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 exampl

Answer 1

Relating to kobers answer I worked it out with more readable clean code and changed the longitudes that crosses the date border:

public bool IsPointInPolygon(List polygon, double latitude, double longitude)
{
  bool isInIntersection = false;
  int actualPointIndex = 0;
  int pointIndexBeforeActual = polygon.Count - 1;

  var offset = calculateLonOffsetFromDateLine(polygon);
  longitude = longitude < 0.0 ? longitude + offset : longitude;

  foreach (var actualPointPosition in polygon)
  {
    var p1Lat = actualPointPosition.Latitude;
    var p1Lon = actualPointPosition.Longitude;

    var p0Lat = polygon[pointIndexBeforeActual].Latitude;
    var p0Lon = polygon[pointIndexBeforeActual].Longitude;

    if (p1Lon < 0.0) p1Lon += offset;
    if (p0Lon < 0.0) p0Lon += offset;

    // Jordan curve theorem - odd even rule algorithm
    if (isPointLatitudeBetweenPolyLine(p0Lat, p1Lat, latitude)
    && isPointRightFromPolyLine(p0Lat, p0Lon, p1Lat, p1Lon, latitude, longitude))
    {
      isInIntersection = !isInIntersection;
    }

    pointIndexBeforeActual = actualPointIndex;
    actualPointIndex++;
  }

  return isInIntersection;
}

private double calculateLonOffsetFromDateLine(List polygon)
{
  double offset = 0.0;
  var maxLonPoly = polygon.Max(x => x.Longitude);
  var minLonPoly = polygon.Min(x => x.Longitude);
  if (Math.Abs(minLonPoly - maxLonPoly) > 180)
  {
    offset = 360.0;
  }

  return offset;
}

private bool isPointLatitudeBetweenPolyLine(double polyLinePoint1Lat, double polyLinePoint2Lat, double poiLat)
{
  return polyLinePoint2Lat <= poiLat && poiLat < polyLinePoint1Lat || polyLinePoint1Lat <= poiLat && poiLat < polyLinePoint2Lat;
}

private bool isPointRightFromPolyLine(double polyLinePoint1Lat, double polyLinePoint1Lon, double polyLinePoint2Lat, double polyLinePoint2Lon, double poiLat, double poiLon)
{
  // lon <(lon1-lon2)*(latp-lat2)/(lat1-lat2)+lon2
  return poiLon < (polyLinePoint1Lon - polyLinePoint2Lon) * (poiLat - polyLinePoint2Lat) / (polyLinePoint1Lat - polyLinePoint2Lat) + polyLinePoint2Lon;
}