Jan 18, 2014

Getting and Displaying The Nearest N Places On a Map in ASP.NET

In my previous tutorial, we implemented to save Geolocation (latitude, longitude) in Geography data type column in Sql Server. Now, we will get the nearest N places from the current geo-location of user.

Environment: VS2012,.NET Framework 4.5, ASP.NET Empty Website template, C#, SQL Server v11.0, Entity Framework 5.0 Database First approach.

HTML Structure:

Add a new Web Form and reference jQuery in head tag

  <script src="http://ajax.aspnetcdn.com/ajax/jquery/jquery-1.9.0.js"></script>

Add following code

  <form id="form1" runat="server">
        <p id="message"></p>
        <asp:HiddenField ID="hdnLocation" runat="server" />
        <asp:Button ID="btnSubmit" runat="server" Text="Get 5 Nearest Places" OnClick="btnSubmit_Click" />
        <asp:GridView ID="GridView1" runat="server" CellPadding="3"></asp:GridView>
        <div id="map" style="width: 600px; height: 400px;"></div>

Hidden field is used to save current user geographical position similar to previous post. On submit, Gridview and Map will display the nearest 5 places.

To Get Current User Geo-Location:

We use same HTML5 Geo-Location API to get current user geographical position.

   $('[id*=btnSubmit]').prop('disabled', true);

        var currentLatLng;
        if (navigator.geolocation) {
            navigator.geolocation.getCurrentPosition(showPosition, showError);
        else { $("#message").html("Geolocation is not supported by this browser."); }

        function showPosition(position) {
            currentLatLng = position.coords;
            var latlon = "Latitude" + currentLatLng.latitude + "," + "Longitude" + currentLatLng.longitude;
            $("[id*=hdnLocation]").val(currentLatLng.longitude + " " + currentLatLng.latitude);

        function showError(error) {
            if (error.code == 1) {
                $("#message").html("User denied the request for Geolocation.");
            else if (error.code == 2) {
                $("#message").html("Location information is unavailable.");
            else if (error.code == 3) {
                $("#message").html("The request to get user location timed out.");
            else {
                $("#message").html("An unknown error occurred.");

By default, submit button is disabled. It is enabled on getting current user geo-location.

Getting The Nearest Places:

First, I would recommend to read my previous post to get idea about database structure. I am using same scene here. The following code is to get the nearest 5 places:

  var currentLocation = DbGeography.FromText("POINT(" + hdnLocation.Value + ")"); 
        string[] latlng = hdnLocation.Value.Split(new char[]{' '}); 
        using ( var context = new SampleDBEntities()){

            var places = (from u in context.PlaceInfoes
                          orderby u.Geolocation.Distance(currentLocation)
                          select u).Take(5).Select(x => new { Name = x.Name, Address = x.Address, City = x.City, State = x.State, Latitude = x.Geolocation.Latitude, Longitude = x.Geolocation.Longitude });


Entity Framework 5 and above supports spatial type operations. Here we are doing order by distance from current location and take top 5 records in LINQ. EF5+ makes it very easy.

To Display Data In GridView:

  //Bind GridView
            GridView1.DataSource = places.ToList();

To Display Data In Map:

We serialize and assign marker points in javascript variable points from server side and on client side, this variable will be used to display markers

 //Set points for map
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            var output = serializer.Serialize(places);
            ClientScript.RegisterClientScriptBlock(GetType(), "points", "var points = " + output + ";var currentLoc = { 'Latitude' : " + latlng[1] + ", 'Longitude':"+latlng[0] +" }", true);

In above code, currentLoc variable is used to hold current geo-location of user.
on client side:

 var map2, infoWindow;

        if (typeof points !== "undefined") {
            var mapOptions = {
                center: new google.maps.LatLng(points[0].Latitude, points[0].Longitude),
                zoom: 9,
                mapTypeId: google.maps.MapTypeId.ROADMAP
            infoWindow = new google.maps.InfoWindow();
            map2 = new google.maps.Map(document.getElementById("map"), mapOptions);

            //marker for current location
            var marker = new google.maps.Marker({
                position: new google.maps.LatLng(currentLoc.Latitude, currentLoc.Longitude),
                map: map2,
                icon: 'https://maps.google.com/mapfiles/kml/shapes/schools_maps.png'

            //marker for nearest places
            for (i = 0; i < points.length; i++) {
                var data = points[i]
                var myLatlng = new google.maps.LatLng(data.Latitude, data.Longitude);
                var marker2 = new google.maps.Marker({
                    position: myLatlng,
                    map: map2,
                    title: data.Name
                (function (marker2, data) {
                    google.maps.event.addListener(marker2, "click", function (e) {
                        infoWindow.open(map2, marker2);
                })(marker2, data);

In above javascript code, current location is defined with different marker icon.


nearest places geolocation


We saw the application of Sql server Geography data type and good support of Entity framework which makes few lines of code to get the nearest N places done.

Enjoy EF !!

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