What is "dead rise angle"?

In the context of the DST100, dead rise angel is the angel between the bottom surface of the DST100 and the horizon. A flat bottom boat has a dead rise angle of 0°, whereas a deep V-hull might cause the DST100 to exit the vessel at an angle. The DST100 needs to be mounted at less than 23° dead rise in order to accurately measure the depth of the water.