Earth Visualization Tools
Within the last few years, remote sensing, geographic information systems, and GPS technology have been integrated into new and exciting Internet tools for visualizing the Earth. Google Earth and World Wind are outstanding examples of these Earth visualization tools.
Google Earth is a program for personal computers that allows users to roam the Earth's surface at will and zoom in on images showing the surface in detail. The program uses the Internet to access a large database of images maintained by Google. The spatial resolution of the images varies from location to location, depending on the Earth imagery available. Basic coverage is provided largely by Landsat satellite data, with pixels 15 or 30 m (50 or 100 ft) on a side. But many areas of much higher spatial detail are present, using other satellite sources as well as air photos. Some states and a number of European countries are covered by images at 1-m spatial resolution. A few locations are covered at resolutions as fine as 15 cm (6 in.). Most of the images are less than three years old.
The images in Google Earth are linked to an elevation database that was also provided by an application of remote sensing using radar mapping technology. As a result, the elevation of each pixel is known to an accuracy of 5–10 m, depending on the location. This allows computation of synthetic three-dimensional views of the landscape, including simulated fly-overs. By placing the viewer in motion over the landscape, the fly-over gives a strong visual impression of three-dimensional terrain, even though it is viewed on a flat computer screen.
Also linked to the image database are layers of GIS information. These include natural features, such as rivers and peaks, as well as political land boundaries and place names. A road network can also be superimposed. A search capability allows the user to type in a location (for example, the name of a city or town) and have the program zoom in to a close view. You can even ask to see restaurants, lodgings, parks, and recreation areas through the GIS linkages.
Because Google Earth provides a view of nearly every point on land, it is a very useful tool for studying physical geography. Our web site provides fly-over tours for each chapter. These are files of placemarks locating views of interest that can be downloaded and opened with the Google Earth application.
OTHER EARTH VISUALIZATION TOOLS
Although Google Earth is at present the most technologically advanced of web-based Earth visualization tools, several other tools are readily available. NASA's World Wind is a similar window on the Earth that starts with a global view and allows zooming in for fine detail. It also uses an elevation database, so it can provide three-dimensional renderings of the surface as well as realistic fly-overs.
TerraServer.com provides overhead high-resolution photos, with spatial resolutions as fine as 30 cm (11.8 in.) per pixel over many urbanized areas of the United States. Outside of the United States, most of the coverage is at 15 m (50 ft) per pixel with some inset areas in higher resolution. The service requires a subscription and payment for downloading images.
TerraServer-USA, a service of the U.S. Geological Survey, provides on-line topographic maps for the United States. Many of these are orthophoto maps that use high-resolution air photos as a base. The Geological Survey also hosts the National Map on the web, an extensive database with a large number of map layers, including administrative boundaries, geographic names, geology, land use and cover, natural hazards, topography, and transportation.