Categories: Java ME | Location Based Services

This page was last modified 10:59, 14 May 2008.

From Forum Nokia Wiki

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
You'll request, from your code, 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. (as pointed out in the Comment page, this is not a fully clear point yet)

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.

Sample application

You can see a sample J2ME application, using the approach described here, on this page: Google Maps J2ME Test
Google Maps J2ME API source code used in this example is also available here: J2ME Google Maps API

Related content

• J2ME Google Maps API
• Location API
• How to get Location Using Location API JSR 179
• Java ME Location API