JSON - Error when collecting data from file using Javascript

I have a jQuery store locator file that collect data using JSON, but it keeps giving an error every time I try to load the page.

Uncaught TypeError: Cannot read property '0' of undefined

The line that shows the code is highlighted below

if(settings.jsonData == true){ dataType = "json"; }
    else{ dataType = "xml"; }

    $.ajax({
    type: "GET",
    url: settings.dataLocation,
    dataType: dataType,
    success: function(data) {

        //After the store locations file has been read successfully
        var i = 0;
    if(settings.jsonData == true)
        {
          //Process JSON
          $.each(data, function() {
            console.log(data);
            var name = this.locname;
            var lat = this.lat;
            var lng = this.lng;
            var address = this.address;
            var address2 = this.address2;
            var city = this.city;
            var postcode = this.postcode;
            var phone = this.phone;
            var web = this.web;
            web = web.replace("http://","");
            var hours1 = this.hours1;
            var hours2 = this.hours2;
            var hours3 = this.hours3;

            var distance = GeoCodeCalc.CalcDistance(orig_lat,orig_lng,lat,lng, GeoCodeCalc.EarthRadiusInMiles);

            //Create the array
            locationset[i] = new Array (distance, name, lat, lng, address, address2, city, postcode, phone, web, hours1, hours2, hours3);

            i++;
          });
        }
        else
        {
          //Process XML
          $(data).find('marker').each(function(){
            //Take the lat lng from the user, geocoded above
            var name = $(this).attr('name');
            var lat = $(this).attr('lat');
            var lng = $(this).attr('lng');
            var address = $(this).attr('address');
            var address2 = $(this).attr('address2');
            var city = $(this).attr('city');
            var postcode = $(this).attr('postcode');
            postcode = postcode.replace(" ","");
            var phone = $(this).attr('phone');
            var web = $(this).attr('web');
            web = web.replace("http://","");
            var hours1 = $(this).attr('hours1');
            var hours2 = $(this).attr('hours2');
            var hours3 = $(this).attr('hours3');

            var distance = GeoCodeCalc.CalcDistance(orig_lat,orig_lng,lat,lng, GeoCodeCalc.EarthRadiusInMiles);

            //Create the array
            locationset[i] = new Array (distance, name, lat, lng, address, address2, city, postcode, phone, web, hours1, hours2, hours3);

            i++;
          });
        }

      //Sort the multi-dimensional array numerically
      console.log(locationset);
      locationset.sort(function(a, b) {
        var x = a[0];
        var y = b[0];
        return ((x < y) ? -1 : ((x > y) ? 1 : 0));
      });

      //Check the closest marker
      **********if(locationset[0][0] > settings.distanceAlert)***********
      {
        alert("Unfortunately, our closest location is more than " + settings.distanceAlert + " miles away.");
      }

even if i try to console.log the locationset, it is blank

EDIT

This is my json code, don't think the header is correct at all

<?php

header('Content-Type: application/json');

$results = mysql_query("SELECT sl_store AS locname, sl_latitude AS lat, sl_longitude AS lng, sl_address AS address, sl_address2 AS address2, sl_city AS city, sl_zip AS postcode, sl_phone AS phone, sl_url AS web, sl_hours AS hours1 FROM `wp_store_locator`");
$rows = array();
while($r = mysql_fetch_assoc($results)) {
$rows[] = $r;
}
echo json_encode($rows);

?>