Mobile games development

I’m really honored that another article, published on Forum Nokia Wiki, has been selected as Article of the Month!

As the previous one, this also is related to Google Maps usage in mobile applications, and you can read it here: How to use Google Maps data in mobile applications.

As its name says, It’s an introductory article on how to use Google Maps services, in particular the geocoding and static maps ones, from a mobile application, where standard Google Maps API code is not suitable (since it is thought for web based and Ajax’d apps). Its content does not focus on any particular programming language, but gives base guidelines to use those services using REST.

So… any kind of feedbacks is welcome!

I’m really happy to announce that my J2ME Google Maps API article on Forum Nokia Wiki has been selected as Article of the Week!

And, to celebrate this event, I’ve added a brand new feature to my article that will allow you to:

• create larger tiled maps
• support map scrolling

How does it work?

You start instantiating a GoogleMaps object as usual:

GoogleMaps gMap = new GoogleMaps("API_KEY");

Then you get your map, for example geocoding a given address:

double[] coords = gMap.geocodeAddress("Leicester square, London");
 
Image mapImage = gMap.retrieveStaticImage(
	150, 150,
	coords[0], coords[1],
	12, "png"
);

Then, let’s say you want to scroll your map 100 pixels up, what you’ll do is:

double[] newCoords = gMap.adjust(
	coords[0], coords[1],
	0, -100, 12
);
 
Image newMapImage = gMap.retrieveStaticImage(
	150, 150,
	newCoords[0], newCoords[1],
	12, "png"
);

As you’ve seen, the adjust method takes these arguments:

• the current latitude and longitude
• the deltaX and deltaY, in pixels
• the current zoom level

and returns the new map center latitude and longitude coordinates.

You can check the full updated source code on Forum Nokia Wiki article: J2ME Google Maps API, and a full-featured example, with the scrolling feature, on the emulator page: J2ME Google Maps API in action.

Note: You can find this article also on Forum Nokia Wiki: How to use Google Maps data in mobile applications

Today we’ll see how to use Google Maps data within a mobile application.
Google Maps offers REST services that allows accessing its data with simple HTTP requests, so we can easily integrate them within our mobile apps.

Signup for a Google Maps API key

First thing you must do is to signup on this page:
http://code.google.com/apis/maps/signup.html
Once done, you’ll get a key (a simple String) you’ll use for all your query to Google Maps services

Static maps

Standard Google Maps code is suited for web applications, since it includes alot of Ajax functionalities, that are not really useful if you’re building a mobile application. So, the solution is to use static maps service, that will allow us to retrieve single images, easily usable within our apps.

Static maps service supports different image formats (png32, gif, jpg) and customizable image size, so that we can get perfect images for all our needs. As an example, suppose we want to retrieve the location at:

• latitude: 41.867878
• longitude: 12.471516

We can simply retrieve this URL with an HTTP GET request:

http://maps.google.com/staticmap?center=41.867878,12.471516&
format=png32&zoom=8&size=240x320&key=<API_KEY>

This way, we’ll get a PNG32 image, with a width of 240 pixels, and a height of 320, centered at point (41.867878,12.471516), and with a zoom level of 8 (zoom can go from 0 to a maximum level of 19).

Geocode an address

From Google Maps docs:
Geocoding is the process of converting addresses (like “1600 Amphitheatre Parkway, Mountain View, CA”) into geographic coordinates (like latitude 37.423021 and longitude -122.083739)

So, let’s assume we want to build an application that displays the address typed by our user. We should firstly geocode its address to geographics coordinates.
To do this, Google Maps offer another REST service easily accessible with simple HTTP requests.

Let’s say you want to geocode this address “Leicester Square, London”, then you’ll request this URL:

http://maps.google.com/maps/geo?q=Leicester%20Square,%20London
&output=csv&key=<API_KEY>

and you’ll get this output:

200,6,51.510605,-0.130728

Where:

• the first number is a code, that in this case (200) means that geocoding has been successfull (for a full list of status codes you can see here: [1])
• the second number gives a measure of geocoding accuracy (from 0 to 9 - maximum accuracy)
• 3rd and 4th numbers represent latitude and longitude of the geocoded address, so these are the coordinate we’ll use to retrieve the map through the static map service we’ve seen before

As you can see, there is an ‘output’ parameter within the geocode request, and this means that we can choose the output format we prefer for our needs. Supported formats are:

• xml
• kml (same as xml, but with different Content-Type)
• json (not really useful for mobile apps)
• csv (comma separated values)

Proxy server, usage limits

Since your Google Maps API key is bound to a specific URL, to access map services you should setup a proxy server that will receive HTTP requests from your mobile application and forward them to Google Maps REST URLs, giving back Google responses to mobile clients.

Also, be aware that there is a limit to the number of requests (both for static maps and geocode service) you can do each day. For personal uses they’re more than enough, anyway consider this point if you plan to develop commercial services.

Google Maps J2ME API and sample application

Now, you want code right? Here it is:

• a J2ME class to access Google Maps REST services: http://wiki.forum.nokia.com/index.php/J2ME_Google_Maps_API
• a sample application using Google Maps data: http://www.jappit.com/index.php?page=emulator&midlet=google_maps