Vector Data Tutorial

Base layers such as OpenStreetMap and Google Maps are generally distributed in pre-rendered tiles using file formats such as PNG or JPG. While these are great for displaying maps, they are not very useful for getting at the data behind a map. They don’t allow you to provide functionality such as informational popups, selection and highlighting of individual features, and editing of data. For these, you need to use vector data, provided through file formats such as KML, GeoJSON, or GML which provide information about each feature on the map, rather than just the pixels to put on the screen.

Note

Web browsers impose a same origin policy on JavaScript code to protect users from cross-site scripting attacks. This means that if your GeoExt application is hosted on a different host or port from your vector data, you will need to configure a proxy service.

Reading KML

As an introduction to using vector data in GeoExt, let’s create a simple map that displays data from a KML. Copy this sample KML file to the same directory with your GeoExt and Ext libraries. Then we can load it with some JavaScript:

var map = new Openlayers.Map();
var bluemarble = new OpenLayers.Layer.WMS(
    "Global Imagery",
    "http://maps.opengeo.org/geowebcache/service/wms",
    {layers: "bluemarble"}
);
var sundials = new OpenLayers.Layer.Vector("Sundials");
map.addLayer(bluemarble);
map.addLayer(sundials);

var store = new GeoExt.data.FeatureStore({
    layer: sundials,
    proxy: new GeoExt.data.ProtocolProxy({
        protocol: new OpenLayers.Protocol.HTTP({
            url: "sundials.kml",
            format: new OpenLayers.Format.KML()
        })
    }),
    fields: [
        {name: 'title', type: 'string'},
        {name: 'description', type: 'string'}
    ],
    autoLoad: true
});

var mapPanel = new GeoExt.MapPanel({
    title: "Sundials",
    map: map,
    renderTo: 'mapPanel',
    height: 400,
    width: 600
});

Here, we set up a map with two layers. bluemarble is a WMS layer, which you should have seen before in other tutorials. sundials is a vector layer, which handles client-side rendering of vector data.

In line 10 we initialize a GeoExt.data.FeatureStore. This class functions as a normal Ext.data.Store to interoperate with ExtJS classes, as well as providing the ability to bind to an OpenLayers.Layer.Vector in order to display features on a map. In this example, we set up the store completely through constructor parameters:

layer: sundials

tells the store to render features using the sundials layer. This is equivalent to calling store.bind(sundials) after initializing the store.

proxy: new GeoExt.data.ProtocolProxy(

tells the store to use a ProtocolProxy for fetching features. ProtocolProxy wraps OpenLayers Protocol objects. Here we use an OpenLayers.Protocol.HTTP to fetch data over the web. The HTTP protocol works with a variety of OpenLayers.Format types; here we use KML to match our dataset. You can see all the available Protocols and Formats in the OpenLayers API documentation.

fields: [...]

tells the store which extra properties (aside from just the geometry) to look for. Here, we know that KML includes a title and a description for each point, and that both are string values.

autoLoad: true

tells the store to go ahead and fetch the feature data as soon as the constructor finishes. This is equivalent to calling store.load() after the store is initialized.

Now we have a map with a background and some data hosted on our server. It looks like any other map; we can pan and zoom normally to navigate around.

However, since GeoExt has access to the data behind the map, we now have some options that weren’t available to us before. For example, we can add a control that allows us to view the features in a tabular format:

new Ext.grid.GridPanel({
    title: 'Sundials',
    store: store
    columns: [{heading: 'Title', dataIndex: 'title'},
              {heading: 'Description', dataIndex: 'description'}],
    renderTo: "grid",
    width: 200,
    height: 600
});