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GCN : September 2013
GCN SEPTEMBER 2013 • GCN.COM 19 PEOPLE HAVE BEEN us- ing maps since the dawn of time as a way to represent, and navigate, the world. Prehistoric hunters probably sketched plans of attack in the dirt. Much of the colonial pe- riod was spent trying to map di erent routes around the world. And today Google and MapQuest do their best get you to the store and back. But all maps have flaws, either based on the tech- niques used to create them or their display constraints. The first world map most children experience in school is prob- ably a Mercator projection. Created in 1569 by Gerardus Mercator, it was renowned for keeping the linear scale correct in all directions and is still the most popular map today. But it tends to distort large objects closer to the poles, so that Greenland looks bigger than Australia, when in fact it s about one third the overall size. Moving to a 3D map ---a globe --- solved the problems of scale and size when repre- senting the Earth. But govern- ment organizations these days are much less interested in knowing the general posi- tion of China or Hawaii rela- tive to the rest of the planet than the detailed physical characteristics of the insides of buildings or the surface of potential battlefields. The Defense Advanced Research Projects Agency not long ago completed a five-year program called the Urban Photonic Sandtable Display that creates a real- time, color, 360-degree 3D holographic display to assist battle planners. Now military planners can view 3D maps of battlefields without even hav- ing to put on special glasses. The 3D map can even be rotated and zoomed, giving maximum control to those tasked with planning danger- ous operations. However, creating a detailed 3D map of an area, especially an indoor struc- ture, requires special tools. Without some way to record what a building looks like, even DARPA s special UPSD hologram would remain blank. One of the best ways to cre- ate a 3D map is to simply send a human or a robot through an area taking pictures, then use software to stitch the images together to create a model that can then be explored virtually by others. That was the concept behind an MIT program last year that took a sensor from the Micro- soft Xbox Kinect videogame console and paired it with positional sensors and map- ping software. The idea is that firefighters entering a burning building trying to find survivors, or soldiers trying to clear a struc- ture of enemies, could benefit from knowing the terrain, if someone or something had gone in there before them and recorded data into a 3D map. However, a drawback with the 3D mapping software from MIT is the same one all robotic 3D mapping pro- grams face, even those that use very precise robots to take measurements. Called loop closure, or drift, it oc- curs when a robot-mounted camera returns to ground it has already covered. Because of slight errors between the path the robot was supposed to take and the path it actu- ally traveled, the software has trouble closing the loop and accurately modeling the complete picture. Doors may be slightly larger or smaller on the map than in reality. Stairway entrances might be represented too far to the left or right than their actual positions. Depending on the circum- stances, those errors can be either troublesome or deadly if they are the sole source of information. For smaller maps, loop errors are relatively minor, nothing like turning Green- land into the eighth continent of the world as with the Mercator projection. But loop errors are incremental, since the farther a robot travels, the more tiny positional errors add up. So after traversing a lot of ground, the loop process can become extremely inaccurate, perhaps even doubling some terrain features. So scientists at MIT and the National University of Ireland at Maynooth went back and found a way to close loop errors altogether. The secret is tracking the posi- tion of the camera in space as the robot moves. Then when the camera gets back to a place that it has already seen, an algorithm compares the robot s path and the projected path and adjusts the model accordingly. "Before the map has been corrected, it s sort of all tangled up in itself," Thomas Whelan, a Ph.D. student at NUI told MIT News. "We use knowledge of where the camera s been to untangle it. The technique we developed allows you to shift the map, so it warps and bends into place." I m pretty sure Mercator would be impressed, but more important, these 3D maps, now that they can be com- pletely accurate, could help save lives. • TEAM CLOSES THE LOOP ON FLAWS IN 3D MAPS BY JOHN BREEDEN II EMERGING TECH The loop process can become extremely inaccurate, even doubling some terrain features.