转自：
http://www.cnblogs.com/gis_gps/archive/2009/02/12/1389071.html备用
一、背景:
如何判断一个指定的经纬度点是否落在一个多边形区域内？
二、实现代码(delphi)
Type
TMyPoint = packed record
X : double;
Y : double;
end;
{*------------------------------------------------------------------------------
判断指定的经纬度坐标点是否落在指定的多边形区域内
@param ALon 指定点的经度
@param ALat 指定点的纬度
@param APoints 指定多边形区域各个节点坐标
@return True 落在范围内 False 不在范围内
------------------------------------------------------------------------------*}
function IsPtInPoly(ALon, ALat: double; APoints: array of TMyPoint): Boolean;
var
iSum, iCount, iIndex: Integer;
dLon1, dLon2, dLat1, dLat2, dLon: double;
begin
Result := False;
if (Length(APoints) < 3) then
begin
Result := False;
Exit;
end;
iSum := 0;
iCount := Length(APoints);
for iIndex :=0 to iCount - 1 do
begin
if (iIndex = iCount - 1) then
begin
dLon1 := APoints[iIndex].X;
dLat1 := APoints[iIndex].Y;
dLon2 := APoints[0].X;
dLat2 := APoints[0].Y;
end
else
begin
dLon1 := APoints[iIndex].X;
dLat1 := APoints[iIndex].Y;
dLon2 := APoints[iIndex + 1].X;
dLat2 := APoints[iIndex + 1].Y;
end;
if ((ALat >= dLat1) and (ALat < dLat2)) or ((ALat>=dLat2) and (ALat < dLat1)) then
begin
if (abs(dLat1 - dLat2) > 0) then
begin
dLon := dLon1 - ((dLon1 -dLon2) * (dLat1 -ALat)) / (dLat1 - dLat2);
if (dLon < ALon) then
Inc(iSum);
end;
end;
end;
if (iSum mod 2 <> 0) then
Result := True;
end;
____________________________________________________________________________________________________________________________
C#算法：
/// <summary>
/// 判断指定的经纬度坐标点是否落在指定的多边形区域内
/// </summary>
/// <param name="ALon">指定点的经度</param>
/// <param name="ALat">指定点的纬度</param>
/// <param name="APoints">指定多边形区域各个节点坐标</param>
/// <returns>True 落在范围内 False 不在范围内</returns>
public bool isPtInPoly(double ALon, double ALat, Point[] APoints)
{
int iSum, iCount, iIndex;
double dLon1, dLon2, dLat1, dLat2, dLon;
if (APoints.Length < 3)
{
return false;
}
iSum = 0;
iCount = APoints.Length;
for (iIndex = 0; iIndex < iCount - 1; iIndex++)
{
if (iIndex == iCount - 1)
{
dLon1 = APoints[iIndex].X;
dLat1 = APoints[iIndex].Y;
dLon2 = APoints[0].X;
dLat2 = APoints[0].Y;
}
else
{
dLon1 = APoints[iIndex].X;
dLat1 = APoints[iIndex].Y;
dLon2 = APoints[iIndex + 1].X;
dLat2 = APoints[iIndex + 1].Y;
}
if (((ALat >= dLat1) && (ALat < dLat2)) || ((ALat >= dLat2) && (ALat < dLat1)))
{
if (Math.Abs(dLat1 - dLat2) > 0)
{
dLon = dLon1 - ((dLon1 - dLon2) * (dLat1 - ALat)) / (dLat1 - dLat2);
if (dLon < ALon)
iSum++;
}
}
}
if ((iSum % 2) != 0)
return true;
return false;
}