Raycasting algorithm with GPS coordinates

Question

I\'m making a small application with Google maps that determines if an entered address is part of a predefined service region.

The user enters an address, and a PHP

Answer 1

Assuming that the $points array contains the corners of a polygon describing the coverage area in clockwise (or counterclockwise order), your code looks correct to me. Basically, it's counting the number of polygon edges that intersect a line drawn due east from the given point to the 180th meridian.

I'd perhaps rewrite it like this, just for clarity:

$p0 = end($points);
foreach ( $points as $p1 ) {
    // ignore edges of constant latitude (yes, this is correct!)
    if ( $p0->lat != $p1->lat ) {
        // scale latitude of $test_point so that $p0 maps to 0 and $p1 to 1:
        $interp = ($test_point->lat - $p0->lat) / ($p1->lat - $p0->lat);
        // does the edge intersect the latitude of $test_point?
        // (note: use >= and < to avoid double-counting exact endpoint hits)
        if ( $interp >= 0 && $interp < 1 ) {
            // longitude of the edge at the latitude of the test point:
            // (could use fancy spherical interpolation here, but for small
            // regions linear interpolation should be fine)
            $long = $interp * $p1->long + (1 - $interp) * $p0->long;
            // is the intersection east of the test point?
            if ( $long < $test_point->long ) {
                // if so, count it:
                $ctr++;
            }
        }
    }
    $p0 = $p1;
}

Note that this code will break in all sorts of interesting ways if the region boundary crosses the 180th meridian, so please don't use it if you have any service regions in the middle of the Pacific Ocean.

If you're still having problems, try plotting the polygon described by the $points array on a map; you may find out that it doesn't look like you thought it would, e.g. if some points are listed in the wrong order.