OpenLayers is an open source map viewing library, originally developed and released by MetaCarta under a BSD license. It is written in pure JavaScript and makes it easy to incorporate interactive maps from a variety of sources into your web pages and applications. As Christopher Schmidt, one of the main project developers, wrote on the OpenLayers users mailing list:
OpenLayers is not designed to be usable out of the box. It is a library designed to help you to build applications, so it's your job as an OpenLayers user to build the box.
Don't be scared! Building the box could be very easy and fun and this book will show you how! The only two things you actually need to write your code and see it up and running are a text editor and a common web browser. With these tools you can create your Hello World web map, even without downloading anything and writing no more than a basic HTML template and a dozen line of JavaScript code.
Going forward, step-by-step, you will realize that OpenLayers is not only easy to learn but also very powerful. So, whether you want to embed a simple web map in your website or you want to develop an advanced mash-up application by importing spatial data from different sources and in different formats, OpenLayers will probably prove to be a very good choice.
The strengths of OpenLayers are many and reside, first of all, in its compliance with the Open Geospatial Consortium (OGC) standards, making it capable to work together with all major and most common spatial data servers. This means you can connect your client application to web services spread as WMS, WFS, or GeoRSS, add data from a bunch of raster and vector file formats such as GeoJSON and GML, and organize them in layers to create your original web mapping applications.
From what has been said until now, it is clear that OpenLayers is incredibly flexible in reading spatial data, but another very important characteristic is that it is also very effective in helping you in the process of optimizing the performances of your web maps by easily defining the strategies with which spatial data are requested and (for vectors) imported on the client side. FastMap and OpenLayers make it possible to obtain them!
As we already said at the beginning, web maps created with OpenLayers are interactive, so users can (and want to) do more than simply looking at your creation. To build this interactivity, OpenLayers provides you with a variety of controls that you can make available to your users. Tools to pan, zoom, or query the map give users the possibility to actually explore the content of the map and the spatial data displayed on it. We could say that controls bring maps to life and you will learn how to take advantage from them in a few easy steps.
Fast loading and interactivity are important, but in many cases a crucial aspect in the process of developing a web map is to make it instantly readable. Isn't it useful to build web maps if the users they are dedicated to need to spend too much time before understanding what they are looking at? Fortunately, OpenLayers comes with a wide range of possibilities to styling features in vector layers. You can choose between different vector features, rendering strategies, and customize every aspect of their graphics to make your maps expressive, actually "talking" and—why not?—cool!
Finally, as you probably remember, OpenLayers is pure JavaScript, and JavaScript is also the language of a lot of fantastic Rich Internet Application (RIA) frameworks. Mixing OpenLayers and one of these frameworks, as we will see later in this book, opens a wide range of possibilities to obtain very advanced and attractive web mapping applications.