How Do I Get Latitude And Longitude Of A Street Address From Where User Has Logged In

<form id="form1" runat="server">
   <div>
       <p id="demo">get location</p>
       <a href ="#" id="get_location">Get Location</a>
       <div id="map">
           <iframe id="google_map" width="700" height="350" src="https://maps.google.com.np?output=embed"></iframe>
       </div>
       <script>
           var x = document.getElementById("demo");

           var c = function (pos) {
               var latitude = pos.coords.latitude;

               var longitude = pos.coords.longitude;
              coords = latitude + ',' + longitude;
               var locality = pos.coords.locality;

                document.getElementById('google_map').setAttribute('src', 'https://maps.google.com.np/?q=' + coords + '&z=15&output=embed');
                x.innerHTML = "lat:" + latitude + "<br /> long: " + longitude + "<br /> locality:" + locality;
           }


               document.getElementById('get_location').onclick = function () {

                   if (navigator.geolocation) {
                       navigator.geolocation.getCurrentPosition(c);
                       return false;
                   } else {
                       alert("Geo Location is not supported on your current browser!");
                   }

       }
       </script>
   </div>
       <asp:HiddenField ID="HiddenField1" runat="server" />
       <asp:HiddenField ID="HiddenField2" runat="server" />
       <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click1"  OnClientClick="ac()"  />
       <asp:TextBox ID="HiddenOffsetBox" runat="server" Visible="true" Text ="<%# latitude  %>" ></asp:TextBox>
   </form>