Where are you? How to write a geosensitive application

Archive - Originally posted on "The Horse's Mouth" - 2013-09-18 19:46:55 - Graham Ellis

Where are you? A question a web server will want to ask of a browser or app client, especially on mobile devices. And modern browsers, through Javascript, can send that vital information to the server. It's really useful in a wide variety of geographic data feeds - I've been working on a prototype listed building locator at http://melksh.am/lb which will find you the 15 nearest listed buildings to where you're located, tell you their direction, and let you move North South East or West to find more. This prototype already lists no fewer that 29,000 listed buildings within 25 miles of Melksham - so it covers Bradford-on-Avon, Chippenham, Malmesbury, Devizes, Trowbridge and Bath to name but a few ... links are provided for each building to the English Heritage site too.

So - how would you write this geolocation into you code?

I have a simple page [here] which uses Javascript to ask the user to permit his location to be used (that's built in to modern browsers) then submits it up as a form. You'll find lots of sophisticated examples on line... I have tried to keep mine simple to show you what's happening.

1. When you load the page, it runs the Javascript:

<body onload='reportwhere()'>

2. The reportwere function runs the built in geolocation, passing the values it gets to two other functions which I've called nextpage and errhandler - the first if the user approves of his location being sent to the server, the second if he does not approve or geolocation isn't working:

  function reportwhere() {

    navigator.geolocation.getCurrentPosition(nextpage,errhandler);

  }

3. These two functions save the status code, and location if it's available, into variables which are passed in to another function which calls up the next page

  function nextpage(position) {

    var latitude = position.coords.latitude;

    var longitude = position.coords.longitude;

    post_to_url("http://www.wellho.net/twapp/railinfo.php",{'scode':0, 'lati':latitude, 'longi':longitude})

  }

  function errhandler(problem) {

    var whatswrong = problem.code;

    post_to_url("http://www.wellho.net/twapp/railinfo.php",{'scode':whatswrong})

  }

4. Here's that function that calls up the next page:

  function post_to_url(url, params) {

    var form = document.createElement('form');

    form.action = url;

    form.method = 'GET';

    for (var i in params) {

      if (params.hasOwnProperty(i)) {

        var input = document.createElement('input');

        input.type = 'hidden';

        input.name = i;

        input.value = params[i];

        form.appendChild(input);

      }

    }

    form.submit();

  }

Moving on, I've pointed that form at a short piece of PHP code (full source [here] which will report on which railway stations you're nearest to, indicating which are busiest and adding a link to live departure boards. Again this is a prototype...

Illustration - Melksham's Spa Houses, dating from 1815.

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Where are you? How to write a geosensitive application