推荐答案

地理围栏只是一个组成多边形的纬度/经度点的数组.获得纬度/经度点列表后，您可以使用点内多边形检查来查看某个位置是否在多边形内.

A geofence is simply an array of lat/long points that form a polygon. Once you have a list of lat/long points, you can use a point-inside-polygon check to see if a location is within the polygon.

这是我在自己的项目中用于对非常大的凹多边形(20K+ 顶点)执行多边形点检查的代码:

This is code I have used in my own projects to perform point-in-polygon checks for very large concave polygons (20K+ vertices):

public class PolygonTest
{
    class LatLng
    {
        double Latitude;
        double Longitude;

        LatLng(double lat, double lon)
        {
            Latitude = lat;
            Longitude = lon;
        }
    }

    bool PointIsInRegion(double x, double y, LatLng[] thePath)
    {
        int crossings = 0;

        LatLng point = new LatLng (x, y);
        int count = thePath.length;
        // for each edge
        for (var i=0; i < count; i++) 
        {
            var a = thePath [i];
            var j = i + 1;
            if (j >= count) 
            {
                j = 0;
            }
            var b = thePath [j];
            if (RayCrossesSegment(point, a, b)) 
            {
                crossings++;
            }
        }
        // odd number of crossings?
        return (crossings % 2 == 1);
    }

    bool RayCrossesSegment(LatLng point, LatLng a, LatLng b)
    {
        var px = point.Longitude;
        var py = point.Latitude;
        var ax = a.Longitude;
        var ay = a.Latitude;
        var bx = b.Longitude;
        var by = b.Latitude;
        if (ay > by)
        {
            ax = b.Longitude;
            ay = b.Latitude;
            bx = a.Longitude;
            by = a.Latitude;
        }
            // alter longitude to cater for 180 degree crossings
        if (px < 0) { px += 360; };
        if (ax < 0) { ax += 360; };
        if (bx < 0) { bx += 360; };

        if (py == ay || py == by) py += 0.00000001;
        if ((py > by || py < ay) || (px > Math.max(ax, bx))) return false;
        if (px < Math.min(ax, bx)) return true;

        var red = (ax != bx) ? ((by - ay) / (bx - ax)) : float.MAX_VALUE;
        var blue = (ax != px) ? ((py - ay) / (px - ax)) : float.MAX_VALUE;
        return (blue >= red);
    }
}

就程序流程而言，您需要后台服务进行位置更新，然后针对您的纬度/经度多边形数据执行此检查，以查看该位置是否在内部.

In terms of program flow, you will want a background service to do location updates and then perform this check against your lat/long polygon data to see if the location is inside.

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