Most aerial and satellite image Web sites like these have similar features and user interfaces, so when you figure out how to use one, using another site is easy. To give you a better idea of image resolution and how the sites work, I use a series of screenshots from each of these three Web sites to shows aerial photos of downtown Seattle, Washington. (If you look really, really, close, you can probably find a Starbuck’s or two.) I personally like these Web sites because they offer a variety of free or reasonably priced images.
The data on each of these Web sites has strengths and weaknesses. Some display free, high-resolution color images with corporate watermarks as well as lower-resolution color images with no watermarks. When I’m looking for aerial or satellite images, I generally visit all three to find an image that meets my needs and budget.
The availability of aerial and satellite images on the Web can change. For example, MapQuest (the popular street map Web site) used to display street maps and aerial photos provided by GlobeXplorer, but the aerial photo service was discontinued in December 2003. Microsoft’s TerraServer project stopped offering satellite imagery for locations all over the globe in 2000. Serving up images on the Web costs for bandwidth and storage; what’s free today — particularly from commercial sites — might not be around tomorrow.
TerraServer-USA
In 1998, Microsoft, Compaq, the USGS, and several other partners launched the TerraServer Web site. The site was a technology demonstration for Microsoft software and Digital (which had recently been purchased by Compaq) hardware and was dubbed "the world’s largest online database." TerraServer had a collection of compressed aerial photos and satellite images that took up over a terabyte (TB) of space. (A terabyte, which is 1,024GB, was a lot of disk space back then.) Anyone with an Internet connection could freely browse through the database of images.
Now renamed TerraServer-USA (www.terraserver-usa.com), this site provides aerial photos and topographic maps supplied by the USGS for just about anywhere in the United States. Although most of the aerial photos that TerraServer-USA provides are black-and-white, TerraServer-USA is adding USGS Urban Areas aerial photography collections of mapping major metropolitan areas with high-resolution, color aerial photos.
Displaying TerraServer-USA aerial photos
To display an aerial photo from TerraServer-USA, you first enter some search criteria. You can search for aerial photos three different ways.
Zoom-in searches
When you visit the main page of the TerraServer-USA site at www. terraserver-usa.com, you’re greeted with a map of the world. Areas displayed in green contain map data. Move the cursor to a location that you want to view and click. Keep clicking and zooming in until the aerial photo you’re interested is displayed.
General searches
Search for a TerraServer-USA aerial photo of a city or well-known location from the Search TerraServer text box. When you click the Go button, a list of the aerial photos that match the text is displayed. Click the text that describes the image you’re interested in to display it.
You can make your search more specific by entering a city name followed by a comma, a space, and the standard two-letter postal abbreviation of the state.
For practice, try searching for a well-known landmark, such as the Space Needle in Seattle, Washington. If you enter Space Needle, you get the aerial photo of downtown Seattle as shown in Figure 20-1. There’s not much detail, but you can find out how to increase the resolution in the section titled "Changing image resolution in TerraServer-USA."
Figure 20-1:
A Terra-ServerUSA aerial photo of downtown Seattle, Washington.
Specific searches
The most accurate way to search TerraServer-USA for aerial photos is to use the Advanced Find options:
Address: Enter a street address for the location that you’d like to view. The City, State, and ZIP Code fields are optional, but using them helps narrow down the search.
Geographic: Enter the latitude and longitude coordinates, either in decimal degrees or degrees, minutes, and seconds.
Place: Enter a place name and the state (and optionally the country). You can enter a partial place name; for example, entering San with California as the state displays all the locations with names beginning with San, such as San Diego, San Francisco, and even Santa Clara.
After you enter your search criteria, click the Go button, and the aerial photo is displayed. If several images match your selection criteria, you can choose the exact photo you’re interested in from a list.
Moving in a TerraSerVer-USA photo
When you view an aerial map in TerraServer-USA, a frame with arrows surrounds the image (refer to Figure 20-1). Click one of the arrows to pan the map in that direction. There’s also a compass rose to the left of the image that works the same way. Click a compass direction to scroll the map; for example, click E (for east) to scroll the map to the right.
Displaying a TerraServer-USA topographic map
In addition to displaying aerial photos, TerraServer-USA also can show a USGS topographic map of the same area that the aerial photo displays. (See Figure 20-2 for a topographic map of the area around the Space Needle.)
To display a map: From the View drop-down list (to the left of the aerial photo and above the compass), select Topo Map. A topographic map in roughly the same scale as the aerial photo is displayed.
To display an aerial photo: To return to the aerial photo image, select Aerial Photo from the View drop-down list.
TerraServer-USA is starting to provide high-resolution, color aerial photos of certain large metropolitan areas in the United States. If these images are available for the area you’re viewing, Urban Areas will appear in the View dropdown list. Select Urban Areas in the list to display a color photo.
Figure 20-2:
A Terra-Server-USA topographic map of downtown Seattle, Washington.
Changing image resolution in TerraSerVer-USA
After you display an aerial photo or a map, you can change the resolution. Resolution, especially when dealing with aerial photos, is expressed in meters (about a yard for nonmetric types). An aerial photo with 1-meter resolution means that you can see an object on the ground that’s one square meter in size. The smaller the resolution number, the more detail the image shows. For example, Figure 20-3 shows the Space Needle viewed at 1-meter resolution.
Topographic maps can be displayed from 2 to 512 meters in resolution, and aerial photos can be shown from 1 to 512 meters. If there’s data for an urban area with high-resolution, color photos, you can go from .25 to 64 meters; .25 meters is enough detail to start seeing people on the street, as shown in Figure 20-4.
A resolution scale is shown immediately below the compass, to the left of the map or aerial photo. To change the image resolution, you can either
Click one of the bars between the plus and minus buttons. The farthest bar on the left provides the least amount of detail and largest area, and the bar on the extreme right shows the greatest amount of detail and the smallest area.
Click the minus button or the plus button. This decreases or increases resolution one step at a time, respectively.
Figure 20-3:
The Space Needle, in the center, viewed at 1-meter resolution.
Figure 20.4:
The Space Needle viewed at .25-meter resolution.
The current resolution is displayed above the scale. In addition, the bar representing the current resolution is highlighted in a different color.
You can change the screen size of an aerial photo or map with the Map Size drop-down list that’s shown under the resolution scale. Images can be displayed in small, medium, or large sizes.
Getting image information from TerraSerVer-USA
When you use aerial photos, it’s always important to know when the photo was taken so you can know how current it is. TerraServer-USA displays the date the photo was snapped (or the date the map was made) above the image.
You can get more detailed image information by clicking the Info item above the displayed image. For an aerial photo, the Image Info window provides additional details such as the image size in pixels, the map projection, and a grid overlaid on the image showing latitude and longitude coordinates.
Another excellent source of free USGS aerial photos is The National Map Web: http://nationalmap.usgs.gov.
TerraSerVer.com
TerraServer.com (not to be confused with TerraServer-USA) is an offshoot of the original Microsoft TerraServer research project. When the original TerraServer was launched in 1998, the site offered SPIN-2 satellite imagery that covered Europe, Asia, and other areas outside the United States. The satellite data was leased for 18 months; in 2000, Aerial Images (the company that owned the leased data) decided to launch its own Web site, TerraServer.com (www.terraserver.com).
TerraServer.com sells aerial photos and satellite images from a number of different image providers. Currently, this site does not offer topographical maps, although that service is supposed to be on the way. You can freely view images that have a maximum 8-meter resolution. (An example image of downtown Seattle in this resolution is shown in Figure 20-5.) With 8-meter resolution, you can easily see large buildings, but the detail isn’t as great as the 1-meter (or lower) resolution USGS aerial photos that TerraServer-USA provides. TerraServer.com does make higher-resolution images available; you just need to subscribe to the service to be able to view them. (Subscription fees range from $6.95 for a day’s use to $99.95 for a year.) There are additional charges for downloading the images or ordering printed copies of them; depending on the size of the option and whether you want a digital or printed copy, expect to pay between $5-$80 per image.
Homeland insecurity
In December 2003, some aerial photos of Washington DC — long available from various Web sites — changed. Photos of such buildings as the White House, the Vice President’s residence, and the Capitol were altered so only blurred pixels appeared instead of sharp detail. (You can view some of the before and after photos at http://cryptome.org/ seatsuneyeball.htm.)
The changes were made by EarthData International, a company that the United States Geological Survey (USGS) contracts to take aerial photos. (The USGS then sells the images to companies for commercial use.) When EarthData went to the U.S. Secret Service to get permission to fly over airspace that’s been restricted since the September 11, 2001, terrorist attacks, the Secret Service requested that the images of certain buildings be degraded.
The Federal government isn’t alone in limiting Internet access to aerial photos. The New York state GIS (Geographic Information System) Clearinghouse (www.nysgis.state.ny.us) offers high-resolution aerial photos of the entire state but also restricts Web access to photos with areas that are deemed sensitive.
Just how much additional security is provided from censoring publicly available aerial photos is debatable. Images of the same government buildings and critical infrastructure facilities can be downloaded and viewed from other sources. And blacking out or reducing the detail of critical facilities might just end up drawing a terrorist’s attention to someplace that domestic authorities have identified as a possible target. Sigh. Welcome to the brave new world of post-9/11.
Figure 20-5:
A Terra Server.com 8-meter resolution image of downtown Seattle, Washington.
TerraServer.com has satellite images and color aerial photos available for many urban areas in and outside the United States that aren’t available on TerraServer-USA. If you don’t need high-resolution images, the free color images might meet your needs. Otherwise, you’ll need to subscribe to the TerraServer.com service and order the images or generate screenshots of the images you’re interested in.
Displaying a TerraSerVer.com image
To find an image, move your cursor over the Search item in the Web site’s menu bar. Click one of the following menu items to see whether the TerraServer.com database has an image of the area you’re interested in:
Coordinates: You can search for an image by latitude and longitude coordinates, either in decimal degrees or degrees, minutes, and seconds.
City (United States): Enter a city in the United States (and optionally, its state).
City (International): TerraServer.com has a collection of international city images. Enter the city you’re interested in viewing and its country.
Zip Code: Enter a ZIP code to display the center point of the ZIP code area. (This applies only to the U.S.)
County: Enter a county in the United States (and optionally, its state). An image with the center of the county will be displayed.
Address: Enter a street address (United States) and the city and state (or ZIP code).
After you enter your search criteria and click Search, TerraServer.com queries its database for matches. The three possible search outcomes are
Bupkis: No images in the TerraServer.com database meet your criteria.
Check your spelling or change your search criteria before you give up.
Multiple matches: A appears with the matching images; you select the one you want. For example, if you search for a city named Bend but don’t specify the state, you get a list of cities in Oregon, Montana, California, Texas, and other states.
Perfect match: TerraServer.com found a single match in its database. Pay dirt! From the list of available image types (with the supplier, date, and resolution), click the image you’d like to view.
TerraServer.com has many international images, but don’t expect photos of every place on Earth. The international images are mostly large urban areas and aren’t available for all metropolitan areas outside the United States.
Moving in a TerraSerVer.com image
After you display an image, you can change the image’s location by panning and its resolution by zooming in and out.
Panning: Panning simply means moving the image to view an unseen area. It’s like scrolling without the scroll bars. To pan the image
1. Make sure that the Pan radio button to the left of the image is selected.
2. Hold down the left mouse button on the image and drag the direction you want to move the image.
For example, to see more area to the north of the image, drag down toward the bottom of the screen.
3. Release the mouse button.
After a moment, the new image is displayed.
Zooming: TerraServer.com has a plus and minus button to the left of the image (refer to Figure 20-5) to control zooming in and out. (There’s also a Zoom In by Click radio button that you can select so that when you click the image, you automatically zoom in.)
Unless you’ve subscribed to the TerraServer.com service, you can’t zoom in on an image beyond 8-meter resolution. However, you can zoom out to view a larger area with less detail.
When you move the cursor over an image, the latitude and longitude coordinates of the location under the cursor are displayed to the left of the image. This is helpful for identifying specific locations.
GlobeXplorer
GlobeXplorer (www.globexplorer.com) specializes in selling online aerial photos, satellite images, and maps of the United States. You can browse through the GlobeXplorer’s database of images for free, but all images are marked with a series of GlobeXplorer watermarks. If you subscribe to the GlobeXplorer service for a fee ($29.95 a month), you can view images without watermarks.
The watermarks aren’t small and subtle. If you want to use the images for something other than personal reference, you’ll need to subscribe to the service or buy a copy of the image without the watermarks. However, you can increase the resolution of images to 1-meter or less for a considerable amount of detail. GlobeXplorer also has a very large database of color images.
Displaying a GlobeXplorer image
GlobeXplorer uses the ImageAtlas Web-based tool to search through the company’s database of aerial images. You start ImageAtlas from GlobeXplorer’s main Web page.
After ImageAtlas is running, you can search for images three different ways:
Map: ImageAtlas starts up with a satellite image, basemap of the United States. Click an area to zoom in, and keep clicking until the location you’re interested in appears. The map to the left of the image shows such features as cities and roads to help you figure out where you are.
Address: Enter the street address of the place you want to see in the text boxes next to the left of the image. At the minimum, you need to fill in a city and state or a ZIP code.
Latitude and longitude: If you know the latitude and longitude of the location, you can use it to display the image. Enter the coordinates in decimal degrees in the text boxes to the left of the image.
After you enter the address or coordinates, click the Find Image button, and the aerial photo is displayed.
GlobeXplorer works only with Microsoft’s Internet Explorer Web browser. If you’re using Netscape, Mozilla, Opera, or another browser, you need to use Internet Explorer to access the Web site.
Zooming in on a GlobeXplorer image
After an image is displayed at GlobeXplorer.com, you can zoom in to see more detail. Underneath the image is a zoom scale, represented by a series of dots.
Clicking the dot to the farthest left displays the largest area of coverage at the least amount of resolution. The dots on the extreme left display satellite images of the area.
Clicking the dot on the farthest right displays the smallest area of coverage with the greatest amount of detail. The dots on the right display aerial photos of the area.
A zoom box lets you draw a rectangle over the part of the photo where you want to see more detail. Follow these steps:
1. Select the Drag a Box to Zoom In radio button.
2. Move the cursor over the area you want to zoom in on.
3. Hold down the left mouse button and draw a rectangle over the area.
When you release the mouse button, the image zooms in to the area that you defined with the rectangle. An example of a zoomed in image at maximum resolution is shown in Figure 20-6.
Black-and-white or color?
Ever wonder why some images are shown in black-and-white while others are displayed in color? The short answer is that black-and-white film was used to create most of the USGS aerial photo data for the United States — and black-and-white film, processing, and photographic paper are cost-effective and stable. Color film, processing, and photographic paper are expensive, and color exposures (positives, negatives, and prints) can lose tonal quality over time. Commercial companies that produce custom aerial photos and images for resale tend to use color film for enhanced resolution and viewing. Most of the large urban areas in the U.S. have color images available because there is a greater opportunity for a commercial aerial photo company to sell the data, thus recouping the costs of taking the aerial photos and making a profit. Sometime in the future, high-resolution color images of the entire U.S. will be available, but for now, if the area you’re interested is far away from a major city, you’ll just have to settle for black-and-white.
Figure 20-6:
ImageAtlas display of the Space Needle, zoomed in at maximum resolution.
Moving around in a GlobeXplorer image
After you display an image, you can move to a new location to focus on the area you want to view.
A frame with compass directions surrounds the photo. Click the compass direction to scroll the image. For example, if you click NORTH, the image scrolls up to the north.
Select the Pan/Recenter radio button beneath the zoom scale. When you click the image, the photo is reorientated so wherever you clicked appears in the center. You can also move the image by holding down the left mouse button while the cursor is over the photo and then dragging. When you release the mouse button, a new location is shown.
Overlaying a street map in GlobeXplorer
GlobeXplorer has a slick feature with which you can overlay a MapQuest street map over a portion of the image. (An example is shown in Figure 20-7.) Because an aerial photo isn’t labeled with streets, this option can help you quickly orientate yourself. To overlay a street map
1. Select the Map & Aerial Overlay Toggle radio button beneath the zoom scale so that (Aerial On) is displayed
2. Move the cursor over the image.
As you move the cursor, a semitransparent image of a street map is overlaid on the image.
Figure 20-7:
A street map overlaid on an aerial photo of downtown Seattle, Washington.
