Point in Polygon algorithm giving wrong results sometimes [closed]

Have been there 🙂 I also travelled through Stackoverflow’s PiP-suggestions, including your reference and this thread. Unfortunately, none of the suggestions (at least those I tried) were flawless and sufficient for a real-life scenario: like users plotting complex polygons on a Google map in freehand, “vicious” right vs left issues, negative numbers and so on.

The PiP-algorithm must work in all cases, even if the polygon consists of hundreds of thousands of points (like a county-border, nature park and so on) – no matter how “crazy” the polygon is.

So I ended up building a new algorithm, based on some source from an astronomy-app:

//Point class, storage of lat/long-pairs
class Point {
    public $lat;
    public $long;
    function Point($lat, $long) {
        $this->lat = $lat;
        $this->long = $long;
    }
}

//the Point in Polygon function
function pointInPolygon($p, $polygon) {
    //if you operates with (hundred)thousands of points
    set_time_limit(60);
    $c = 0;
    $p1 = $polygon[0];
    $n = count($polygon);

    for ($i=1; $i<=$n; $i++) {
        $p2 = $polygon[$i % $n];
        if ($p->long > min($p1->long, $p2->long)
            && $p->long <= max($p1->long, $p2->long)
            && $p->lat <= max($p1->lat, $p2->lat)
            && $p1->long != $p2->long) {
                $xinters = ($p->long - $p1->long) * ($p2->lat - $p1->lat) / ($p2->long - $p1->long) + $p1->lat;
                if ($p1->lat == $p2->lat || $p->lat <= $xinters) {
                    $c++;
                }
        }
        $p1 = $p2;
    }
    // if the number of edges we passed through is even, then it's not in the poly.
    return $c%2!=0;
}

Illustrative test :

$polygon = array(
    new Point(1,1), 
    new Point(1,4),
    new Point(4,4),
    new Point(4,1)
);

function test($lat, $long) {
    global $polygon;
    $ll=$lat.','.$long;
    echo (pointInPolygon(new Point($lat,$long), $polygon)) ? $ll .' is inside polygon<br>' : $ll.' is outside<br>';
}

test(2, 2);
test(1, 1);
test(1.5333, 2.3434);
test(400, -100);
test(1.01, 1.01);

Outputs :

2,2 is inside polygon 
1,1 is outside
1.5333,2.3434 is inside polygon 
400,-100 is outside
1.01,1.01 is inside polygon

It is now more than a year since I switched to the above algorithm on several sites. Unlike the “SO-algorithms” there have not been any complaints so far. See it in action here (national mycological database, sorry for the Danish). You can plot a polygon, or select a “kommune” (a county) – ultimately compare a polygon with thousands of points to thousands of records).

Update
Note, this algorithm is targeting geodata / lat,lngs which can be very precise (n’th decimal), therefore considering “in polygon” as inside polygon – not on border of polygon. 1,1 is considered outside, since it is on the border. 1.0000000001,1.01 is not.