C# 计算地图上某个坐标点的是否在多边形内

作者：vkvi 来源：ITPOW（原创）日期：2019-6-15
判断是否在多边形内，原理是画条射线，看射线与多边形相交的点是奇数还是偶数，看本代码是我从网上抄的，并没有使用和检查过，原文见：https://www.cnblogs.com/yushuo/p/9304471.html。
public class location
    {
        public double lat;
        public double lng;
    }

    public class GpsPolygonHelper
    {
        /// <summary>
        /// 坐标点是否在多边形内判断
        /// </summary>
        /// <param name="point"></param>
        /// <param name="pts"></param>
        /// <returns></returns>
        public static bool isPointInPolygon(location point, List<location> pts)
        {

            //检查类型
            if (point == null || pts == null)
                return false;

            var N = pts.Count;
            var boundOrVertex = true; //如果点位于多边形的顶点或边上，也算做点在多边形内，直接返回true
            var intersectCount = 0; //cross points count of x 
            var precision = 2e-10; //浮点类型计算时候与0比较时候的容差
            location p1, p2; //neighbour bound vertices
            var p = point; //测试点
            p1 = pts[0]; //left vertex        
            for (var i = 1; i <= N; ++i)
            {
//check all rays            
                if (p.lat.Equals(p1.lat) && p.lng.Equals(p1.lng))
                {
                    return boundOrVertex; //p is an vertex
                }

                p2 = pts[i % N]; //right vertex            
                if (p.lat < Math.Min(p1.lat, p2.lat) || p.lat > Math.Max(p1.lat, p2.lat))
                {
//ray is outside of our interests                
                    p1 = p2;
                    continue; //next ray left point
                }

                if (p.lat > Math.Min(p1.lat, p2.lat) && p.lat < Math.Max(p1.lat, p2.lat))
                {
//ray is crossing over by the algorithm (common part of)
                    if (p.lng <= Math.Max(p1.lng, p2.lng))
                    {
//x is before of ray                    
                        if (p1.lat == p2.lat && p.lng >= Math.Min(p1.lng, p2.lng))
                        {
//overlies on a horizontal ray
                            return boundOrVertex;
                        }

                        if (p1.lng == p2.lng)
                        {
//ray is vertical                        
                            if (p1.lng == p.lng)
                            {
//overlies on a vertical ray
                                return boundOrVertex;
                            }
                            else
                            {
//before ray
                                ++intersectCount;
                            }
                        }
                        else
                        {
//cross point on the left side                        
                            var xinters =
                                (p.lat - p1.lat) * (p2.lng - p1.lng) / (p2.lat - p1.lat) +
                                p1.lng; //cross point of lng                        
                            if (Math.Abs(p.lng - xinters) < precision)
                            {
//overlies on a ray
                                return boundOrVertex;
                            }

                            if (p.lng < xinters)
                            {
//before ray
                                ++intersectCount;
                            }
                        }
                    }
                }
                else
                {
//special case when ray is crossing through the vertex                
                    if (p.lat == p2.lat && p.lng <= p2.lng)
                    {
//p crossing over p2                    
                        var p3 = pts[(i + 1) % N]; //next vertex                    
                        if (p.lat >= Math.Min(p1.lat, p3.lat) && p.lat <= Math.Max(p1.lat, p3.lat))
                        {
//p.lat lies between p1.lat & p3.lat
                            ++intersectCount;
                        }
                        else
                        {
                            intersectCount += 2;
                        }
                    }
                }
                p1 = p2; //next ray left point
            }

            if (intersectCount % 2 == 0)
            {
//偶数在多边形外
                return false;
            }
            else
            {
                //奇数在多边形内
                return true;
            }

        }