Question

I have added a WFS layer to a map and can see (using Fiddler) a request being made to the server for the layer data. The server uses GML as the data format and the data being returned is valid. However, OpenLayers does not display the data. Here is the code that I use.

$(document).ready(
        function () {
            // allow testing of specific renderers via "?renderer=Canvas", etc
            var renderer = OpenLayers.Util.getParameters(window.location.href).renderer;
            renderer = (renderer) ? [renderer] : OpenLayers.Layer.Vector.prototype.renderers;
            var geographic = new OpenLayers.Projection("EPSG:4326");
            var mercator = new OpenLayers.Projection("EPSG:900913");

            map = new OpenLayers.Map({
                div: "map",
                layers: [
                    new OpenLayers.Layer.WMS("OpenLayers WMS",
                        "http://vmap0.tiles.osgeo.org/wms/vmap0",
                        { layers: "basic" }
                    ),
                    new OpenLayers.Layer.Vector("GML", {
                        strategies: [new OpenLayers.Strategy.Fixed()],
                        protocol: new OpenLayers.Protocol.WFS({
                                url: "http://localhost/MapServer/Default.aspx",
                                featureType: "Layer_ACTIVE",
                                featureNS: "http://www.tstgis.org/gml",
                                version: "1.1.0",
                                geometryName: "line"
                            }),
                            renderers: renderer
                        })
                ],
                zoom: 15
            });

            var bb = new OpenLayers.Bounds(-179.821327209473, 12.1057098342161, -56.5289154052734, 78.1442901657839);
            map.zoomToExtent(bb);
});

This test script is part of an HTML shell that is running under localhost/mapserver, so it rules out the familiar cross-domain issue.

What is going on? Note: the returned data set is pretty big (1.5 MB) and I wonder if that has anything to do with the missing display.

Answer 1

I had no doubt that this was a configuration issue with OpenLayers. From what I knew about OpenLayers, I had it configured properly. Turns out, it is essential, in my case, to set the featurePrefix option. The reason for this is that the namespace and prefix in the XML response (GML) for each FeatureMember Node is used to identify the appropriate reader for the response. By default, the featurePrefix is set to ‘feature’. If the response’s namespace + prefix does not match the configuration in OpenLayers, the features are not added to the layer and hence not displayed. In my case, the prefix is set to an empty string as the server does not add a prefix to the response.

Also, setting the geometryName is critical. The default value for geometryName is ‘the_geom’ for WFS version 1.0, and null for higher versions. The geometryName is used by the server to actually locate the features. In my case, geometryName used on the server was “msGeometry”.

Here is the working code.

$(document).ready(
function () {
    // allow testing of specific renderers via "?renderer=Canvas", etc
    var renderer = OpenLayers.Util.getParameters(window.location.href).renderer;
    renderer = (renderer) ? [renderer] : OpenLayers.Layer.Vector.prototype.renderers;
    var geographic = new OpenLayers.Projection("EPSG:4326");
    var mercator = new OpenLayers.Projection("EPSG:900913");

    map = new OpenLayers.Map({
        div: "map",
        layers: [
            new OpenLayers.Layer.WMS("OpenLayers WMS",
                "http://vmap0.tiles.osgeo.org/wms/vmap0",
                { layers: "basic" }
            ),
            new OpenLayers.Layer.Vector("GML", {
                strategies: [new OpenLayers.Strategy.Fixed()],
                protocol: new OpenLayers.Protocol.WFS({
                        url: "http://localhost/MapServer/Default.aspx",
                        featureType: "Data_ACTIVE",
                        version: "1.1.0",
                        geometryName: "msGeometry",
                        featurePrefix: ""
                    }),
                    renderers: renderer
                })
        ],
        zoom: 15
    });

    var bb = new OpenLayers.Bounds(-179.821327209473, 12.1057098342161, -56.5289154052734, 78.1442901657839);
    map.zoomToExtent(bb);

});