Although many Internet users know about street mapping Web sites like MapQuest, fewer people are aware of Web sites that provide free aerial photos (taken from aircraft) and satellite data. These images complement traditional maps, displaying features that don’t show up on topographic or street maps, such as vegetation and unmarked roads or trails. In addition, aerial photos and satellite images are often more up-to-date than topographic maps and provide more timely and accurate information.
In this topic, I present several popular Web sites that you can visit to view aerial photos and satellite images; I also describe the basics of using three of my favorite aerial photo and satellite services.
I treat both aerial photos and satellite images as simply graphics images in this topic because that’s how you view them on the Web. Unless there’s a need to specifically differentiate between an aerial photo and a satellite image, the word image in this topic refers to both.
One of the biggest advantages to using an image captured from an airplane or satellite is that it shows reality — not a cartographer’s representation drawn on a map. You can use aerial and satellite images to
11 Complement a topographic map to help determine your current location.
1 Find roads and trails not shown on maps.
Better understand how thick tree and brush cover will be in an area you plan to visit.
Don’t rely solely on these images for navigation. Aerial and satellite images typically don’t have natural or man-made features labeled or show elevation contours.
View neighborhoods in urban areas.
Augment marketing endeavors, Web sites, or presentations. Aerial photos are always attention-getters.
The two types of aerial photos and satellite images available on the Web are
Government: Governments use aircraft and satellites to map land for administrative and research purposes. Some maps are for intelligence gathering, but these images usually aren’t available to the public. Most government-produced images are usually free on the Internet. Keep in mind that many governments aren’t as liberal as the United States in distributing images because of national security reasons.
Commercial: A number of businesses specialize in taking aerial photographs or reselling photos and satellite images from other vendors. Some of these companies let users browse their Web sites freely for reduced-scale or watermarked images (images with an overlaid logo or text that might obscure detail), selling printed, full-size, unwatermarked versions of these images.
You can download free or commercial aerial photos for mapping programs,but Web-hosted aerial and satellite image services work well for a quick look at an area you’re interested in. Some advantages to using Web-hosted image services include
No need to have special software installed on a PC to view the images. (All you need is a Web browser.)
Some Web sites might require you to use a specific Web browser. For example, to access GlobeXplorer images, you need to use Internet Explorer.
Access of images from any computer connected to the Internet.
Simplified user interfaces of the Web pages that make viewing the images easy.
To save and edit free aerial and satellite images displayed on Web sites.However, purchasing a downloaded image from a commercial site might be quicker and easier.
Airplane or satellite?
You might think that the photos displayed on TerraServer-USA and other Web-hosted mapping sites come from satellites, but these photos are actually taken from airplanes flying at precise altitudes (up to 25,000 feet) with specially mounted cameras. Images that come from satellites (or the Space Shuttle) can be photos, but they can also be composite images put together from data that comes from sensors that detect differences in moisture, vegetation, or terrain. In general, satellite images don’t show as much detail as aerial photos. That’s changing; commercial services (such as DigitalGlobe’s QuickBird satellite) can produce high-resolution, color photos on demand for anywhere in the world. (See www.digitalglobe.com for some amazing examples of high-resolution satellite images.) In general, if you see images labeled Landsat, SPOT, SPIN-2, or QuickBird, they came from satellites. Most other images are photos taken from airplanes.
Don’t expect real-time, spy-satellite accuracy from free Web images. These images can be over five years old and not show current roads or other features. (Image accuracy is better in wilderness, rural, or urban areas that haven’t experienced a lot of natural or man-made change.)
