How to Determine if a Point Lies Within a Polygon Using PHP?

Finding Points Within a Polygon Using PHP

Problem:

Given a polygon defined by an array of latitude and longitude coordinates and a point (vertex) with known coordinates, determine whether the vertex lies within the polygon.

Solution:

To solve this problem, we utilize an adapted version of a polygon intersection algorithm:

<code class="php">function is_in_polygon($points_polygon, $vertices_x, $vertices_y, $longitude_x, $latitude_y)
{
  $i = $j = $c = 0;
  for ($i = 0, $j = $points_polygon ; $i < $points_polygon; $j = $i++) {
    if ( (($vertices_y[$i]  >  $latitude_y != ($vertices_y[$j] > $latitude_y)) &amp;&amp;
     ($longitude_x < ($vertices_x[$j] - $vertices_x[$i]) * ($latitude_y - $vertices_y[$i]) / ($vertices_y[$j] - $vertices_y[$i]) + $vertices_x[$i]) ) )
       $c = !$c;
  }
  return $c;
}</code>

This function iterates through the vertices of the polygon, calculating the intersection of lines formed by the vertex and subsequent vertices with a horizontal line passing through the point to be tested. If the intersection point lies to the left of the vertex for all lines, the point is outside the polygon; otherwise, it is inside.

Example:

<code class="php">$vertices_x = array(37.628134, 37.629867, 37.62324, 37.622424);
$vertices_y = array(-77.458334,-77.449021,-77.445416,-77.457819);
$points_polygon = count($vertices_x) - 1;
$longitude_x = $_GET["longitude"];
$latitude_y = $_GET["latitude"];

if (is_in_polygon($points_polygon, $vertices_x, $vertices_y, $longitude_x, $latitude_y)){
  echo "Is in polygon!";
}
else echo "Is not in polygon";</code>

Additional Notes:

For more complex operations involving polygons, consider using the polygon.php class available online.

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