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Google has created a searchable index of Google Earth data files, a feature that should make it easier for users to find and adopt third-party overlays for this popular mapping application.

Google Earth's search engine now returns KML (Keyhole Markup Language) files which developers have created to add data to the application's maps, the company said Wednesday.

"Users can now search through all of the world's KML files, making the millions of Google Earth layers on the Web instantly accessible for geobrowsing and exploration," wrote Chikai Ohazama, a Google Earth product manager in an official company blog. Google expects to later extend this capability to its mapping Web site Google Maps, Ohazama wrote.

Google Earth is a free, downloadable PC application that taps a multiterabyte database of aerial and satellite images to let users "fly" around the globe using a video-game type user interface. By creating overlays in the KML file format, users can create markers to pinpoint places and provide all sorts of information about an area, making Google Earth a repository of local business listings, homes for sale, photos, architectural drawings, videos, historical facts and geographical data.

KML users range from casual, individual users to large organizations like Discovery Networks, the U.S. National Park Service, the Smithsonian Institution and National Geographic. KML can also be used to add data files to Google Maps, a mapping Web site that is one of the most popular among developers for creating mashups, which are Web applications that use data and features from an existing site or online service via application programming interfaces (APIs).

Online maps, in combination with local search engines, have become very popular in recent years, giving people a way to discover local businesses and attractions, obtain driving directions and view satellite images of an area, among other things. Google competitors like Microsoft, Yahoo, AOL and Ask.com have made their mapping sites a high priority for development. Advertising revenue in local search engines is expected to grow from US$3.4 billion in 2005 to US$13 billion in 2010, according to The Kelsey Group.