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copied!<p>The formula you used, seems to be the <em>arccosine</em> instead of the <em>haversine</em> formula. The haversine formula is indeed more appropriate to calculate the distance on a sphere, because it is not prone to rounding errors with antipodal points.</p> <pre><code>/** * Calculates the great-circle distance between two points, with * the Haversine formula. * @param float $latitudeFrom Latitude of start point in [deg decimal] * @param float $longitudeFrom Longitude of start point in [deg decimal] * @param float $latitudeTo Latitude of target point in [deg decimal] * @param float $longitudeTo Longitude of target point in [deg decimal] * @param float $earthRadius Mean earth radius in [m] * @return float Distance between points in [m] (same as earthRadius) */ function haversineGreatCircleDistance( $latitudeFrom, $longitudeFrom, $latitudeTo, $longitudeTo, $earthRadius = 6371000) { // convert from degrees to radians $latFrom = deg2rad($latitudeFrom); $lonFrom = deg2rad($longitudeFrom); $latTo = deg2rad($latitudeTo); $lonTo = deg2rad($longitudeTo); $latDelta = $latTo - $latFrom; $lonDelta = $lonTo - $lonFrom; $angle = 2 * asin(sqrt(pow(sin($latDelta / 2), 2) + cos($latFrom) * cos($latTo) * pow(sin($lonDelta / 2), 2))); return $angle * $earthRadius; } </code></pre> <p>P.S. I couldn't find an error in your code, so is it just a typo that you wrote <code>$lat= 41.9133741000 $lat= 12.5203944000</code> ? Maybe you just calculated with $lat=12.5203944000 and $long=0 because you overwrote your $lat variable.</p> <p>Edit:</p> <p>Tested the code and it returned a correct result:</p> <pre><code>$center_lat = 41.8350; $center_lng = 12.470; $lat = 41.9133741000; $lng = 12.5203944000; // test with your arccosine formula $distance =( 6371 * acos((cos(deg2rad($center_lat)) ) * (cos(deg2rad($lat))) * (cos(deg2rad($lng) - deg2rad($center_lng)) )+ ((sin(deg2rad($center_lat))) * (sin(deg2rad($lat))))) ); print($distance); // prints 9.662174538188 // test with my haversine formula $distance = haversineGreatCircleDistance($center_lat, $center_lng, $lat, $lng, 6371); print($distance); // prints 9.6621745381693 </code></pre>

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