Google Maps, Ajax XML data

I am learning to access data from an XML file and place on a Google map but keep getting the error: 'TypeError: xml is undefined http://ift.tt/12uaPjV Line 138'

Can anyone see why this error is occurring?

All code is on this example

var map;

    var Data = [];                           // this array stores all data           
    var viewportMarkersKS2 = [];
    var viewportMarkersP16 = [];
    var viewportMarkersHealthcare = [];
    var KS2MarkersVisible = document.getElementById('Checkbox1');
    var P16MarkersVisible = document.getElementById('Checkbox2');
    var HealthcareMarkersVisible = document.getElementById('Checkbox3');
    var infoWindow;
    var handle1, handle2;
    var iconBlue = 'icons/blue-dot.png';
    var iconGreen = 'icons/green-dot.png';
    var iconPink = 'icons/pink-dot.png';
    //var zoomLevel = map.getZoom();

    // property and map center
    var iniLat = 51.401997;
    var iniLon = -1.276934;
    var iniZoom = 15;
    var ResetZoom = 13;
    var maxZoom = 12;
    var propertyLatLng = new google.maps.LatLng(iniLat, iniLon);

    function initialize() {

        // change map size dependent on useragent
        detectBrowser()

        // map options
        var mapOptions = {
            center: propertyLatLng,
            zoom: iniZoom
        };

        map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);




        //********************************************************************************************************************
        //********************************************************************************************************************
        //**********************************************                         *********************************************
        //**********************************************    SELECTED PROPERTY    *********************************************
        //**********************************************                         *********************************************
        //********************************************************************************************************************
        //********************************************************************************************************************

        // add the property to the map
        var propertyIcon = new google.maps.Marker({
            position: propertyLatLng
        });




        // infoWindows
        var infoWindowContent = '11 Derwent Road<br />Thatcham<br />Berkshire<br />RG19 3UT';

        var infowindow = new google.maps.InfoWindow({
            content: infoWindowContent
        });

        google.maps.event.addListener(propertyIcon, 'click', function () {
            infowindow.open(map, propertyIcon);
        });


        // add markers to the map
        propertyIcon.setMap(map);





        // when user drags the map or zooms in/out, update list of airports in range
        google.maps.event.addListenerOnce(map, 'bounds_changed', function () { showGetResultXml() });
        google.maps.event.addListener(map, 'dragend', function () { showGetResultXml() });
        google.maps.event.addListener(map, 'zoom_changed', function () { showGetResultXml() });


    }





    function showGetResultXml() {
        var result = null;
        var scriptUrl = "http://ift.tt/1zQEzBF";
        $.ajax(
        {
            url: scriptUrl,
            type: 'get',
            dataType: 'xml',
            async: false,
            success: function(data) {
                result = data;
                var xml = data.responseXML;
                var markers = xml.documentElement.getElementsByTagName("Row");

                for (var i = 0; i < markers.length; i++) {
                    var name = markers[i].getAttribute("SchoolName");
                    var address = markers[i].getAttribute("Address1");
                    var type = markers[i].getAttribute("type");
                    var point = new google.maps.LatLng(parseFloat(markers[i].getAttribute("Coordinates")));

                    var html = "<b>" + name + "</b> <br/>" + address;
                    var icon = customIcons[type] || {};

                    var marker = new google.maps.Marker(
                    {
                        map: map,
                        position: point,
                        icon: iconBlue
                    });

                    bindInfoWindow(marker, map, infoWindow, html);
                }
            },
            error: function onXmlError() {
                alert("An Error has occurred.");
            }
        });







        function bindInfoWindow(marker, map, infoWindow, html) {
            google.maps.event.addListener(marker, 'click', function () {
                infoWindow.setContent(html);
                infoWindow.open(map, marker);
            });
        }

        return result;
    }



    function detectBrowser() {
        var useragent = navigator.userAgent;
        var mapdiv = document.getElementById("map-canvas");

        if (useragent.indexOf('iPhone') != -1 || useragent.indexOf('Android') != -1) {
            mapdiv.style.width = '100%';
            mapdiv.style.height = '100%';
        } else {
            mapdiv.style.width = '100%';
        }
    }





    google.maps.event.addDomListener(window, 'load', initialize);

Thanks for any help.