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

I translated c# method in Php and I added many comments to understand code.

Description of PolygonHelps:
Check if a point is inside or outside of a polygon. This procedure uses gps coordinates and it works when polygon has a little geographic area.


INPUT:
$poly: array of Point: polygon vertices list; [{Point}, {Point}, ...];
$point: point to check; Point: {"lat" => "x.xxx", "lng" => "y.yyy"}


When $c is false, the number of intersections with polygon is even, so the point is outside of polygon;
When $c is true, the number of intersections with polygon is odd, so the point is inside of polygon;
$n is the number of vertices in polygon;
For each vertex in polygon, method calculates line through current vertex and previous vertex and check if the two lines have an intersection point.
$c changes when intersection point exists.
So, method can return true if point is inside the polygon, else return false.

class PolygonHelps {

    public static function isPointInPolygon(&$poly, $point){

        $c = false; 
        $n = $j = count($poly);


        for ($i = 0, $j = $n - 1; $i < $n; $j = $i++){

            if ( ( ( ( $poly[$i]->lat <= $point->lat ) && ( $point->lat < $poly[$j]->lat ) ) 
                || ( ( $poly[$j]->lat <= $point->lat ) && ( $point->lat < $poly[$i]->lat ) ) ) 

            && ( $point->lng <   ( $poly[$j]->lng - $poly[$i]->lng ) 
                               * ( $point->lat    - $poly[$i]->lat ) 
                               / ( $poly[$j]->lat - $poly[$i]->lat ) 
                               +   $poly[$i]->lng ) ){

                $c = !$c;
            }
        }

        return $c;
    }
}