C# 計算地圖上某個座標點的是否在多邊形內

這個方法引用自羣友的博客 https://www.xiaofengyu.com/?p=143

使用百度地圖的時候，經常會用到判斷一個點是否在一個多邊形的範圍內，該方法用到的是射線法，

經過修改Javascrpit的代碼過來的，射線法的意思就是從點出發和任意的一邊的交叉點數爲奇數則爲在改區域內，

參考文檔http://erich.realtimerendering.com/ptinpoly/

 

    public class location
    {
        public double lat;
        public double lng;
    }

    public class GpsPolygonHelper
    {
        /// <summary>
        /// 座標點是否在多邊形內判斷
        /// </summary>
        /// <param name="point"></param>
        /// <param name="pts"></param>
        /// <returns></returns>
        public static bool isPointInPolygon(location point, List<location> pts)
        {

            //檢查類型
            if (point == null || pts == null)
                return false;

            var N = pts.Count;
            var boundOrVertex = true; //若是點位於多邊形的頂點或邊上，也算作點在多邊形內，直接返回true
            var intersectCount = 0; //cross points count of x 
            var precision = 2e-10; //浮點類型計算時候與0比較時候的容差
            location p1, p2; //neighbour bound vertices
            var p = point; //測試點
            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)
            {
//偶數在多邊形外
                return false;
            }
            else
            {
                //奇數在多邊形內
                return true;
            }

        }