Javascript ~ calculate distance by latitude longitude

function distance(lat1, lon1, lat2, lon2, unit) {
var radlat1 = Math.PI * lat1/180;
var radlat2 = Math.PI * lat2/180;
var radlon1 = Math.PI * lon1/180;
var radlon2 = Math.PI * lon2/180;
var theta = lon1-lon2;
var radtheta = Math.PI * theta/180;
var dist = Math.sin(radlat1) * Math.sin(radlat2) + Math.cos(radlat1) * Math.cos(radlat2) * Math.cos(radtheta);
dist = Math.acos(dist);
dist = dist * 180/Math.PI;
dist = dist * 60 * 1.1515;
if (unit=="K") { dist = dist * 1.609344 }
if (unit=="N") { dist = dist * 0.8684 }
return dist;
}

example
distance(24.983179467184524, 121.45857810974121, 24.75337880339563, 121.24134063720703, 'K');

return
33.6620849036904 (公里)


'M' is statute miles
'K' is kilometers (default)
'N' is nautical miles


source
http://www.zipcodeworld.com/samples/distance.js.html