Humminbird revolutionized fish finding sonar when they introduced Side Imaging. Because Side Imaging lets you clearly see and identify fish-holding structure on either side of the boat, it greatly reduces the time it takes to put a deep bend in your rod. Down Imaging soon followed.
Prior to this, imaging sonar had been used by the military, Homeland Security and commercial fishermen. Imaging sonar sends out a narrow band of high-frequency signals that produce video-like images. A tree looks like a tree, a rock looks like a rock and a submerged bridge is shown in glorious detail. Imaging sonar eliminates the guesswork associated with traditional 2D sonar.