Canadian Postal Codes Radius

Question

I am using the following scripting that I found on the net to grab all postal codes between a given set coordinates.

When using it my concern is that when some postal codes being grab are greater than the distance entered; not by much - about 20 KM off.

function GetPostalCodes($latitude, $longitude, $range) {
    $radius = 3959;
    $north = rad2deg(asin(sin(deg2rad($latitude)) * cos($range / $radius) + cos(deg2rad($latitude)) * sin($range / $radius) * cos(deg2rad(0))));
    $south = rad2deg(asin(sin(deg2rad($latitude)) * cos($range / $radius) + cos(deg2rad($latitude)) * sin($range / $radius) * cos(deg2rad(180))));
    $east = rad2deg(deg2rad($longitude) + atan2(sin(deg2rad(90)) * sin($range / $radius) * cos(deg2rad($latitude)), cos($range / $radius) - sin(deg2rad($latitude)) * sin(deg2rad($north))));
    $west = rad2deg(deg2rad($longitude) + atan2(sin(deg2rad(270)) * sin($range / $radius) * cos(deg2rad($latitude)), cos($range / $radius) - sin(deg2rad($latitude)) * sin(deg2rad($north))));
    $return = DBSelectAllArrays("SELECT postal FROM postalcodes WHERE (latitude <= $north AND latitude >= $south AND longitude <= $east AND longitude >= $west)");
    krsort($return);
    if (empty($return)) return false;
    return $return;
}

Is there something I am missing to get a more accurate result?

Answer 1

Given your comments:

$radius = 6371.0; // mean radius of Earth in km

This is taken from wikipedia, but I've seen it within a +/- 3km tolerance from other sources. I began to question whether you were using great circle distance calculations, but this is more important for accuracy over longer distances due to the curvature of the earths surface.

Answer 2

Tim, you started by using a bounding box (rectangle) and then with the Haversine formula, you'll get a radius (circle), which generally is much better if you just want people within a certain distance. You don't state your purpose, but if you're looking for people who may travel a certain distance to you, you may want to consider metropolitan areas, which vary in shape. If so, look at: Canadian Metro Areas data