Google has upgraded Google Earth with a revamped user interface and a larger index of images.

There have been over 100 million unique downloads of the free application in the past year. The application taps into a multiterabyte database of satellite images and, with a video-game-like interface, lets users travel around the world, virtually flying from destination to destination and zooming in and out of cities.

Google acquired the software in 2004 when it bought Keyhole Corp. The Google Earth product family also includes more advanced, fee-based versions of the free application designed for commercial uses.

On Monday, Google announced that Google Earth's index of high-resolution satellite and aerial imagery has been quadrupled. Google Maps, the online mapping service, will begin tapping into this expanded index soon. This, in turn, will benefit the Google Local business-listings search service with which Google Maps is tightly integrated.

Meanwhile, the more streamlined user interface includes new tools to create and display content generated by users and external applications. This is a nod to the now popular practice among internet companies of encouraging users and external developers to extend their services and products with contributions such as plug-ins, applications, tags and reviews.

For the first time, Google Earth is now available for Linux as well as Windows and Mac OS X. Spanish, Italian, German and French versions are also now available.

Also new in Google Earth is support for the creation of textured building drawings with the Google SketchUp application for creating, viewing and modifying 3D images.

Lastly, Google has updated its Google Maps application programming interface (API) to tie address data into applications that use Google Maps. Meanwhile, Google also unveiled a fee-based licence for businesses interested in tapping Google Maps for web or internal applications.