GEOG 585
Open Web Mapping

Programming patterns with web mapping APIs


If you distinguish yourself within your organization as a person who can develop web maps, it's likely you'll eventually be called upon to use more than just one of the APIs described in the previous section. As noted, project circumstances and requirements can force the selection of different APIs for differing endeavors. As a programmer, it's important to understand the general structures, patterns, and architectures behind the APIs and languages you use, so that you can learn new ones on the fly. Technology is always changing, and you will limit your utility if you tie yourself to a single development mode.

The following section describes some patterns and features that are held in common among many (but not all) of the web mapping APIs described earlier. I include this section before diving into Leaflet, so that you get an idea of what things are not unique to Leaflet when we begin looking at the code. However, I also include example snippets to show how the concept is implemented in Leaflet.

Nearly all pages that use web mapping APIs include the following:

References to JavaScript files and stylesheets

Before you can get off the ground, your HTML page needs to include a <script> tag pointing at the web mapping API's JavaScript files. Be aware that the more JavaScript you are referencing, the longer it will take to load your page. Some APIs are slimmer than others (hence a name like ModestMaps) but may offer fewer features. When adopting one of the larger APIs, like OpenLayers, some developers build and reference their own smaller version of the API that contains just the functions they want to offer.

There are a couple of ways to reference the API. One approach is to download and host the API on your own server, thus minimizing load times and allowing you to customize the API. The second approach is to reference the API on someone else's server. Sites called content delivery networks (CDNs) specialize in hosting commonly-referenced APIs. This is what a CDN URL looks like for Leaflet, referenced within a script tag within the page head:

<script src=""></script>

In this course, we'll reference Leaflet through the CloudFlare CDN in this manner, for simplicity. This requires you to maintain an Internet connection while you are testing your code. Be aware that if you were developing an internal application, needed full control over the APIs hardware, or needed to customize the API in any way, you would need to download and host the API yourself.

Many web mapping APIs offer some stylesheets that can get you started with making nice-looking applications. You can reference these stylesheets in the form of CSS files, either by hosting them on your own server or using a CDN. You can bring a Leaflet stylesheet into your page from the CloudFlare CDN within the head using syntax like this:

<link rel="stylesheet" href="" type="text/css" crossorigin="">

The map div and object

When you want to put a map in your page, you will typically carve out some space using an HTML <div> tag. You then create a map object using the API and relate it to your div.

For example, with Leaflet, you can create a map div like this in the body of your page.

<div id="mapid"></div>

Elsewhere in your page, in your JavaScript code, you can create a Leaftlet Map object and relate it to the div. The constructor takes the div name as an argument (L stands for the Leaflet library here).

var myMap;
myMap = new'mapid');

The map contains methods for getting the current layer set, centering the map, zooming to given coordinates, and so forth. In many web mapping APIs, the map is one of the most powerful objects. Whenever you adopt a new mapping API, take a look at the reference documentation for the map object to understand what it can do and the syntax for invoking the most common methods.

Layer objects

Most web mapping APIs provide ways to define layer objects, which you add to the map object one by one to create your mashup. It is very important to note that a layer, in this sense, could represent a web service such as a WMS or tiled service that itself contains many underlying data layers. However, you would only need one layer object to bring this type of web service into your map. Other layer objects that reference single data files, such as KML or GeoJSON, are simpler to conceptualize.

In many web mapping APIs, the layer is an abstract (or "base") class offering a set of common properties, and is exposed to the developer only through more specific classes. Take a look at the methods and events offered in Leaftlet's Layer base class. These include methods to add and remove the layer from the map and manage elements like popups and tooltips. Now look at the properties of some of the more specific derived classes such as TileLayer, WMS, and GeoJSON to see some of the more specific methods associated with these layer types. You will typically be working with the API help documents at this lower level; however, it is important to remember that you still have all the properties and methods of the Layer class available to you whenever you use any of these specialized layer types.

When you create a new layer in Leaflet, you're generally expected to provide the URL or file path containing the source data for the layer. The layer will not show up on the map until you call its addTo(...) method. Adapted from the Leaflet documentation, here's one way you could create a layer referencing a WMS and add it onto your basemap:

var nexrad = L.tileLayer.wms("", {
    layers: 'nexrad-n0r-900913',
    format: 'image/png',
    transparent: true,
    attribution: "Weather data © 2012 IEM Nexrad"


Don't worry if the meaning of all the parameters above is not immediately apparent. The important thing is to recognize that the layer required you to supply a URL before it could recognize the WMS. You were also able to toggle a transparency option and supply attribution text for the layer.

If you call the addTo(...) method on some other layer, it will be placed on top of the first layer.

Many types of tiled layers include the tile zoom level, row, and column number in each URL. When you create these layers in Leaflet, it would not make sense to supply a specific URL, since this would only point to one tile. Instead, you supply a general format using {x}, {y}, and {z} for the column, row, and zoom levels, respectively. This is sort of like what you did when you added your tiles to the ArcGIS Online map viewer in the previous lesson.

// create and add OpenStreetMap tile layer
var osm = L.tileLayer('http://{s}{z}/{x}/{y}.png', {
    maxZoom: 19,
    attribution: '&copy; <a href="">OpenStreetMap</a>'


Layer styling mechanisms

Layers that come already drawn by the server, such as tiled maps and WMS images, already have styling applied, but for layers drawn by the browser such as GeoJSON or GeoRSS, you must define how the layer will be styled. Web mapping APIs typically offer a group of properties that you can set on a layer like this to define how the browser should draw it. These properties include things like fill width, fill color, outline width, outline color, and so forth. In Leaflet, it's common to define styles as functions that can then be referenced in the layer constructor, like so:

function parkStyle(feature) {
    return {
        fillColor: '#FF00FF',
        fillOpacity: 1,
        color: '#B04173',

var gardenLayer = new L.GeoJSON.AJAX('gardens.geojson', {
    style: parkStyle


The style function returns a set of style properties (fillColor, fillOpacity, and color in this case) that will be applied when drawing the features from the layer. In this simple case, all features will look exactly the same. However, since the style function has the feature to be drawn as a parameter (function parameter feature), we can use properties of the feature to calculate its style property values. e.g., to have all features colored differently based on one of their attributes. You will learn how to do this later on in this course.  

Many APIs allow you to use a custom image for a marker, rather than placing a simple dot. The Leaflet example below shows how you could add a GeoJSON layer of grocery stores to your map and style it with a shopping cart icon saved in an SVG (scalable vector graphics) file called grocery.svg. This could just as easily be a PNG file or other rasterized image type.

// create icons for supermarkets
var groceryIcon = L.icon({
    iconUrl: 'grocery.svg',
    iconSize: [20,20]

// create a vector layer of grocery stores from GeoJSON data
var groceryLayer = new L.GeoJSON.AJAX('supermarkets.geojson',{
    pointToLayer: function (feature, latlng) {
        return L.marker(latlng, {icon: groceryIcon});


The result might look something like this when put on top of a basemap layer:

Screen Capture: Grocery store icon
Figure 6.1

Don't worry if all the code above is not completely clear to you yet. Just make sure you can see where the style is being defined via a JavaScript object. In Lesson 7, you will get some more exposure to GeoJSON and styling browser-drawn graphics.

Events and interactive elements

Web mapping APIs offer interactive elements that help your map become more than just a static picture on an HTML page. The map and layer objects described above typically allow you to run code in response to certain user actions, such as clicking the mouse. The user action is called an event (or firing an event). The code you run in response is called the event handler, and it typically goes into its own function block of code. Sometimes the event can supply arguments to the handler function via an "event object" containing properties of the event, such as the screen coordinates of the mouse click that fired the event.

For example, you might instruct the map to "listen" for a mouse click event. You could then write a handler function that takes the screen coordinates of the event argument (in other words, the coordinates of the clicked pixel) and converts them to map coordinates, then writes the coordinates into a label in the HTML page so they can be seen by the user. An even greater measure of interactivity could be achieved by wiring up this handler function to a hover event, instead of a click. You would then get the effect of always seeing the coordinates of your mouse as you moved it around the screen.

Web map users often want to obtain more detailed information about specific features in the map. It is very common to handle a click event by showing a popup window with more information about the clicked feature; so common, in fact, that many web APIs have special classes and methods for popups that don't require you to write the typical amount of event listening logic. You will learn more about popups in Leaflet in future lessons in this course.

Sometimes popups are too limited in space or complexity for the content you want to show. Even if your mapping API allows you to cram large images or tabbed interfaces into a popup, it often makes more sense to show this type of content in an HTML div elsewhere on the page. Web mapping APIs allow you to perform queries on a clicked point and retrieve attribute information from the clicked features (often as part of a handler function like the ones mentioned above). You can then do anything you want with this handler function in order to display the information in HTML. You might even pass the information to a different specialized API that draws charts, queries Wikipedia, finds nearby homes for sale, and so forth.

Another common piece of desired interactivity is the ability to switch layers off and on. Remember that in this sense, a "layer" is an entire web service. It is obviously not possible to toggle the visibility of individual sublayers inside a tiled map because all the layers are "burned into" the tiled image. However, you could switch to a different tiled base map, or turn off a WMS or GeoJSON layer placed on top of it.

Leaflet offers a Layers Control that acts like a table of contents for toggling visibility. You create your layers as usual, but instead of adding them to the map individually, you organize them in JavaScript objects for your base layers and overlays (thematic or business layers). Suppose you have two layer variables, grayscale and streets, representing tiled maps, and one layer variable cities representing a layer of city points. Again, from the Leaflet doc, here is how you can add the layer switching control with these defined as base layers and overlays:

var baseLayers = {
  "Grayscale": grayscale,
  "Streets": streets

var overlays = {
  "Cities": cities

L.control.layers(baseLayers, overlays).addTo(map);

Now that you are familiar with the different elements of a web mapping API and have seen how these are expressed in Leaflet, we'll move forward and take a look at some fully functioning examples.