Google Earth

The globe-imaging software Google Earth has become a cult web product since its release last June. Using the web-based tool, users can fly around the globe and zoom in on both natural features and whole worlds of information added by other users

To slash the amount of data they have to transmit across the Internet, virtual globes such as Google Earth approximate the sphere of the planet's surface with a polygon made up of flat tiles. The further away your viewpoint is from the surface, the fewer tiles are needed to create the illusion of roundness, and the lower the resolution of these tiles can be.

As you zoom in, the computer explodes each tile into smaller sub-tiles, each with higher resolution, and re-forms the polygon into a ball. The process continues as you zoom. This means that the virtual globe only has to download high-resolution data when the viewer is actively zooming towards it.

Virtual globes also use another trick to speedthings up further: a disk cache. Images for places you have already looked at are stored locally on your hard drive, so when you fly over this area again the software does not need to re-download the images, but instead quickly calls them up from your hard disk.

EarthStream technology consists of client software (what is used to view the Earth), integration software (what is used to build the Earth), and server software (what is used to stream out the Earth).

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