Geolocation watchPosition() API

The HTML Geolocation watchPosition() method retrieves periodic updates about the current geographic location of the device. The location is expressed as a set of geographic coordinates together with information about heading and speed.

The location information is returned in a Position object. Each update returns a new Position object.

Syntax

The syntax of HTML Geolocation watchPosition() method is as follows −

watchPosition(showLocation, ErrorHandler, options);

Parameters

The HTML Geolocation watchPosition() method accepts three parameters −

  • showLocation − This specifies the callback method that retrieves the location information. This method is called asynchronously with an object corresponding to the Position object which stores the returned location information.

  • ErrorHandler − This optional parameter specifies the callback method that is invoked when an error occurs in processing the asynchronous call. This method is called with the PositionError object that stores the returned error information.

  • options − This optional parameter specifies a set of options for retrieving the location information. You can specify (a) Accuracy of the returned location information (b) Timeout for retrieving the location information and (c) Use of cached location information.

Return value

The watchPosition() method returns a unique transaction ID (number) associated with the asynchronous call. Use this ID to cancel the watchPosition call and to stop receiving location updates.

Example

In the code below, we are using the watchPosition() method to automatically update the location of user's device.

<!DOCTYPE HTML>

<head>
   <html>
   
      <script type = "text/javascript">
         var watchID;
         var geoLoc;
         
         function showLocation(position) {
            var latitude = position.coords.latitude;
            var longitude = position.coords.longitude;
            alert("Latitude : " + latitude + " Longitude: " + longitude);
         }
         
         function errorHandler(err) {
            if(err.code == 1) {
               alert("Error: Access is denied!");
            } else if( err.code == 2) {
               alert("Error: Position is unavailable!");
            }
         }
         
         function getLocationUpdate(){
            
            if(navigator.geolocation){
               
               // timeout at 60000 milliseconds (60 seconds)
               var options = {timeout:60000};
               geoLoc = navigator.geolocation;
               watchID = geoLoc.watchPosition(showLocation, errorHandler, options);
            } else {
               alert("Sorry, browser does not support geolocation!");
            }
         }
      </script>
   </head>
   <body>
   
      <form>
         <input type = "button" onclick = "getLocationUpdate();" value = "Watch Update"/>
      </form>
      
   </body>
</html>

On executing the above HTML code, a button named "Watch Update" is displayed, if you click on it, the Latitude and Longitude of the user's device will be prompted.

html_geolocation.htm
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