Working with Vector Layers

The base OpenLayers.Layer.Vector constructor provides a fairly flexible layer type. By default, when you create a new vector layer, no assumptions are made about where the features for the layer will come from. In addition, a very basic style is applied when rendering those features. Customizing the rendering style is addressed in an upcoming section. This section introduces the basics of vector data formats, the protocols used to read and write feature data, and various strategies for engaging with those protocols.

In dealing with vector layers and features, it is somewhat useful to consider a postal analogy. When writing a letter, you have to know some of the rules imposed by the postal service, such as how addresses are formatted or what an envelope can contain. You also have to know something about your recipient: primarily what language they speak. Finally, you have to make a decision about when to go to the post office to send your letter. Having this analogy in mind may help in understanding the concepts below.

OpenLayers.Format

The OpenLayers.Format classes in OpenLayers are responsible for parsing data from the server representing vector features. Following the postal analogy, the format you choose is analogous to the language in which you write your letter. The format turns raw feature data into OpenLayers.Feature.Vector objects. Typically, format is also responsible for reversing this operation.

Consider the two blocks of data below. Both represent the same OpenLayers.Feature.Vector object (a point in Barcelona, Spain). The first is serialized as GeoJSON (using the OpenLayers.Format.GeoJSON). The second is serialized as GML (using the OpenLayers.Format.GML.v3).

GeoJSON Example

{
    "type": "Feature",
    "id": "OpenLayers.Feature.Vector_107",
    "properties": {},
    "geometry": {
        "type": "Point",
        "coordinates": [2.17, 41.39]
    }
}

GML Example

<?xml version="1.0" encoding="utf-16"?>
<gml:featureMember
    xsi:schemaLocation="http://www.opengis.net/gml http://schemas.opengis.net/gml/3.1.1/profiles/gmlsfProfile/1.0.0/gmlsf.xsd"
    xmlns:gml="http://www.opengis.net/gml"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <feature:feature fid="OpenLayers.Feature.Vector_107" xmlns:feature="http://example.com/feature">
        <feature:geometry>
            <gml:Point>
                <gml:pos>2.17 41.39</gml:pos>
            </gml:Point>
        </feature:geometry>
    </feature:feature>
</gml:featureMember>

See the vector formats example for a demonstration of translation between a few OpenLayers formats.

OpenLayers.Protocol

The OpenLayers.Protocol classes refer to specific communication protocols for reading and writing vector data. A protocol instance may have a reference to a specific OpenLayers.Format. So, you might be working with a service that communicates via HTTP and deals in GeoJSON features. Or, you might be working with a service that implements WFS and deals in GML. In these cases you would construct an OpenLayers.Protocol.HTTP with a reference to an OpenLayers.Format.GeoJSON object or an OpenLayers.Protocol.WFS with a reference to an OpenLayers.Format.GML object.

Back on the postal analogy, the protocol is akin to the rules about how an envelope must be addressed. Ideally, a protocol doesn’t specify anything about the format of the content being delivered (the post office doesn’t care about the language used in a letter; ideally, they only need to read the envelope).

Neither protocols nor formats are explicitly tied to an OpenLayers.Layer.Vector instance. The layer provides a view of the data for the user, and the protocol shouldn’t have to bother itself about that view.

OpenLayers.Strategy

Loosely speaking, the OpenLayers.Strategy classes tie together the layer and the protocol. Strategies deal with when to make requests for data (or when to send modifications). Strategies can also determine how to prepare features before they end up in a layer.

The OpenLayers.Strategy.BBOX strategy says “request new features whenever the map bounds are outside the bounds of the previously requested set of features.”

The OpenLayers.Strategy.Cluster strategy says “before passing any new features to the layer, clump them together in clusters based on proximity to other features.”

In creating a vector layer, you choose the mix: one protocol (typically) with a reference to one format, and any number of strategies. And, all of this is optional. You can very well create a vector layer without protocol or strategies and manually make requests for features, parse those features, and add them to the layer.

Having dispensed with the basics of formats, protocols, and strategies, we’re ready to start creating new features.