OpenLayers OpenLayers

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OpenLayers Library Documentation

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Understanding OpenLayers Syntax

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Getting Started

Creating Your First Map

The OpenLayers API has two concepts which are important to understand in order to build your first map: ‘Map’, and ‘Layer’. An OpenLayers Map stores information about the default projection, extents, units, and so on of the map. Inside the map, data is displayed via ‘Layer’s. A Layer is a data source – information about how OpenLayers should request data and display it.

Crafting HTML

Building an OpenLayers viewer requires crafting HTML in which your viewer will be seen. OpenLayers supports putting a map inside of any block level element – this means that it can be used to put a map in almost any HTML element on your page.

In addition to a single block level element, it is also required to include a script tag which includes the OpenLayers library to the page.

<html>
<head>
  <title>OpenLayers Example</title>
    <script src="http://openlayers.org/api/OpenLayers.js"></script>
    </head>
    <body>
      <div style="width:100%; height:100%" id="map"></div>
    </body>
</html>

Ex. 1: Creating your first HTML Page

Creating the Map Viewer

In order to create the viewer, you must first create a map. The OpenLayers.Map constructor requires one argument: This argument must either be an HTML Element, or the ID of an HTML element. This is the element in which the map will be placed.

var map = new OpenLayers.Map('map');

Ex. 2: Map Constructor

The next step to creating a viewer is to add a layer to the Map. OpenLayers supports many different data sources, from WMS to Yahoo! Maps to WorldWind. In this example, the WMS layer is used. The WMS layer is an example provided by MetaCarta.

var wms = new OpenLayers.Layer.WMS(
  "OpenLayers WMS",
  "http://vmap0.tiles.osgeo.org/wms/vmap0",
  {'layers':'basic'} );
map.addLayer(wms);

Ex. 3: Layer Constructor

The first parameter in this constructor is the name of the layer, which is used for some display purposes. The second argument is the URL of the WMS server. The third argument is an object containing the parameterss to be appended to the WMS request.

Finally, in order to display the map, you must set a center and zoom level. In order to zoom to fit the map into the window, you can use the zoomToMaxExtent function, which will zoom as close as possible while still fitting the full extents within the window.

Putting it All Together

The following code block puts all the pieces together to create an OpenLayers viewer.

<html>
<head>
  <title>OpenLayers Example</title>
    <script src="http://openlayers.org/api/OpenLayers.js"></script>
    </head>
    <body>
      <div style="width:100%; height:100%" id="map"></div>
      <script defer="defer" type="text/javascript">
        var map = new OpenLayers.Map('map');
        var wms = new OpenLayers.Layer.WMS( "OpenLayers WMS",
            "http://vmap0.tiles.osgeo.org/wms/vmap0", {layers: 'basic'} );
        map.addLayer(wms);
        map.zoomToMaxExtent();
      </script>

</body>
</html>

Ex. 4: Full HTML and Javascript for simple WMS browser

Adding an Overlay WMS

WMS layers have the capability to be overlaid on top of other WMS layers in the same projection. There are several ways to mark a layer as an overlay, rather than a base layer. With WMS, the best way to do this is by setting the ‘transparent’ parameter to ‘true’. The example here uses a political borders WMS to demonstrate overlaying a transparent WMS.

var dm_wms = new OpenLayers.Layer.WMS(
    "Canadian Data",
    "http://www2.dmsolutions.ca/cgi-bin/mswms_gmap",
    {
        layers: "bathymetry,land_fn,park,drain_fn,drainage," +
                "prov_bound,fedlimit,rail,road,popplace",
        transparent: "true",
        format: "image/png"
    },
    {isBaseLayer: false}
);
map.addLayer(dm_wms);

Ex. 5: How to add a transparent WMS overlay to your map.

Using the transparent: ‘true’ parameter sets two flags automatically:

  • format parameter. The format option of the WMS layer is set to image/png if the browser supports transparent PNG images. (This is all browsers except for Internet Explorer 6.) In Internet Explorer 6, this will instead be set to image/gif.
  • isBaseLayer option. The isBaseLayer option controls whether the layer can be displayed at the same time as other layers. This option defaults to false for the WMS layer, but setting transparent to true changes it to true by default.

Putting this code together with our earlier example, we get the following:

<html>
<head>
  <title>OpenLayers Example</title>
    <script src="http://openlayers.org/api/OpenLayers.js"></script>
    </head>
    <body>
      <div style="width:100%; height:100%" id="map"></div>
      <script defer="defer" type="text/javascript">
        var map = new OpenLayers.Map('map');
        var wms = new OpenLayers.Layer.WMS( "OpenLayers WMS",
            "http://vmap0.tiles.osgeo.org/wms/vmap0", {layers: 'basic'} );
        var dm_wms = new OpenLayers.Layer.WMS(
            "Canadian Data",
            "http://www2.dmsolutions.ca/cgi-bin/mswms_gmap",
            {
                layers: "bathymetry,land_fn,park,drain_fn,drainage," +
                        "prov_bound,fedlimit,rail,road,popplace",
                transparent: "true",
                format: "image/png"
            },
            {isBaseLayer: false}
        );
        map.addLayers([wms, dm_wms]);
        map.zoomToMaxExtent();
      </script>

</body>
</html>

Ex. 6: How to add a transparent WMS overlay to your map.

One thing to note here is that we have used addLayers on the map object to add both layers at the same time. This allows us to save a line of code in this case, and may be useful in other cases when you need to add multiple layers to the map at the same time.

Adding a Vector Marker to the Map

To add a single marker at a latitude and longitude to the map, you can use a Vector Layer to add an overlay.

var vectorLayer = new OpenLayers.Layer.Vector("Overlay");
var feature = new OpenLayers.Feature.Vector(
 new OpenLayers.Geometry.Point(-71, 42),
 {some:'data'},
 {externalGraphic: 'img/marker.png', graphicHeight: 21, graphicWidth: 16});
vectorLayer.addFeatures(feature);
map.addLayer(vectorLayer);

This is a simple demonstration – more information is available on overlays, how to interact with them, and how to control and style them via the Styling and Overlays documentation.