Interpreting objects on the sonar display

The size at which a target appears on the sonar display is dependent on many factors, and may not be proportional to its actual physical size.

Moving targets (fish)

Moving targets (such as fish) appear as objects in the water column.

The shape and size of a displayed target do not necessarily reflect actual target shape and size. Target shape and size are influenced by the sonar technology in use and the velocity of the target.

  1. When using traditional non-CHIRP sonar: Fish targets appear as blob-like shapes on the screen.

  2. When using traditional CHIRP sonar: Fish targets appear as arches on the screen.

Traditional non-CHIRP vs CHIRP

Target’s length

The length of targets is determined by the duration of time in which the detected target is in the transducer’s beam. If the target passes through the transducer beam quickly, the target will be narrow. The longer the target is within the transducer beam, the longer the target will appear on the screen.

Fish arches

With CHIRP sonar, targets travelling through the transducer beam will appear as arches (1). This is because the transducer’s beam is weaker at the edges than in the middle. Either side of the arch represents the weaker signal, and the center of the arch represents the target passing through the strongest part of the transducer beam. Targets that look like half an arch, have passed through the beam too quickly for the complete arch to be formed. The slower a target moves through the beam, the flatter and longer it becomes (2). If the target remains under the transducer beam for a long time, the target will appear as a flat line stretching across the screen (3).

Target's vertical width

The vertical width of a target can be used to help differentiate a target’s size. The bigger the vertical width, the bigger the target.

For fish, the size of the displayed target is dependent on the size of the fish’s swim bladder, rather than its overall size. Fish swim bladder size varies between different species of fish. The fish bladder is represented by the vertical width of the target, at or near its center.

  1. Larger vertical width = larger fish.

  2. Smaller vertical width = smaller fish.