Google Latitude

Google Latitude is a location-aware mobile app developed by Google. Latitude allows a mobile phone user to allow certain people to track their location. Via their own iGoogle accounts, the user's cell phone location is mapped on Google Maps. The user can control the accuracy and details of what each of the other users can see — an exact location can be allowed, or it can be limited to identifying the city only. For privacy, it can also be turned off by the user, or a location can be manually entered. Users have to explicitly opt in to Latitude, and may only see the location of friends who have decided to share their location.

Google's 2005 acquisition, Dodgeball, offered similar facility to users by way of SMS. With Google Latitude, the service has expanded to PCs (it uses IP geolocation as well as user driven input) and automated location detection on mobile phones using Cell-ID, Cellular Positioning, and GPS.

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Source : Wikipedia