3D mapping of underwater caves

Wakulla Springs-a group of deep, underground, water-filled caves south of Tallahassee, FL, USA-is an example where mapping has proved challenging. The Wakulla 2 expedition of the US Deep Caving Team Inc. had one primary goal-to automatically build the first fully 3D cave map. To make the map, divers had to survive the hostile environment. The divers' attention mostly focuses on staying alive, so the more automatic the surveying, the better. The critical piece of equipment for the Wakulla 2 expedition was the Digital Wall Mapper (DWM). The device was designed specifically for the project to gather survey data to make the 3D map. 32 sonar transducers were spirally arrayed around the nose of the 2-m long, 150-kg instrument. Thus, four times a second, the DWM sends and receives 32 equally spaced radial readings. The distance to the walls was important but not useful unless we knew the DWM's exact position and orientation. To record this information, we used an inertial measurement unit (IMU), which is located in the center of the DWM