The reality of measuring anything in the marine environment is uncertainty; the medium (and the sensors, themselves) are moving, it is not possible to measure the properties of either the seafloor or the water column spatially or temporally to the preciseness required. Key sensors, like GPS receivers cannot be used underwater. Terrestrially, it is possible to reach a “shovel width” accuracy, or about 1 foot. Underwater, 1 meter is a challenge. The calibration line provides both a learning experience for the DCL developer and a tool to monitor DCL system performance. An example calibration line is shown below for MuST (Multi-Sensor Towbody).