Can someone tell me what the highlighted iine is running just above the bottom?

[Team Colibri]

You might get the line even if the tranducers are perfectly aligned, like when using 3 in 1. The different coneangle of 2D and DownScan means they will only draw the same bottom-line as long as that bottom is 100% flat and even within both coneangles.

[MichaelJ3]

You might get the line even if the tranducers are perfectly aligned, like when using 3 in 1. The different coneangle of 2D and DownScan means they will only draw the same bottom-line as long as that bottom is 100% flat and even within both coneangles.

[bloodman]

You might get the line even if the tranducers are perfectly aligned, like when using 3 in 1. The different coneangle of 2D and DownScan means they will only draw the same bottom-line as long as that bottom is 100% flat and even within both coneangles.

Exactly!!!! Image interpretation is the key in any sonar setup.