ShopKick's app offers a good demonstration of what greater indoor location accuracy can offer shoppers and stores, Marti says. He acknowledges that it does require some effort from the retailer. "Using an acoustic signal makes perfect sense," he says, "but anecdotally our big-box customers are not interested in putting extra stuff in their stores. Our belief is that existing infrastructure is a better route."

Point Inside provides a mobile mapping experience like that of Google Maps, but its maps are indoors and are created with wireless access points to get a location fix. Companies like Skyhook Wireless, Google, and Apple operate databases of wireless access points detectable from the street to help refine location estimates of applications like Google Maps for Mobile. Point Inside works with retailers and mall operators to gather accurate measurements of Wi-Fi signals from transmitters already in stores and their locations indoors and submits the data to Skyhook and others for inclusion in their databases.

Point Inside can then create maps for those places and apps that allow people to locate themselves inside a building. The density of wireless points inside a retail store--a typical Walmart will have 24 spread across its floor--enough to pinpoint a phone to within five meters in some situations, says Marti. Last week, Point Inside released an app that makes it possible to locate a particular product inside department stores operated by Meijer.

Store owners are becoming more aware of the possibilities of encouraging people to share where they are inside their businesses, says Marti--and future cell phones will make indoor location easier. In particular, he thinks a technique called "dead reckoning" will become more common. This technique, recently demonstrated by Microsoft, involves using location and orientation sensors to keep track of how far a phone has moved since its last location fix.

Marti also expects more sensitive radios to become a typical feature as cell-phone designers look to help users to locate themselves indoors and out. Such radios will improve triangulation from cell towers, Wi-Fi, and other sources.

"Our opportunity is really all those gray spaces you see on Google Maps," says Marti, referring to the areas on online maps between streets that contain buildings--and businesses that are keen to know more about the whereabouts of their customers.