Over-the-air distance calculator

This calculator connects with the Google Maps API in order to get geographic coordinates of two addresses and then calculate distance “as the crow flies” (in miles and kilometres)

The calculator uses Vincenty’s formula, which is extremely accurate and precise on the WGS-84 World Geodetic System.

Source code for this form:

<label>Starting address:
[address start]</label>

<label>Destination address:
[address finish]</label>

[calculate_button calculate_button-1 "Calculate"]
[calculation distance_m_direct cf7-hide "fn_php(
  $lat1 = deg2rad((float) $_POST{'start_lat'});
  $lat2 = deg2rad((float) $_POST{'finish_lat'});
  $lon1 = deg2rad((float) $_POST{'start_lng'});
  $lon2 = deg2rad((float) $_POST{'finish_lng'});
  // Equatorial Radius
  $a = 6378137.0;
  // Polar Radius
  $b = 6356752.31424518;
  // Flattening of the ellipsoid
  $f = 0.00335281066;
  // Difference in longitude
  $L = $lon2 - $lon1;  
  $U1 = atan((1 - $f) * tan($lat1));  //U is 'reduced latitude'
  $U2 = atan((1 - $f) * tan($lat2));
  $sinU1 = sin($U1);
  $sinU2 = sin($U2);
  $cosU1 = cos($U1);
  $cosU2 = cos($U2);

  $lambda = $L;
  $lambdaP = 2 * pi();
  $i = 20;

  while (abs($lambda - $lambdaP) > 1e-12 && --$i > 0) {
    $sinLambda = sin($lambda);
    $cosLambda = cos($lambda);
    $sinSigma = sqrt(($cosU2 * $sinLambda) * ($cosU2 * $sinLambda) + ($cosU1 * $sinU2 - $sinU1 * $cosU2 * $cosLambda) * ($cosU1 * $sinU2 - $sinU1 * $cosU2 * $cosLambda));

    if ($sinSigma == 0)
      return 0;  //co-incident points

    $cosSigma = $sinU1 * $sinU2 + $cosU1 * $cosU2 * $cosLambda;
    $sigma = atan2($sinSigma, $cosSigma);
    $sinAlpha = $cosU1 * $cosU2 * $sinLambda / $sinSigma;
    $cosSqAlpha = 1 - $sinAlpha * $sinAlpha;
    $cos2SigmaM = $cosSigma - 2 * $sinU1 * $sinU2 / $cosSqAlpha;
    if (is_nan($cos2SigmaM))
      $cos2SigmaM = 0;  //equatorial line: cosSqAlpha=0 (6)
    $c = $f / 16 * $cosSqAlpha * (4 + $f * (4 - 3 * $cosSqAlpha));
    $lambdaP = $lambda;
    $lambda = $L + (1 - $c) * $f * $sinAlpha * ($sigma + $c * $sinSigma * ($cos2SigmaM + $c * $cosSigma * (-1 + 2 * $cos2SigmaM * $cos2SigmaM)));
  }

  if ($i == 0)
    return 0;  //formula failed to converge

  $uSq = $cosSqAlpha * ($a * $a - $b * $b) / ($b * $b);
  $A = 1 + $uSq / 16384 * (4096 + $uSq * (-768 + $uSq * (320 - 175 * $uSq)));
  $B = $uSq / 1024 * (256 + $uSq * (-128 + $uSq * (74 - 47 * $uSq)));
  $deltaSigma = $B * $sinSigma * ($cos2SigmaM + $B / 4 * ($cosSigma * (-1 + 2 * $cos2SigmaM * $cos2SigmaM) - $B / 6 * $cos2SigmaM * (-3 + 4 * $sinSigma * $sinSigma) * (-3 + 4 * $cos2SigmaM * $cos2SigmaM)));
  $d = $b * $A * ($sigma - $deltaSigma);
  return $d;
)"]
<label>Distance "as the crow flies":</label>
[calculation distance_km_direct precision:2 "distance_m_direct / 1000"] km</label>
[calculation distance_mi_direct precision:2 "distance_m_direct / 1609.34"] miles
[calculation distance_nm_direct precision:2 "distance_m_direct / 1852"] nautical miles