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Obtaining an API Key
All Maps API applications should load the Maps API using an API key. Using an API key enables you to monitor your application's Maps API usage, and ensures that Google can contact you about your application if necessary. If your application's Maps API usage by exceeds the Usage Limits, you must load the Maps API using an API key in order to purchase additional quota.
To create your API key, visit the APIs Console at https://code.google.com/apis/console and log in with your Google Account.
Click the Services link from the left-hand menu, then activate the Google Maps API v2 service.
Once the service has been activated, your API key is available from the API Access page, in the Simple API Access section. Maps API applications use the Key for browser apps.
By default a key can be used on any site. We strongly recommend that you restrict use of your key only to domains you administer, to prevent use on unauthorized sites. You can specify which domains are allowed to use your API key by clicking the Edit allowed referrers... link for your key.