Note: this page was once monitored by SevenTwentyFour, a linkchecking company that claims to have a map of the web. I don't know about this, but according to their website, their robot will visit pages I nominate, nightly, and verify they still exist. It will send me an email if the page cannot be found. It's a subscription (paid-for) service. Curious thing is, I never bought anything from them. Never even heard of them. But someone else could sign up for their service and monitor my page with it. Their website says that monitoring external pages is a feature of their service, as you don't want a page you're linking to to go down. Fair enough... but according to Alta Vista, nobody was linking to this page ...... and according to my logs, the robot visited weekly, not nightly.
Curious? Perhaps it's because satellite imagery gets used for this.
TerraServer gets its images from two locations; United States Geological Survey (USGS) provides georectified digitized aerial photographs (known as digital orthophotoquads) from EarthExplorer, and SPIN-2 provides high-resolution satellite images.
SPIN-2 imagery is the highest resolution satellite imagery commercially available anywhere. This branded two-meter imagery is the result of a joint venture of SOVINFORMSPUTNIK (the Interbranch Association of Russian space agencies), based in Moscow; Aerial Images Inc. of Raleigh, N.C.; and Central Trading Systems Inc. of Huntington Bay, N.Y. The satellite imagery is distributed globally by SPIN-2 Marketing, which offers to the public cartographic quality, high-resolution images from Russian mapping satellite systems and new satellite imagery collected in early 1998.
The SPIN-2 images are digital, panchromatic, two-meter resolution images that have been geo-referenced and orthorectified. Currently, no other commercial system can provide two-meter resolution satellite imagery. The SPIN-2 image library contains extensive coverage of the Earth's surface--more than two million square kilometers--with images being added regularly from new satellite missions.
An original SPIN-2 image is a 40-kilometer-by-160 kilometer photographic swath taken by Russian mapping satellites. Because these broad images are too large to scan at once, each photograph is scanned into four separate 40-kilometer-square images. Each digital scan is then separated into four 20-kilometer square files. The image-editing process ultimately creates a "TerraServer SPIN-2" image that is 1/48th of degree wide by 1/96th of a degree high.
SPIN-2 images are ideal for use in digital mapping. SPIN-2 images are valuable for professional applications because of their exceptional clarity, image quality, and lack of distortion. These images are ideal for use in digital mapping; enhanced 911 and other public-safety system maps; environmental assessments; and management of forests and other natural resources.
The Microsoft TerraServer acts as an online catalog for the SPIN-2 image library. The images on the TerraServer are at full two-meter resolution, geo-referenced but not orthorectified. Orthorectified images, which are extremely useful for digital mappers, environmentalists, natural resource managers, and others using Geographic Information Systems may be ordered directly from SPIN-2 Marketing. Purchased images can be downloaded and selectively printed from TerraServer using the electronic commerce capabilities in TerraServer.
source: Microsoft TerraServer, USGS, Spin-2 Marketing