Geolocation - HTML5 and JavaScript Web Apps - page 132

var geolocationID ;
( function getLocation () {
var count = 0 ;
geolocationID = window . setInterval (
function () {
count ++ ;
if ( count > 3 ) { //when count reaches a number, reset interval
window . clearInterval ( geolocationID );
getLocation ();
} else {
navigator .
geolocation . getCurrentPosition ( successCallback , errorCallback ,
{ enableHighAccuracy : true , timeout : 10000 });
}
},
600000 ); //end setInterval;
})();
Another issue with the specific WebKit Geolocation implementation, is that accessing
geolocation activates the Geolocation service, which currently blocks page caching
( https://bugs.webkit.org/show_bug.cgi?id=43956 ). If you simply check the geolocation
property, you can avoid this issue:
function supports_geolocation () {
try {
return 'geolocation' in navigator &&
navigator [ 'geolocation' ] !== null ;
} catch ( e ) {
return false ;
}
}
You can view a live demo with all implemented workarounds at http://html5e.org/exam
ple/geo .
A Practical Use Case: User Tracking
To track a user over a set of latitude and longitude coordinates, you can use the Haversine
formula. With it, your application can calculate the shortest distance over the Earth's
surface and provide an as-the-crow-flies distance between the points. The code you need
is:
function calculateDistance ( lat1 , lon1 , lat2 , lon2 ) {
var R = 6371 ; // km
var dLat = ( lat2 - lat1 ). toRad ();
var dLon = ( lon2 - lon1 ). toRad ();
var a = Math . sin ( dLat / 2 ) * Math . sin ( dLat / 2 ) +
Math . cos ( lat1 . toRad ()) * Math . cos ( lat2 . toRad ()) *
Math . sin ( dLon / 2 ) * Math . sin ( dLon / 2 );
var c = 2 * Math . atan2 ( Math . sqrt ( a ), Math . sqrt ( 1 - a ));
var d = R * c ;