One additional note...

Most all sonar units that have 2d (HB SI, HDS, etc)....get the "digital depth" from the cone shape 2d beam (and usually from just 1 of the 2 frequencies)...

Dedicated DI units that do not have a 2d piezo...get digital depth from the DI beam...

Some HB DI units...(dedicated DI HB's)...get digital depth from the DI piezo until a certain depth and then switches to get digital depth from the 2d piezo...(I still don't understand this one myself)...

Regardless of which piezo's sound pulse the unit gets digital depth from, the concept is the same...

Digital depth comes from the "strongest reoccurring echo return" in the emitted sound pulses ("pings")...then the processor "interprets" those "reoccuring" returns as the bottom...

And (as Pap said)...it is easily confused by dense brush, surface clutter, thermoclines, thick bait balls, etc...

Rickie