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Deliver On-the-Fly Mapping Services to Your Rich Desktop Java Application, Part 1
by Dan Andrews
In October of 2004, Google acquired the Australian start-up Where 2 Technologies. By early 2005, OpenStreetMaps was up and running and Jesse James Garrett coined the term AJAX. By mid-2005, Google and Yahoo! were beginning to share their mapping APIs with developers and Microsoft launched Virtual Earth. By late 2005, Andre Charland had asked the question, "Will AJAX replace the desktop?"
Since that time, the interest in mapping services has exploded. The popularity of AJAX has been fueled, in part, by developer access to mapping JavaScript APIs. These two technologies work especially well when the geographic data of interest is distributed on the Internet.
Data is not always distributed on the Internet. Geographic data may be about your business or your personal whereabouts. Depending on the situation, desktop applications may be more appropriate than Rich Internet Applications for your data. Additionally, an increasing need for rich desktop mapping applications will be driven by increasing numbers of consumer GPS devices—things like watches, cell phones, hand held navigation tools, and vehicle navigation aids. Altogether, people are collecting megabytes worth of data, all pertaining to their daily whereabouts—and none of it uploads easily into an AJAX application.
In this series, you'll learn about about building a rich client Java application to visualize mapping data. You'll examine how to keep current with the inevitable explosion of free mapping services without recompiling Java code. But before you start building that client, let's examine some of the conventions that are used by OpenStreetMaps, Microsoft's Virtual Earth, Google Maps, Yahoo! Maps, and others.
Tile Conventions
Because this series is about building a rich desktop (client) Java application, we'll be using the term "client" where normally one might use the term "browser." A request for a map tile will simply be a properly constructed URL to a tile image which is hosted on a server that we'll call a "tile server."
The client application requests map tiles for a particular location and zoom level from one or more tile servers. The tile server is then used to respond quickly to client requests by organizing and clustering map tiles according to their location and zoom level. When a client receives a tile, the tile is positioned, along with other tiles, to present a map view to the end user.
| Author's Note: Knowing how to form requests for map tiles will help you understand this process. While there is no need to master these low-level details, the more you know, the more comfortable you will feel modifying and debugging a client mapping application. |
Every tile, including the entire world map, is a perfect square. Starting from zoom level 1, the world square can be broken into four square quadrants. The grid is numbered similar to screen pixel coordinates: the x is the horizontal coordinate and y is the vertical coordinate, starting in the top left. Table 1 shows the tile grid for Zoom Level 1. You access thetiles using the URL http://tile.openstreetmap.org/z/x/y.png, where z is the zoom level, x is the horizontal position and y is the vertical position.
OpenStreetMaps, Google's regular map type, and Yahoo! all use the x, y, and z notation for processing tile requests—although, Yahoo! uses a slight variation of this notation. To find the parent tile URL from the x, y, z notation, divide x and y in half and add one to z. Neighboring tiles can easily be found by adding or subtracting 1 to either or both x and y. The following URLs provide an example for each of these servers:
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