SciTech

Google Earth builds textured area maps

At some point, you’ve probably used Google Maps to figure out how to get somewhere. If you went to menu and clicked “Satellite,” you were able to see physically what your surroundings look like, so you could look for key landmarks. In fact, the concept of satellite imaging was so popular that Google created a whole new application dedicated to it: Google Earth.

Google Earth emerged from Earth Viewer 3-D, a program created by Keyhole Corp., a digital mapping company based in Mountain View, California. Keyhole Corp. was also funded in part by the Central Intelligence Agency. According to a Google press release, Google acquired Keyhole Corp. back in 2004, and thereafter released Google Earth in 2005.

The present application highlights 3-D images that allow the user to explore cities, buildings, bridges, monuments — you name it. Distant galaxies can be explored using Google Sky, a feature introduced in Google Earth 4.2 in August 2007, and the ocean can be explored using Google Ocean, a feature introduced in Google Earth 5.0 in February 2009.

Google Earth images are so detailed that you can clearly see your house, objects in your yard, all pretty accurately. But how does Google Earth get all of these high-resolution graphics? The answer lies in satellite and aerial imaging. We’ll focus on satellite imaging here.

Satellite imaging is a process that uses satellites to scan the Earth and gain information about it. Satellites use different sensors to collect electromagnetic radiation reflected from objects on the Earth — passive sensors collect radiation from the Sun reflected on the Earth, while active sensors send out radiation themselves and analyze it after it has been reflected off of the Earth. The difference in reflectivity allows us to identify objects using remote sensing.

Water, for example, reflects little infrared or visible light, while vegetation absorbs visible light, but strongly reflects infrared light.

Satellite images are the result of thousands of pixels that the satellite scanned into rows and columns. The satellite gathers these pixels into a computer file, and the area the file covers is called a scene. Scene sizes vary depending on the type and size of the sensor.

Satellite imaging has many applications — personal, environmental, militaristic. It can be used for weather predictions; it can also help map out enemy terrain.

Google Earth gets their images from satellites like TeleAtlas and EarthSat, which compile photographs and maps into digital form for commercial use. Google Earth images differ from images collected by major satellites in many ways.

With Google Earth images, the image is sharpest in the center and becomes increasingly blurry towards the edges. There is also no timestamp on Google Earth images, so although they are one to three years old, it is impossible to tell the exact time and date the images were taken, which is essential with images from major satellites.

While compiling images is a major feature of Google Earth, the most challenging thing is transferring those images to your computer quickly and efficiently. One way Google Earth cuts down transfer time is by using your computer’s disk cache to store images for places you’ve already looked at so that when you look at them again, Google Earth doesn’t need to re-download the images.

Google also has a patent called Universal Texture, which uses two methods for getting large amounts of information to your computer — mipmapping and clip stacking.

Mipmaps are collections of bitmap images that create the illusion of depth by working within a texture in an inverted pyramid structure. These images stack onto one another in layers, with each layer having twice the resolution of the layer below it. Clip stacks are portions of giant mipmaps that are clipped to a specified maximum size, which speeds up the process since Google Earth relies on the fact that a user only wants to see a portion of the mipmap at a time.

Since the data comes from different sources, it is provided at different resolutions, which is why your town may appear really crisp, while a town in, say, Mexico, is blurry. Google Earth has several countries including the United States, Canada, and the United Kingdom mapped out clearly to the street level, and there is a good amount of information on other regions like Western Europe, India, and Japan, but everything else is hit or miss. While you can zoom in to look at the Egyptian Pyramids in great detail, for example, you can’t see where the local grocery is.

There is also controversy that it may provide unwanted information, whether in regard to personal privacy or national and international security. Google has blocked Google Earth in Iran and Sudan since 2007 due to U.S. government export restrictions, and Maroc Telecom, a major service provider in Morocco, has also blocked the application since 2006 for unknown reasons. Other countries such as India, South Korea, and Israel have expressed concern that Google Earth made highly classified locations visible to terrorist organizations, some of which have since been censored and pixelated by Google.

Regardless of how you use it, the images were brought to you through the use of satellite imaging.