Geo Fencing - point inside/outside polygon

Question

I would like to determine a polygon and implement an algorithm which would check if a point is inside or outside the polygon.

Does anyone know if there is any example available of any similar algorithm?

Answer 1

Just a heads up (using answer as I can't comment), if you want to use point-in-polygon for geo fencing, then you need to change your algorithm to work with spherical coordinates. -180 longitude is the same as 180 longitude and point-in-polygon will break in such situation.

Answer 2

You can try this simple class https://github.com/xopbatgh/sb-polygon-pointer

It is easy to deal with it

1. You just insert polygon coordinates into array
2. Ask library is desired point with lat/lng inside the polygon

$polygonBox = [
    [55.761515, 37.600375],
    [55.759428, 37.651156],
    [55.737112, 37.649566],
    [55.737649, 37.597301],
];

$sbPolygonEngine = new sbPolygonEngine($polygonBox);

$isCrosses = $sbPolygonEngine->isCrossesWith(55.746768, 37.625605);

// $isCrosses is boolean

(answer was returned from deleted by myself because initially it was formatted wrong)

Answer 3

I think there is a simpler and more efficient solution.

Here is the code in C++. I should be simple to convert it to C#.

int pnpoly(int npol, float *xp, float *yp, float x, float y)
{
  int i, j, c = 0;
  for (i = 0, j = npol-1; i < npol; j = i++) {
    if ((((yp[i] <= y) && (y < yp[j])) ||
         ((yp[j] <= y) && (y < yp[i]))) &&
        (x < (xp[j] - xp[i]) * (y - yp[i]) / (yp[j] - yp[i]) + xp[i]))
      c = !c;
  }
  return c;
}

Answer 4

The complete solution in asp.Net C#, you can see the complete detail here, you can see how to find point(lat,lon) whether its inside or Outside the Polygon using the latitude and longitudes ? Article Reference Link

private static bool checkPointExistsInGeofencePolygon(string latlnglist, string lat, string lng) {

    List<Loc> objList = new List<Loc>();
    // sample string should be like this strlatlng = "39.11495,-76.873259|39.114588,-76.872808|39.112921,-76.870373|";
    string[] arr = latlnglist.Split('|');
    for (int i = 0; i <= arr.Length - 1; i++)
    {
        string latlng = arr[i];
        string[] arrlatlng = latlng.Split(',');

        Loc er = new Loc(Convert.ToDouble(arrlatlng[0]), Convert.ToDouble(arrlatlng[1]));
        objList.Add(er);
    }
    Loc pt = new Loc(Convert.ToDouble(lat), Convert.ToDouble(lng));

    if (IsPointInPolygon(objList, pt) == true)
    {
          return true;
    }
    else
    {
           return false;
    }
}
private static bool IsPointInPolygon(List<Loc> poly, Loc point)
{
    int i, j;
    bool c = false;
    for (i = 0, j = poly.Count - 1; i < poly.Count; j = i++)
    {
        if ((((poly[i].Lt <= point.Lt) && (point.Lt < poly[j].Lt)) |
            ((poly[j].Lt <= point.Lt) && (point.Lt < poly[i].Lt))) &&
            (point.Lg < (poly[j].Lg - poly[i].Lg) * (point.Lt - poly[i].Lt) / (poly[j].Lt - poly[i].Lt) + poly[i].Lg))
            c = !c;
    }
    return c;
}