I am not 100% sure how we mark the bottom and if I knew the particulars I would not be able to say – but I don’t so what I say here is my own $0.02. I know that it could be the nearest point to the transducer but we also look at the returned signal and use algorithms to determine the actual bottom. Example: if there was a softer sonar return from say 9.6 feet deep and a stronger one at 10.0 feet deep, we may display the water depth as 10.0 feet deep while showing the weaker sonar return level from 9.6 to 10.0 feet deep. In the image off of the Vexilar web site; it could be possible to display the fish shown in what they label as the “DEAD ZONE”. This could happen if the fish reflect more sonar back to the unit than the slope does. The sonar returns from the fish would than be shown along with the weaker sonar returns from the slope. To simplify things though we can assume (uh oh!) that the unit would show the shallower sonar return as the bottom and not show the fish at all.

For your drop off you would see a thicker sonar return from the bottom with (possibly) the shallowest point being displayed as the digital depth readout. On a flatter bottom that had the same depth (and bottom composition) the same sonar return for the bottom reading would be thinner but would have a better chance of showing a second (or more) sonar return.