There are many ways to judge an online map -- the panoramic sweep of its street views, how accurately it gives directions, the number of restaurant reviews it has.
But an increasingly popular way is by studying the fine-grained detail of its birds-eye photographs. Usually taken by satellite, they provide the real-world detail that computer-generated maps lack.
The arms race over overhead images heated up earlier this month as the two Web cartographers battling for the mantle of technical supremacy, Google Inc. and Microsoft Corp., announced a pair of key deals.
On Oct. 7, Microsoft signed a multi-year contract with DigitalGlobe Inc. to use its entire library of more than 177 million square miles of earth photos in its Virtual Earth and Live Search Maps.
Until last month, access to DigitalGlobe's entire library of images, some of them taken at resolutions as fine as 2 feet-per-screen-pixel, had been exclusive only to Google for use in its Google Maps and Google Earth services.
"It's safe to say that Microsoft's high-res coverage layer was not as extensive as Google's over the last couple of years," Michael McCarthy, senior director of business development, said in a phone interview earlier this month. Microsoft's deal with DigitalGlobe, which is non-exclusive, is "an effort on their part to catch up."
The following day, arch-rival GeoEye Inc. showed off the first color photos taken by its recently-launched GeoEye-1 satellite at a stunning 1.3 feet-per-pixel resolution. (See them at GeoEye's public web site. )
With exclusive commercial access to GeoEye-1's images for years to come, Google's lead over Microsoft on this closely-watched map benchmark seems certain.
Or is it? For one, when GeoEye-1 starts officially taking photos later this fall, it will be able to shoot up to 350,000 square miles a day, or an area about half the size of Texas. But the actual number of usable photos is always far lower, said GeoEye vice-president of marketing, Mark Brender.
"Half of the Earth is covered by clouds at any given time," said Brender. "Over Iraq, it's wonderful. Over Ecuador, it's terrible. They call it a rain forest for a reason."
Google has thrown in its lot with GeoEye so heavily that its logo adorned the rocket that blasted GeoEye-1 into space in September. But Google is not GeoEye's only customer, nor likely its most important one. Most of GeoEye's customers are government and military organizations.
While GeoEye intends to "be very responsive to what area of the world Google wants," says Brender, it won't always have first dibs.
As a result, it will "take some time," admitted a Google spokeswoman, before the new GeoEye maps begin trickling into Google Earth. In the meantime, Google will continue to use DigitalGlobe maps -- the same maps Microsoft now has access -- for many parts of the world.
Even when the GeoEye maps become available, Google won't be able to display them at their maximum resolution because of U.S. security rules governing satellite photos. Rather, Google will have to ratchet the resolution down to 1.6 meters per pixel.
Also, DigitalGlobe plans to launch its WorldView-2 satellite in the third quarter of next year, according to McCarthy. It will take color photos that at least match GeoEye-1's resolution, and possibly even best it, if U.S. rules are liberalized by then, he said.
The war of words over who has the best satellite imagery is also moot because U.S. government rules allow Google and Microsoft to use imagery that's better than 1.6-feet-per-pixel resolution -- provided they are taken by airplane or balloon.
Microsoft, for example, displays aerial photos of more than 200 cities. They were taken at resolutions of 6 inches per-pixel.
Google also has aerial photos of selected cities at the same resolution, said the spokeswoman. A few of them are cities that donated their high-res images to be imported into Google Earth for marketing reasons. The tourist city of Nanaimo, British Columbia, is a trailblazer in this.
Microsoft is also asking cities and local governments to do the same thing.
Finally, there is another map provider that at least in its home turf, appears to best both Google and Microsoft: 192.com, a UK-based online equivalent to the Yellow and White Pages in the United States. It displays high-res aerial photos for 280 square miles of London that users can super-zoom up to 1.5 inches per pixel. For the rest of the UK, 192.com's aerial images provide 4.9-inch-per-pixel resolution.
This story, "In Satellite Photo Resolution Race, Who's Winning?" was originally published by Computerworld.