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 and Microsoft, announced a pair of key deals.

On October 7, Microsoft signed a multi-year contract with DigitalGlobe 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 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.